Plasma membranes are subject to continuous deformations. Strikingly, some of these transient membrane undulations yield membrane‐associated signaling hubs that differ in composition and function, depending on membrane geometry and the availability of co‐factors. Here, recent advancements on this ubiquitous type of receptor‐independent signaling are reviewed, with a special focus on emerging concepts and technical challenges associated with studying these elusive signaling sites.

Signal transduction via receptors for N‐formylmethionyl peptide chemoattractants (FPR) on human neutrophils is a highly regulated process which involves participation of cytoskeletal elements. Evidence exists suggesting that the cytoskeleton and/or the membrane skeleton controls the distribution of FPR in the plane of the plasma membrane, thus controlling the accessibility of FPR to different proteins in functionally distinct domains. In desensitized cells, FPR are restricted to domains which are depleted of G proteins but enriched in cytoskeletal proteins such (...) as actin and fodrin. Thus, the G protein signal transduction partners of FPR become inaccessible to the agonist‐occupied receptor, preventing cell activation. The mechanism of interaction of FPR with the membrane skeleton is poorly understood but evidence is accumulating that suggests a direct binding of FPR (and other receptors) to cytoskeletal proteins such as actin. (shrink)

The common neurotrophin receptor (p75NTR) regulates various functions in the developing and adult nervous system. Cell survival, cell death, axonal and growth cone retraction, and regulation of the cell cycle can be regulated by p75NTR‐mediated signals following activation by either mature or pro‐neurotrophins and in combination with various co‐receptors, including Trk receptors and sortilin. Here, we review the known functions of p75NTR by cell type, receptor‐ligand combination, and whether regulated intra‐membrane proteolysis of p75NTR is required for signalling. (...) We highlight that the generation of the intracellular domain fragment of p75NTR is associated with many of the receptor functions, regardless of its ligand and co‐receptor interactions. (shrink)

The majority of G protein-coupled receptors (GPCRs) self-assemble in the form dimeric/oligomeric complexes along the plasma membrane. Due to the molecular interactions they participate, GPCRs can potentially provide the framework for discriminating a wide variety of intercellular signals, as based on some kind of combinatorial receptor codes. GPCRs can in fact transduce signals from the external milieu by modifying the activity of such intracellular proteins as adenylyl cyclases, phospholipases and ion channels via interactions with specific G-proteins. However, in (...) spite of the number of cell functions they can actually control, both GPCRs and their associated signal transduction pathways are extremely well conserved, for only a few alleles with null or minor functional alterations have so far been found. This would seem to suggest that, beside a mechanism for DNA repairing, there must be another level of quality control that may help maintaining GPCRs rather stable throughout evolution. We propose here receptor oligomerization to be a basic molecular mechanism controlling GPCRs redundancy in many different cell types, and the plasma membrane as the first hierarchical cell structure at which selective categorical sensing may occur. Categorical sensing can be seen as the cellular capacity for identifying and ordering complex patterns of mixed signals out of a contextual matrix, i.e., the recognition of meaningful patterns out of ubiquitous signals. In this context, redundancy and degeneracy may appear as the required feature to integrate the cell system into functional units of progressively higher hierarchical levels. (shrink)

Long‐term potentiation (LTP) of excitatory synapses is a leading model to explain the concept of information storage in the brain. Multiple mechanisms contribute to LTP, but central amongst them is an increased sensitivity of the postsynaptic membrane to neurotransmitter release. This sensitivity is predominantly determined by the abundance and localization of AMPA‐type glutamate receptors (AMPARs). A combination of AMPAR structural data, super‐resolution imaging of excitatory synapses, and an abundance of electrophysiological studies are providing an ever‐clearer picture of how (...) AMPARs are recruited and organized at synaptic junctions. Here, we review the latest insights into this process, and discuss how both cytoplasmic and extracellular receptor elements cooperate to tune the AMPAR response at the hippocampal CA1 synapse. (shrink)

Receptor‐mediated endocytosis occurs via clathrin‐coated pits and is therefore coupled to the dynamic cycle of assembly and disassembly of the coat constituents. These coat proteins comprise part, but certainly not all, of the machinery involved in the recognition of membrane receptors and their selective packaging into transport vesicles for internalization. Despite considerable knowledge about the biochemistry of coated vesicles and purified coat proteins, little is known about the mechanisms of coated pit assembly, receptor‐sorting and coated vesicle formation. Cell‐free (...) assays which faithfully reconstitute these events provide powerful new tools with which to elucidate the overall mechanism of receptor‐mediated endocytosis. (shrink)

In the context of 1960s research on biological membranes, scientists stumbled upon a curiously coloured material substance, which became called the “purple membrane.” Interactions with the material as well as chemical analyses led to the conclusion that the microbial membrane contained a photoactive molecule similar to rhodopsin, the light receptor of animals’ retinae. Until 1975, the find led to the formation of novel objects in science, and subsequently to the development of a field in the molecular life sciences (...) that comprised biophysics, bioenergetics as well as membrane and structural biology. Furthermore, the purple membrane and bacteriorhodopsin, as the photoactive membrane transport protein was baptized, inspired attempts at hybrid bio-optical engineering throughout the 1980s. A central motif of the research field was the identification of a functional biological structure, such as a membrane, with a reactive material substance that could be easily prepared and manipulated. Building on this premise, early purple membrane research will be taken as a case in point to understand the appearance and transformation of objects in science through work with material substances. Here, the role played by a perceptible material and its spontaneous change of colour, or reactivity, casts a different light on objects and experimental practices in the late twentieth century molecular life sciences. With respect to the impact of chemical working and thinking, the purple membrane and rhodopsins represent an influential domain straddling the life and chemical sciences as well as bio- and material technologies, which has received only little historical and philosophical attention. Re-drawing the boundary between the living and the non-enlivened, these researches explain and model organismic activity through the reactivity of macromolecular structures, and thus palpable material substances. (shrink)

Receptor oligomerization plays a key role in maintaining genome stability and restricting protein mutagenesis. When properly folded, protein monomers assemble as oligomeric receptors and interact with environmental ligands. In a gene-centered view, the ligand specificity expressed by these receptors is assumed to be causally predetermined by the cell genome. However, this mechanism does not fully explain how differentiated cells have come to express specific receptor repertoires and which combinatorial codes have been explored to activate their associated signaling pathways. (...) It is our contention that the plasma membrane acts as the locus where several contextual cues may be integrated. As such it allows the semiotic selection of those receptor configurations that provide cells with the minimum essential requirements for agency. The occurrence of protein misfolding makes it impossible for receptor monomers to assemble along the membrane and to sustain meaningful relationships with environmental ligands. How could a cell lineage deal with these loss-of-function mutations during evolution and restrain gene redundancy accordingly? In this paper, we will be arguing that the easiest way for bacteria clones to accomplish this goal is by getting rid of cells expressing mutated receptor proteins. The mechanism sustaining this cell selection is also occurring in many somatic tissues and its function is currently believed to counteract in vivo protein mutagenesis. Our discussion will be mainly focused on the significance and semiotic nature of the interplay between membrane receptors and the epigenetic control of gene expression, as mediated by the control of mismatched repairing and protein folding mechanisms. (shrink)

Basement membranes are thin sheets of extracellular proteins situated in close contact with cells at various locations in the body. They have a great influence on tissue compartmentalization and cellular phenotypes from early embryonic development onwards. The major constituents of all basement membranes are collagen IV and laminin, which both exist as multiple isoforms and each form a huge irregular network by self assembly. These networks are connected by nidogen, which also binds to several other components (proteoglycans, fibulins). Basement membranes (...) are connected to cells by several receptors of the integrin family, which bind preferentially to laminins and collagen IV, and via some lectin‐type interactions. The formation of basement membranes requires cooperation between different cell types since nidogen, for example, is usually synthesized by cells other than those exposed to the basement membranes. Thus many molecular interactions, of variable affinities, determine the final shape of basement membranes and their preferred subanatomical localization. (shrink)

The role of membrane receptors is regarded as being to transduce the signal represented by ligand binding from the external cell surface across the membrane into the cell. Signals are subsequently conveyed from the cytoplasm to the nucleus through a combination of second-messenger molecules, kinase/phosphorylation cascades, and transcription factor (TF) translocation to effect changes in gene expression. Mounting evidence suggests that through direct targeting to the nucleus, polypeptide ligands and their receptors may have an important additional (...) signaling role. Ligands such as those of the platelet-derived and fibroblast growth factor classes, as well as cytokines such as interferon-γ and interleukins-1 and -5, have been found to localize in the nucleus through the action of nuclear localization sequences (NLSs). Where tested, these NLSs appear to be essential for full signaling activity and may be responsible for cotranslocating receptors to the nucleus in complexes with their ligands. The implication is that, subsequent to endocytosis at the membrane, particular polypeptide ligands or their receptors, or both, may translocate to the nucleus to participate directly in gene regulation. BioEssays 20:400–411, 1998.© 1998 John Wiley & Sons Inc. (shrink)

A model of an intentional self-observing system is proposed based on the structure and functions of astrocyte-synapse interactions in tripartite synapses. Astrocyte-synapse interactions are cyclically organized and operate via feedforward and feedback mechanisms, formally described by proemial counting. Synaptic, extrasynaptic and astrocyte receptors are interpreted as places with the same or different quality of information processing described by the combinatorics of tritograms. It is hypothesized that receptors on the astrocytic membrane may embody intentional programs that select corresponding (...) synaptic and extrasynaptic receptors for the formation of receptor-receptor complexes. Basically, the act of self-observation is generated if the actual environmental information is appropriate to the intended observation processed by receptor-receptor complexes. This mechanism is implemented for a robot brain enabling the robot to experience environmental information as “its own”. It is suggested that this mechanism enables the robot to generate matches and mismatches between intended observations and the observations in the environment, based on the cyclic organization of the mechanism. In exploring an unknown environment the robot may stepwise construct an observation space, stored in memory, commanded and controlled by the intentional self-observing system. Finally, the role of self-observation in machine consciousness is shortly discussed. (shrink)

The role of membrane receptors is regarded as being to transduce the signal represented by ligand binding from the external cell surface across the membrane into the cell. Signals are subsequently conveyed from the cytoplasm to the nucleus through a combination of second-messenger molecules, kinase/phosphorylation cascades, and transcription factor (TF) translocation to effect changes in gene expression. Mounting evidence suggests that through direct targeting to the nucleus, polypeptide ligands and their receptors may have an important additional (...) signaling role. Ligands such as those of the platelet-derived and fibroblast growth factor classes, as well as cytokines such as interferon-γ and interleukins-1 and -5, have been found to localize in the nucleus through the action of nuclear localization sequences (NLSs). Where tested, these NLSs appear to be essential for full signaling activity and may be responsible for cotranslocating receptors to the nucleus in complexes with their ligands. The implication is that, subsequent to endocytosis at the membrane, particular polypeptide ligands or their receptors, or both, may translocate to the nucleus to participate directly in gene regulation. BioEssays 20:400–411, 1998.© 1998 John Wiley & Sons Inc. (shrink)

Recent findings necessitate revision of the traditional view of G protein‐coupled receptor (GPCR) signaling and expand the diversity of mechanisms by which receptor signaling influences cell behavior in general. GPCRs elicit signals at the plasma membrane and are then rapidly removed from the cell surface by endocytosis. Internalization of GPCRs has long been thought to serve as a mechanism to terminate the production of second messengers such as cAMP. However, recent studies show that internalized GPCRs can continue to either (...) stimulate or inhibit cAMP production in a sustained manner. They do so by remaining associated with their cognate G protein subunit and adenylyl cyclase at endosomal compartments. Once internalized, the GPCRs produce cellular responses distinct from those elicited at the cell surface. (shrink)

This paper tells the story of G-protein coupled receptors, one of the most important scientific objects in contemporary biochemistry and molecular biology. By looking at how cell membrane receptors turned from a speculative concept into a central element in modern biochemistry over the past 40 years, we revisit the role of manipulability as a criterion for entity realism in wet-lab research. The central argument is that manipulability as a condition for reality becomes meaningful only once scientists have (...) decided how to conceptually coordinate measurable effects distinctly to a specific object. We show that a scientific entity, such as GPCRs, is assigned varying degrees of reality throughout different stages of its discovery. The criteria of its reality, we further claim, cannot be made independently of the question about how this object becomes a standard by which the reality of neighbouring elements of enquiry is evaluated. (shrink)

You are what you eat – this well‐known phrase properly describes the phenomenon of the effects of diet on acute and chronic inflammation. Several lipids and lipophilic compounds that are delivered with food or are produced in situ in pathological conditions exert immunomodulatory activity due to their interactions with the plasma membrane. This group of compounds includes cholesterol and its oxidized derivatives, fatty acids, α‐tocopherol, and polyphenols. Despite their structural heterogeneity, all these compounds ultimately induce changes in plasma (...) class='Hi'>membrane architecture and fluidity. By doing this, they modulate the dynamics of plasma membrane receptors, such as TLR4. This receptor is activated by lipopolysaccharide, triggering acute inflammation during bacterial infection, which often leads to sepsis and is linked with diverse chronic inflammatory diseases. In this review, we discuss how the impact on plasma membrane properties contributes to the immunomodulatory activity of dietary compounds, pointing to the therapeutic potential of some of them.Also watch the Video Abstract. (shrink)

MUC1 and MUC4 are the two membrane mucins that have been best characterized. Although they have superficially similar structures and have both been shown to provide steric protection of epithelial surfaces, recent studies have also implicated them in cellular signaling. They act by substantially different mechanisms, MUC4 as a receptor ligand and MUC1 as a docking protein for signaling molecules. MUC4 is a novel intramembrane ligand for the receptor tyrosine kinase ErbB2/HER2/Neu, triggering a specific phosphorylation of the ErbB2 in (...) the absence of other ErbB ligands and potentiating phosphorylation and signaling through the ErbB2/ErbB3 heterodimeric receptor complex formed in the presence of neuregulin. In contrast, MUC1 has a highly conserved cytoplasmic tail, which binds β‐catenin, a key component of adherens junctions and a regulator of transcription, in a process that is tightly regulated by MUC1 phosphorylation. The specific localization of these membrane mucins to the apical surfaces of epithelial cells suggests that their signaling functions may be important as sensor mechanisms in response to invasion or damage of epithelia. BioEssays 25:66–71, 2003. © 2002 Wiley Periodicals, Inc. (shrink)

CB1 receptors are functionally present within brain mitochondria, although they are usually considered specifically targeted to plasma membrane. Acute activation of mtCB1 alters mitochondrial ATP generation, synaptic transmission, and memory performance. However, the detailed mechanism linking disrupted mitochondrial metabolism and synaptic transmission is still uncharacterized. CB1 receptors are among the most abundant G protein-coupled receptors in the brain and impact on several processes, including fear coping, anxiety, stress, learning, and memory. Mitochondria perform several key physiological processes (...) for neuronal homeostasis, including production of ATP and reactive oxygen species, calcium buffering, metabolism of neurotransmitters, and apoptosis. It is therefore possible that acute activation of mtCB1 impacts on these different mitochondrial functions to modulate synaptic transmission. In reviewing and integrating across the literature in this area, we describe the possible mechanisms involved in the regulation of brain physiology by mtCB1 receptors. Activation of mitochondrial CB1 but not of plasma membrane CB1 decreases brain mitochondrial activity, glutamatergic synaptic transmission, and memory performance. Several processes can be involved in this alteration of brain physiology, including mitochondrial ATP and ROS production, axonal mitochondrial transport, Ca2+ homeostasis, and/or metabolism of the neurotransmitter glutamate. (shrink)

Extracellular ATP released from necrotic cells in inflamed tissues activates the P2X7 receptor, stimulates the exposure of phosphatidylserine, and causes cell lysis. Recent findings indicated that XK, a paralogue of XKR8 lipid scramblase, forms a complex with VPS13A at the plasma membrane of T cells. Upon engagement by ATP, an unidentified signal(s) from the P2X7 receptor activates the XK‐VPS13A complex to scramble phospholipids, followed by necrotic cell death. P2X7 is expressed highly in CD25+CD4+ T cells but weakly in CD8+ (...) T cells, suggesting a role of this system in the activation of the immune system to prevent infection. On the other hand, a loss‐of‐function mutation in XK or VPS13A causes neuroacanthocytosis, indicating the crucial involvement of XK‐VPS13A‐mediated phospholipid scrambling at plasma membranes in the maintenance of homeostasis in the nervous and red blood cell systems. (shrink)

SNAREs (soluble N‐ethylmaleimide‐sensitive factor attachment protein receptors) are a large family of proteins that are present on all organelles involved in intracellular vesicle trafficking and secretion. The interaction of complementary SNAREs found on opposing membranes presents an attractive lock‐and‐key mechanism, which may underlie the specificity of vesicle trafficking. Moreover, formation of the tight complex between a vesicle membrane SNARE and corresponding target membrane SNAREs could drive membrane fusion. In synapses, this tight complex, also referred to as (...) the synaptic core complex, is essential for neurotransmitter release. However, recent observations in knockout mice lacking major synaptic SNAREs challenge the prevailing notion on the executive role of these proteins in fusion and open up several questions about their exact role(s) in neurotransmitter release. Persistence of a form of regulated neurotransmitter release in these mutant mice also raises the possibility that other cognate or non‐cognate SNAREs may partially compensate for the loss of a particular SNARE. Future analysis of SNARE function in central synapses will also have implications for the role of these molecules in other vesicle trafficking events such as endocytosis and vesicle replenishment. Such analysis can provide a molecular basis for synaptic processes including certain forms of short‐term synaptic plasticity. BioEssays 24:926–936, 2002. © 2002 Wiley Periodicals, Inc. (shrink)

Wnt signaling plays pivotal roles during our entire lives, from conception to death, through the regulation of morphogenesis in developing embryos and the maintenance of tissue homeostasis in adults. The regulation of Wnt signaling occurs on several levels: at the receptor level on the plasma membrane, at the β‐catenin protein level in the cytoplasm, and through transcriptional regulation in the nucleus. Several recent studies have focused on the mechanisms of Wnt receptor regulation, following the discovery that the Wnt receptor (...) frizzled (Fzd) is a target of the ubiquitin ligases, RNF43 and ZNRF3. RNF43 and ZNRF3 are homologous genes that are mutated in several cancers. The details underlying their mechanism of action continue to unfold, while at the same time raising many new questions. In this review, we discuss advances and controversies in our understanding of Wnt receptor regulation. (shrink)

A new high‐resolution structure of a pain‐sensing ion channel, TRPA1, provides a molecular scaffold to understand channel function. Unexpected structural features include a TRP‐domain helix similar to TRPV1, a novel ligand‐binding site, and an unusual C‐terminal coiled coil stabilized by inositol hexakisphosphate (IP6). TRP‐domain helices, which structurally act as a nexus for communication between the channel gates and its other domains, may thus be a feature conserved across the entire TRP family and, possibly, other allosterically‐gated channels. Similarly, the TRPA1 antagonist‐binding (...) site could also represent a druggable location in other ion channels. Combined with known TRPA1 functional properties, the structural role for IP6 leads us to propose that polyphosphate unbinding could act as a molecular kill switch for TRPA1 inactivation. Finally, although packing of the TRPA1 membrane‐proximal region hints at a mechanism for electrophile sensing, the details of how TRPA1 responds to noxious reactive electrophiles and temperature await future studies.Also watch the Video Abstract. (shrink)

Glycine is a major inhibitory neurotransmitter in the spinal cord and in the brain stem, where it acts by activating a chloride conductance. The postsynaptic glycine receptor has been purified and contains two transmembrane subunits of 48 kDa (α) and 58 kDa (β), and a peripheral membrane protein of 93 kDa. cDNA sequencing of the α and β subunits has revealed a common structural organization and a strong homology between these polypeptides and the nicotinic acetylcholine and GABAA receptor proteins. (...) The glycine receptor exhibits a heterogeneity resulting from the existence of several α subtypes with distinct functional properties and different developmental expressions. When present in the central nervous system in situ, as well as in primary cultures of spinal cord neurons, these receptors are localized at the postsynaptic membrane adjacent to the presynaptic release sites, thus forming functional microdomains at the neuronal surface. This distribution raises the question of the formation and the maintenance of the heterogeneity of the somato‐dendritic plasma membrane. (shrink)

Cell adhesion complexes are critical for the physical coordination of cell–cell interactions and the morphogenesis of tissues and organs. Many adhesion receptors are anchored to the plasma membrane by a glycosylphosphatidylinositol (GPI) moiety and are thereby partitioned into membrane rafts. In this review, we focus on reciprocal interactions between rafts and adhesion molecules, leading to receptor clustering and raft expansion and stability. A model for a three‐stage adhesion complex assembly process is also proposed. First, GPI‐anchored adhesion molecules (...) are recruited into rafts, which in turn promote receptor cis‐oligomerization and thereby produce precursory complexes primed for avid trans‐interactions. Second, trans‐interactions of the receptors cross‐link and stabilize large amalgams of rafts at sites of adhesion complex assembly. Finally, the enlarged and stabilized rafts acquire enhanced abilities to recruit the cytoskeleton and induce signaling. This process exemplifies how the domain structure of the plasma membrane can impact the function of its receptors. BioEssays 24:996–1003, 2002. © 2002 Wiley‐Periodicals, Inc. (shrink)

Embryogenesis requires the precise movement and reorganization of many cell and tissue types. Presumably, cell surface receptors allow cells to interact selectively with adjacent cells and with the extracellular environment, as well as initiate differentiative events by transducing appropriate signals across the plasma membrane. One cell surface component that serves as a receptor during a variety of cellular interactions is β1,4‐galactosyltransferase. Cell surface galactosyltransferase participates in diverse cellular interactions by binding its specific glycoconjugate substrate on adjacent cell surfaces (...) or in the extracellular matrix. Biochemical, immunological, and molecular probes are being used to better define cell surface galactosyl‐transferase functional in cellular interactions during fertilization, intercellular adhesion, cell migration, and growth control. (shrink)

Interaction of ligands such as epidermal growth factor and interferon‐γ with the extracellular domains of their plasma membrane receptors results in internalization followed by translocation into the nucleus of the ligand and/or receptor. There has been reluctance, however, to ascribe signaling importance to this, the focus instead being on second messenger pathways, including mobilization of kinases and inducible transcription factors (TFs). The latter, however, fails to explain the fact that so many ligands stimulate the same second messenger cascades/TFs, (...) and yet show distinct gene activation profiles. This is particularly apt in the case of the seven STAT TFs that are held to be the mediators of the distinct cellular functions of over 60 ligands. The current review focuses on five representative nuclear localizing ligands for which there is documentation of translocation into the cytosol and nucleus through well‐characterized pathways, in addition to a role in gene activation by ligand/receptor in the nucleus. BioEssays 26:993‐1004, 2004. © 2004 Wiley Periodicals, Inc. (shrink)

The common neurotrophin receptor (p75NTR) regulates various functions in the developing and adult nervous system. Cell survival, cell death, axonal and growth cone retraction, and regulation of the cell cycle can be regulated by p75NTR‐mediated signals following activation by either mature or pro‐neurotrophins and in combination with various co‐receptors, including Trk receptors and sortilin. Here, we review the known functions of p75NTR by cell type, receptor‐ligand combination, and whether regulated intra‐membrane proteolysis of p75NTR is required for signalling. (...) We highlight that the generation of the intracellular domain fragment of p75NTR is associated with many of the receptor functions, regardless of its ligand and co‐receptor interactions. (shrink)

Outer membrane proteins (OMPs) maintain the viability of Gram‐negative bacteria by functioning as receptors, transporters, ion channels, lipases, and porins. Folding and assembly of OMPs involves synchronized action of chaperones and multi‐protein machineries which escort the highly hydrophobic polypeptides to their target outer membrane in a folding competent state. Previous studies have identified proteins and their involvement along the OMP biogenesis pathway. Yet, the mechanisms of action and the intriguing ability of all these molecular machines to work (...) without the typical cellular energy source of ATP, but solely based on thermodynamic principles, are still not well understood. Here, we highlight how different single‐molecule studies can shed additional light on the mechanisms and kinetics of OMP biogenesis. (shrink)

Membrane proteins possess certain features that make them susceptible to the electric fields generated at the level of the plasma membrane. A reappraisal of cell signalling, taking into account the protein interactions with the membrane electrostatic profile, suggests that an electrical dimension is deeply involved in this fundamental aspect of cell biology. At least three types of potentials can contribute to this dimension: (1) the potential across the compact layer of water adherent to membrane surfaces; this (...) potential is affected by classical inducers of cell differentiation, like dimethylsulfoxide and hexamethylenebisacetamide; (2) the potential across the Gouy‐Chapman double layer, which accounts for the effects of extracellular cations in the modulation of differentiation; and (3) the resting potential. This last potential and its governing ion currents can be exploited in localised mechanisms of cell signalling centred on the functional association of integrin receptors with ion channels. (shrink)

Cell proliferation in response to growth factors is mediated by specific high affinity receptors. Ligand‐binding by receptors of the protein tyrosine kinase family results in the stimulation of several intracellular signal transduction pathways. Key signalling enzymes are recruited to the plasma membrane through the formation of stable complexes with activated receptors. These interactions are mediated by the conserved, non‐catalytic SH2 domains present in the signalling molecules, which bind with high affinity and specificity to tyrosine‐phosphorylated sequences on (...) the receptors. The assembly of enzyme complexes is emerging as a major mechanism of signal transduction and may regulate the pleiotropic effects of growth factors. (shrink)

Although the Ca2+‐dependent proteinase (calpain) system has been found in every vertebrate cell that has been examined for its presence and has been detected in Drosophila and parasites, the physiological function(s) of this system remains unclear. Calpain activity has been associated with cleavages that alter regulation of various enzyme activities, with remodeling or disassembly of the cell cytoskeleton, and with cleavages of hormone receptors. The mechanism regulating activity of the calpain system in vivo also is unknown. It has been (...) proposed that binding of the calpains to phospholipid in a cell membrane lowers the Ca2+ concentration, [Ca2+], required for the calpains to autolyze, and that autolysis converts an inactive proenzyme into an active protease. Recent studies, however, show that the calpains bind to specific proteins and not to phospholipids, and that binding to cell membranes does not affect the [Ca2+] required for autolysis. It seems likely that calpain activity is regulated by binding of Ca2+ to specific sites on the calpain molecule, with binding to each site eliciting a response (proteolytic activity, calpastatin binding, etc.) specific for that site. Regulation must also involve an, as yet, undiscovered mechanism that increases the affinity of the Ca2+‐binding sites for Ca2+. (shrink)

Wnt proteins form a family of secreted signaling proteins that play a key role in various developmental events such as cell differentiation, cell migration, cell polarity and cell proliferation. It is currently thought that Wnt proteins activate at least three different signaling pathways by binding to seven transmembrane receptors of the Frizzled family and the co-receptor LRP6. Despite our growing knowledge of intracellular components that mediate a Wnt signal, the molecular events at the membrane have remained rather unclear. (...) Now several publications1-4 indicate that Frizzled receptors are G-protein coupled and kinases were identified that phosphorylate the co-receptor LRP6. These data deepen our understanding of Wnt-mediated signal transduction and provide more insight into how specificity may be achieved. BioEssays 28: 339–343, 2006. © 2006 Wiley Periodicals, Inc. (shrink)

It has long been believed that membrane proteins present in degradative compartments such as endolysosomes or vacuoles would be destined for destruction. Now however, it appears that mechanisms and machinery exist in simple eukaryotes such as yeast and more complex organisms such as mammals that can rescue potentially “doomed” membrane proteins by retrieving them from these “late” compartments and recycling them back to the Golgi complex. In yeast, a sorting nexin dimer containing Snx4p can recognize and retrieve the (...) Atg27p membrane protein while in mammals, the AP5 complex (with SPG11 and SPG15) directs the recycling of Golgi‐localized proteins along with the cation‐independent mannose 6‐phosphate receptor (CIMPR). Although the respective machinery is different, there is much commonality between yeast and mammals regarding the mechanisms of retrieval and the physiological importance of these late recycling pathways. (shrink)

Interleukin‐15 (IL‐15) is a pleiotropic cytokine of the 4 α‐helix bundle family, which binds to a receptor complex that displays common elements with the IL‐2 receptor and a unique high‐affinity α chain. This review focuses on juxtacrine and reverse signaling levels in the IL‐15/IL‐15R system. Specifically, we discuss how agonistic stimulation of membrane‐bound IL‐15 induces phosphorylation of members of the MAP kinase family and of focal adhesion kinase (FAK), thereby upregulating processes including cytokine secretion, cell adhesion and migration. In (...) addition, we explore IL‐15 trans‐presentation and intracellular signaling, and define promising molecular targets for future pharmacological intervention in infectious diseases and immunological disorders. These frontiers in IL‐15/IL‐15Rα research serve as highly instructive examples for key concepts, unsolved problems and therapeutic opportunities in juxtacrine and reverse signaling in general. BioEssays 28: 362–377, 2006. © 2006 Wiley Periodicals, Inc. (shrink)

PDZ (also called DHR or GLGF) domains are found in diverse membraneassociated proteins including members of the MAGUK family of guanylate kinase homologues, several protein phosphatases and kinases, neuronal nitric oxide synthase, and several dystrophin‐associated proteins, collectively known as syntrophins. Many PDZ domain‐containing proteins appear to be localised to highly specialised submembranous sites, suggesting their participation in cellular junction formation, receptor or channel clustering, and intracellular signalling events. PDZ domains of several MAGUKs interact with the C‐terminal polypeptides of a subset (...) of NMDA receptor subunits and/or with Shaker‐type K+ channels. Other PDZ domains have been shown to bind similar ligands of other transmembrane receptors. Recently, the crystal structures of PDZ domains, with and without ligand, have been determined. These demonstrate the mode of ligand‐binding and the structural bases for sequence conservation among diverse PDZ domains. (shrink)

Most mitochondrial proteins are synthesized in the cytosol as preproteins with a cleavable presequence and are delivered to the import receptors on the mitochondria by cytoplasmic import factors. The proteins are then imported to the intramitochondrial compartments by the import systems of the outer and inner membranes, TOM and TIM. Mitochondrial outer membrane proteins are synthesized without a cleavable presequence and most of them contain hydrophobic transmembrane domains, which, in conjunction with the flanking segments, function as the mitochondria (...) import signals. Some of the proteins are inserted into the outer membrane by the TOM machinery; the import signal probably arrests further translocation and is released from the translocation channel to the lipid bilayer. The other proteins are inserted into the membrane by a novel pathway independent of the TOM machinery. This article reviews recent developments in the biogenesis of mitochondrial outer membrane proteins. BioEssays 22:364–371, 2000. © 2000 John Wiley & Sons, Inc. (shrink)

Hepatocyte growth factor, a potent mitogen for epithelial and other cell types, and scatter factor, a stimulant of epithelial cell motility are identical. In addition to these mitogenic and motogenic functions, the factor has been shown to be an epithelial morphogen and also has antiproliferative effects in some cancer cell lines. The membrane receptor for hepatocyte growth factor/scatter factor has been identified as the c‐met proto‐oncogene product.

Secreted Frizzled‐related proteins (SFRPs) are modulators of the intermeshing pathways in which signals are transduced by Wnt ligands through Frizzled (Fz) membrane receptors. The Wnt networks influence biological processes ranging from developmental cell fate, cell polarity and adhesion to tumorigenesis and apoptosis. In the five or six years since their discovery, the SFRPs have emerged as dynamically expressed proteins able to bind both Wnts and Fz, with distinctive structural properties in which cysteine‐rich domains from Fz‐ and from netrin‐like (...) proteins are juxtaposed. The abundant expression of SFRP genes in the early embryo, altered expression patterns in disease states, and potential significance in the evolution of the vertebrate body plan, make these intriguing molecules relevant to investigations in diverse fields of biology and biomedical sciences. BioEssays 24:811–820, 2002. © 2002 Wiley Periodicals, Inc. (shrink)

Stomata, the most influential components in gas exchange with the atmosphere, represent a revealing system for studying cell fate determination. Studies in Arabidopsis thaliana have demonstrated that many of the components, functioning in a signaling cascade, guide numerous cell fate transitions that occur during stomatal development. The signaling cascade is initiated at the cell surface through the activation of the membrane receptors TOO MANY MOUTHS (TMM) and/or ERECTA (ER) family members by the secretory peptide EPIDERMAL PATTERNING FACTOR1 (EPF1) (...) and/or a substrate processed proteolytically by the subtilase STOMATAL DENSITY AND DISTRIBUTION1 (SDD1) and transduced through cytoplasmic MAP kinases (YODA (YDA), MKK4/MKK5, and MPK3/MPK6) towards the nucleus. In the nucleus, these MAP kinases regulate the activity of the basic helix‐loop‐helix (bHLH) proteins SPEECHLESS (SPCH), MUTE, and FAMA, which act in concert with the bHLH‐Leu zipper protein SCREAM (SCRM) (and/or its closely related paralog, SCREAM2). This article reviews current insights into the role of this signaling cascade during stomatal development. (shrink)

The vav proto‐oncogene encodes a 95 kDa protein which is expressed exclusively in hematopoietic cells. Analysis of the deduced amino acid sequence has revealed the presence of a src‐homology 2 (SH2) domain, 2 SH3 domains, a cysteine‐rich region with similarity to protein kinase C, and a region highly similar to proteins with guanine nucleotide exchange activity on ras‐like GTPases. Recent work has shown that vav is tyrosine phosphorylated in response to stimulation of surface membrane receptors in a variety (...) of hematopoietic cell lines. Vav may play a role in hematopoietic cell signaling by coupling tyrosine kinase pathways to ras‐like GTPases through the regulation of guanine nucleotide exchange. (shrink)

Cytokine receptors fall into two basic classes: those with their own intrinsic protein tyrosine kinase (PTK) domain, and those lacking a PTK domain. Nonetheless, PTK activity plays a fundamental role in the signal transduction processes lying downstream of both classes of receptor. It now seems likely that many of those cytokine receptors that lack their own PTK domain use members of the JAK family of PTKs to propagate their intracellular signals. Moreover, the involvement of the JAK kinases in (...) a newly defined pathway which links membrane receptors directly to the activation of nuclear genes, via latent cytoplasmic transcription factors known as STATs (for Signal Transducers and Activators of Transcription), appears to be a theme common to cytokine receptors of both classes. (shrink)

The direction and specificity of endolysosomal membrane trafficking is tightly regulated by various cytosolic and membrane‐bound factors, including soluble NSF attachment protein receptors (SNAREs), Rab GTPases, and phosphoinositides. Another trafficking regulatory factor is juxta‐organellar Ca2+, which is hypothesized to be released from the lumen of endolysosomes and to be present at higher concentrations near fusion/fission sites. The recent identification and characterization of several Ca2+ channel proteins from endolysosomal membranes has provided a unique opportunity to examine the roles (...) of Ca2+ and Ca2+ channels in the membrane trafficking of endolysosomes. SNAREs, Rab GTPases, and phosphoinositides have been reported to regulate plasma membrane ion channels, thereby suggesting that these trafficking regulators may also modulate endolysosomal dynamics by controlling Ca2+ flux across endolysosomal membranes. In this paper, we discuss the roles of phosphoinositides, Ca2+, and potential interactions between endolysosomal Ca2+ channels and phosphoinositides in endolysosomal dynamics. (shrink)

To understand how the photoreceptor protein rhodopsin performs in its role as a receptor, its structure needs to be determined at the atomic level. Upon receiving a photon of light, rhodopsin undergoes a change in conformation that allows it to bind and activate the C-protein, transducin. An important future goal should be to determine the structure of both the inactive and the photoactivated state of rhodopsin, R*. This should provide the groundwork necessary for experiments on how rhodopsin achieves its signaling (...) state R*, and how R* functions to activate transducin. To do this, the crystal structure of both rhodopsin and R* must be determined. Few membrane proteins have been successfully crystallized, so this is not a trivial undertaking. Two- or three dimensional crystals of rhodopsin must be prepared that are well ordered, to produce a high-resolution structure. Rhodopsin must be purified to homogeneity and the appropriate detergent(s) selected for crystallization experiments. Long-term thermal stability of the rhodopsin-detergent complex must be achieved in the presence of a precipitant. Two-dimensional crystals may offer advantages in investigating the structure of R*, but the structure obtained may be limited in resolution. It is necessary to work with rhodopsin in the dark, unless suitable light-stable retinal derivatives are developed. Protein engineering of rhodopsin offers attractive opportunities to improve its ability to crystallize, but is presently hindered by the absence of a high-yielding expression system. Knowledge of the structure of rhodopsin will have general importance. Because rhodopsin is a member of the family of C-protein-coupled receptors, knowledge of the structure and the mechanism of action of rhodopsin suggests by analogy how other members of the receptor family may function. (shrink)

Allosteric and transmembrane signaling are among the major questions of structural biology. Here, we review and discuss signal transduction in four-helical TM bundles, focusing on histidine kinases and chemoreceptors found in two-component systems. Previously, piston, scissors, and helical rotation have been proposed as the mechanisms of TM signaling. We discuss theoretically possible conformational changes and examine the available experimental data, including the recent crystallographic structures of nitrate/nitrite sensor histidine kinase NarQ and phototaxis system NpSRII:NpHtrII. We show that TM helices can (...) flex at multiple points and argue that the various conformational changes are not mutually exclusive, and often are observed concomitantly, throughout the TM domain or in its part. The piston and scissoring motions are the most prominent motions in the structures, but more research is needed for definitive conclusions. Models of allosteric transmembrane signaling in microbial sensor histidine kinases and chemoreceptors are discussed in light of recent crystallographic structures. It is shown that the TM α-helices are flexible and various conformational changes are often observed concomitantly. The piston and scissoring motions are the most prominent in the structures. (shrink)

Neurons of organisms with complex and flexible behavior, especially humans, must precisely control protein localization and activity to support higher brain functions such as learning and memory. In contrast, simpler organisms generally have simpler individual neurons, less complex nervous systems and display more limited behaviors. Strikingly, however, many key neuronal proteins are conserved between organisms that have very different degrees of behavioral complexity. Here we discuss a possible mechanism by which conserved neuronal proteins acquired new attributes that were crucial in (...) the evolution of complexity of nervous system structure and function. Specifically, we hypothesize that vertebrate‐specific post‐translational palmitoylation sites and PDZ‐binding protein‐protein interaction motifs act as gain‐of‐function mutations, increasing the regulatory potential of conserved proteins without affecting their core functions. We further hypothesize that the additional regulation of neurotransmitter receptors and other membrane proteins made possible by these sites and motifs is critical for the function of complex nervous systems. (shrink)

The tricarboxylic acid (TCA) or Krebs cycle, which takes place in prokaryotic cells and in the mitochondria of eukaryotic cells, is central to life on Earth and participates in key events such as energy production and anabolic processes. Despite its relevance, it is not perceived as tightly regulated compared to other key metabolisms such as glycolysis/gluconeogenesis. A better understanding of the functioning of the TCA cycle is crucial due to mitochondrial function impairment in several diseases, especially those that occur with (...) neurodegeneration. This article revisits what is known about the regulation of the Krebs cycle and hypothesizes the need for large‐scale, rapid regulation of TCA cycle enzyme activity. Evidence of mitochondrial enzyme activity regulation by activation/deactivation of protein kinases and phosphatases exists in the literature. Apart from indirect regulation via G protein‐coupled receptors (GPCRs) at the cell surface, signaling upon activation of GPCRs in mitochondrial membranes may lead to a direct regulation of the enzymes of the Krebs cycle. Hormonal‐like regulation by posttranscriptional events mediated by activable kinases and phosphatases deserve proper assessment using isolated mitochondria. Also see the video abstract here: https://youtu.be/aBpDSWiMQyI. (shrink)

Specific extracellular interaction between glycophosphatidylinositol (GPI)‐anchored proteins and adhesion G protein‐coupled receptors (aGPCRs) plays an important role in unique biological functions. GPI‐anchored proteins are derived from a novel post‐translational modification of single‐span membrane molecules, while aGPCRs are bona fide seven‐span transmembrane proteins with a long extracellular domain. Although various members of the two structurally‐distinct protein families are known to be involved in a wide range of biological processes, many remain as orphans. Interestingly, accumulating evidence has pointed to a (...) complex interaction and functional synergy between these two protein families. I discuss herein current understanding of specific functional partnerships between GPI‐anchored proteins and aGPCRs. (shrink)

Different anesthetics are known to modulate different types of membrane-bound receptors. Their common mechanism of action is expected to alter the mechanism for consciousness. Consciousness is hypothesized as the integral of all the units of internal sensations induced by reactivation of inter-postsynaptic membrane functional LINKs during mechanisms that lead to oscillating potentials. The thermodynamics of the spontaneous lateral curvature of lipid membranes induced by lipophilic anesthetics can lead to the formation of non-specific inter-postsynaptic membrane functional LINKs (...) by different mechanisms. These include direct membrane contact by excluding the inter-membrane hydrophilic region and readily reversible partial membrane hemifusion. The constant reorganization of the lipid membranes at the lateral edges of the postsynaptic terminals (dendritic spines) resulting from AMPA receptor-subunit vesicle exocytosis and endocytosis can favor the effect of anesthetic molecules on lipid membranes at this location. Induction of a large number of non-specific LINKs can alter the conformation of the integral of the units of internal sensations that maintain consciousness. Anesthetic requirement is reduced in the presence of dopamine that causes enlargement of dendritic spines. Externally applied pressure can transduce from the middle ear through the perilymph, cerebrospinal fluid, and the recently discovered glymphatic pathway to the extracellular matrix space, and finally to the paravenular space. The pressure gradient reduce solubility and displace anesthetic molecules from the membranes into the paravenular space, explaining the pressure reversal of anesthesia. Changes in membrane composition and the conversion of membrane hemifusion to fusion due to defects in the checkpoint mechanisms can lead to cytoplasmic content mixing between neurons and cause neurodegenerative changes. The common mechanism of anesthetics presented here can operate along with the known specific actions of different anesthetics. (shrink)