Physical contact between organelles are widespread, in part to facilitate the shuttling of protein and lipid cargoes for cellular homeostasis. How do protein‐protein and protein‐lipid interactions shape organelle subdomains that constitute contact sites? The endoplasmic reticulum (ER) forms extensive contacts with multiple organelles, including lipid droplets (LDs) that are central to cellular fat storage and mobilization. Here, we focus on ER‐LD contacts that are highlighted by the conserved protein seipin, which promotes LD biogenesis and expansion. Seipin is enriched (...) in ER tubules that form cage‐like structures around a subset of LDs. Such enrichment is strongly dependent on polyunsaturated and cyclopropane fatty acids. Based on these findings, we speculate on molecular events that lead to the formation of seipin‐positive peri‐LD cages in which protein movement is restricted. We hypothesize that asymmetric distribution of specific phospholipids distinguishes cage membrane tubules from the bulk ER. (shrink)

Calcium storage is one of the functions commonly attributed to the endoplasmic reticulum (ER) in non‐muscle cells. Several recent studies have added support to this concept. Analysis of reticuloplasm, the luminal ER content, has shown that it contains several proteins (reticuloplasmins) which are prospective calcium storage proteins. One of these, calreticulin, is also present in the sarcoplasmic reticulum (SR). In sea urchin eggs, a calsequestrin‐like protein has been clearly localised to the ER. The recent demonstration that the (...) IP3 receptor, which has similarities with the calcium release channel in the SR is also localised in the ER membrane suggests that calcium stored in the ER is important for intracellular signalling. The alternative view, that the physiologically important calcium store is a specialised organelle, the calciosome, is not supported by these observations. Recent evidence also suggests that ER calcium might be important in ER structure and in the retention of the luminal ER proteins. (shrink)

During mitosis, cells comprehensively restructure their interior to promote the faithful inheritance of DNA and cytoplasmic contents. In metazoans, this restructuring entails disassembly of the nuclear envelope, redistribution of its components into the endoplasmic reticulum (ER) and eventually nuclear envelope reassembly around the segregated chromosomes. The microtubule cytoskeleton has recently emerged as a critical regulator of mitotic nuclear envelope and ER dynamics. Microtubules and associated molecular motors tear open the nuclear envelope in prophase and remove nuclear envelope remnants (...) from chromatin. Additionally, two distinct mechanisms of microtubule‐based regulation of ER dynamics operate later in mitosis. First, association of the ER with microtubules is reduced, preventing invasion of ER into the spindle area, and second, organelle membrane is actively cleared from metaphase chromosomes. However, we are only beginning to understand the role of microtubules in shaping and distributing ER and other organelles during mitosis. (shrink)

Sarco/endoplasmic reticulum Ca2+ ATPase 2b (SERCA2b), a member of the SERCA family, is expressed ubiquitously and transports Ca2+ into the sarco/endoplasmic reticulum using the energy provided by ATP binding and hydrolysis. The crystal structure of SERCA2b in its Ca2+‐ and ATP‐bound (E1∙2Ca2+‐ATP) state and cryo‐electron microscopy (cryo‐EM) structures of the protein in its E1∙2Ca2+‐ATP and Ca2+‐unbound phosphorylated (E2P) states have provided essential insights into how the overall conformation and ATPase activity of SERCA2b is regulated by the (...) transmembrane helix 11 and the subsequent luminal extension loop, both of which are specific to this isoform. More recently, our cryo‐EM analysis has revealed that SERCA2b likely adopts open and closed conformations of the cytosolic domains in the Ca2+‐bound but ATP‐free (E1∙2Ca2+) state, and that the closed conformation represents a state immediately prior to ATP binding. This review article summarizes the unique mechanisms underlying the conformational and functional regulation of SERCA2b. (shrink)

The endoplasmic reticulum (ER) uses various mechanisms to ensure that only properly folded proteins enter the secretory pathway. For proteins that oligomerize in the ER, the proper tertiary and quaternary structures must be achieved before their release. Although some proteins fold before oligomerization, others initiate oligomerization cotranslationally. Here, we discuss these different strategies and some of the unique problems they present for the ER quality control system. One mechanism used by the ER is thiol retention. Thiol retention operates (...) by monitoring the redox state of specific cysteine residue(s) and was discovered in studies on the assembly of IgM, a complex oligomeric glycoprotein. This system is also involved in retaining other unassembled proteins in the ER. Mutations that result in uneven numbers of cysteine residues can subject yet other proteins to thiol retention, altering their oligomerization status and function. The implications of these results on the effects of thiol retention on protein function and cell fate are discussed. BioEssays 20:546–554, 1998. © 1998 John Wiley & Sons Inc. (shrink)

The cellular machinery responsible for conveying proteins between the endoplasmic reticulum and the Golgi is being investigated using genetics and biochemistry. A role for vesicles in mediating protein traffic between the ER and the Golgi has been established by characterizing yeast mutants defective in this process, and by using recently developed cell‐free assays that measure ER to Golgi transport. These tools have also allowed the identification of several proteins crucial to intracellular protein trafficking. The characterization and possible functions (...) of several GTP‐binding proteins, peripheral membrane proteins, and an integral membrane protein during ER to Golgi transport are discussed here. (shrink)

The synthesis of biological membranes requires the insertion of proteins into a lipid bilayer. The rough endoplasmic reticulum of eukaryotic cells is a principal site of membrane biogenesis. The insertion of proteins into the membrane of the endoplasmic reticulum is mediated by a resident proteinaceous machinery. Over the last five years several different experimental approaches have provided information about the components of the machinery and how it may function.

ER‐associated degradation (ERAD) is a component of the protein quality control system, ensuring that aberrant polypeptides cannot transit through the secretory pathway. This is accomplished by a complex sequence of events in which unwanted proteins are selected in the ER and exported to the cytosol for degradation by the proteasome. Given that protein quality control can be essential for cell survival, it is not surprising that ERAD is linked to numerous disease states. Here we review the molecular mechanisms of ERAD, (...) its role in metabolic regulation and biomedical implications, and the unanswered questions regarding this process. BioEssays 25:868–877, 2003. © 2003 Wiley Periodicals, Inc. (shrink)

The endoplasmic reticulum (ER) organelle is the key intracellular site of both protein and lipid biosynthesis. ER dysfunction, termed ER stress, can result in protein accretion within the ER and cell death; a pathophysiological process contributing to a range of metabolic diseases and cancers. ER stress leads to the activation of a protective signalling cascade termed the Unfolded Protein Response (UPR). However, chronic UPR activation can ultimately result in cellular apoptosis. Emerging evidence suggests that cells undergoing ER stress (...) and UPR activation can release extracellular signals that can propagate UPR activation to target tissues in a cell non‐autonomous signalling mechanism. Separately, studies have determined that the UPR plays a key regulatory role in the biosynthesis of bioactive signalling lipids including sphingolipids and ceramides. Here we weigh the evidence to combine these concepts and propose that during ER stress, UPR activation drives the biosynthesis of ceramide lipids, which are exported and function as cell non‐autonomous signals to propagate UPR activation in target cells and tissues. (shrink)

Lipid droplets (LDs) are ubiquitous, neutral lipid storage organelles that act as hubs of metabolic processes. LDs are structurally unique with a hydrophobic core that mainly consists of neutral lipids, sterol esters, and triglycerides, enclosed within a phospholipid monolayer. Nascent LD formation begins with the accumulation of neutral lipids in the endoplasmic reticulum (ER) bilayer. The ER membrane proteins such as seipin, LDAF1, FIT, and MCTPs are reported to play an important role in the formation of nascent LDs. (...) As the LDs grow, they unmix from the highly charged ER membrane to form mature LDs. LD biogenesis is an exciting, emerging research area, and herein, we discuss the recent progress in our understanding of the formation of eukaryotic nascent LDs. We focus on the role of ER membrane shaping proteins such as reticulons and reticulon‐like proteins, membrane lipids, and cytoskeleton proteins such as septin in the formation of nascent LDs. (shrink)

The segregation of damaged components at cell division determines the survival and aging of cells. In cells that divide asymmetrically, such as Saccharomyces cerevisiae, aggregated proteins are retained by the mother cell. Yet, where and how aggregation occurs is not known. Recent work by Zhou and collaborators shows that the birth of protein aggregates, under specific stress conditions, requires active translation, and occurs mainly at the endoplasmic reticulum. Later, aggregates move to the mitochondrial surface through fis1‐dependent association. During (...) replicative aging, aggregate association with the mother‐cell mitochondria contributes to the asymmetric segregation of aggregates, because mitochondria in the daughter cell do not carry aggregates. With increasing age of mother cells, aggregates lose their connection to the mitochondria, and segregation is less asymmetric. Relating these findings to other mechanisms of aggregate segregation in different organisms, we postulate that fusion between aggregates and their tethering to organelles such as the vacuole, nucleus, ER, or mitochondria are common principles that establish asymmetric segregation during stress resistance and aging. (shrink)

The secretory pathway delivers proteins synthesized at the rough endoplasmic reticulum (RER) to various subcellular locations via the Golgi apparatus. Currently, efforts are focused on understanding the molecular machineries driving individual processes at the RER and Golgi that package, modify and transport proteins. However, studies are routinely performed using non‐dividing cells. This obscures the critical issue of how the secretory pathway is affected by cell division. Indeed, several studies have indicated that protein trafficking is down‐regulated during mitosis. Moreover, (...) the RER and Golgi apparatus exhibit gross reorganization in mitosis. Here I provide a relatively neglected perspective of how the mitotic cyclin‐dependent kinase (CDK1) could regulate various stages of the secretory pathway. I highlight several aspects of the mitotic control of protein trafficking that remain unresolved and suggest that further studies on how the mitotic CDK1 influences the secretory pathway are necessary to obtain a deeper understanding of protein transport. (shrink)

In eukaryotic cells terminally misfolded proteins of the secretory pathway are retarded in the endoplasmic reticulum (ER) and subsequently degraded in a ubiquitin‐proteasome‐dependent manner. This highly conserved process termed ER‐associated protein degradation (ERAD) ensures homeostasis in the secretory pathway by disposing faulty polypeptides and preventing their deleterious accumulation and eventual aggregation in the cell. The focus of this paper is the functional description of membrane‐bound ubiquitin ligases, which are involved in all critical steps of ERAD. In the end (...) we want to speculate on how the modular architecture of these entities ensures the specificity of substrate selection and possibly accomplishes the transport of misfolded polypeptides from the ER into the cytoplasm. (shrink)

Autophagy functions in both selective and non‐selective ways to maintain cellular homeostasis. Endoplasmic reticulum autophagy (ER‐phagy) is a subclass of autophagy responsible for the degradation of the endoplasmic reticulum through selective encapsulation into autophagosomes. ER‐phagy occurs both under physiological conditions and in response to stress cues, and plays a crucial role in maintaining the homeostatic control of the organelle. Although specific receptors that target parts of the ER membrane, as well as, internal proteins for lysosomal degradation (...) have been identified, the molecular regulation of ER‐phagy has been elusive. Recent work has uncovered novel regulators of ER‐phagy that involve post‐translational modifications of ER‐resident proteins and functional cross‐talk with other cellular processes. Herein, we discuss how morphology affects the function of the peripheral ER, and how ER‐phagy modulates the turnover of this organelle. We also address how ER‐phagy is regulated at the molecular level, considering implications relevant to human diseases. (shrink)

The biochemistry of geotropism in plants and gravisensing in e.g. cyanobacteria or paramacia is still not well understood today [1]. Perhaps there are more ways than one for organisms to sense gravity. The two best known relatively old explanations for gravity sensing are sensing through the redistribution of cellular starch statoliths and sensing through redistribution of auxin. The starch containing statoliths in a gravity field produce pressure on the endoplasmic reticulum of the cell. This enables the cell to (...) sense direction. Alternatively, there is the redistribution of auxin under the action of gravity. This is known as the Cholodny-Went hypothesis [2], [3]. The latter redistribution coincides with a redistribution of electrical charge in the cell. With the present study the aim is to add a mathematical unified field explanation to gravisensing. (shrink)

Mitochondria are increasingly being recognized as information hubs that sense cellular changes and transmit messages to other cellular components, such as the nucleus, the endoplasmic reticulum (ER), the Golgi apparatus, and lysosomes. Nonetheless, the interaction between mitochondria and the nucleus is of special interest because they both host part of the cellular genome. Thus, the communication between genome‐bearing organelles would likely include gene expression regulation. Multiple nuclear‐encoded proteins have been known to regulate mitochondrial gene expression. On the contrary, (...) no mitochondrial‐encoded factors are known to actively regulate nuclear gene expression. MOTS‐c (mitochondrial open reading frame of the 12S ribosomal RNA type‐c) is a recently identified peptide encoded within the mitochondrial 12S ribosomal RNA gene that has metabolic functions. Notably, MOTS‐c can translocate to the nucleus upon metabolic stress (e.g., glucose restriction and oxidative stress) and directly regulate adaptive nuclear gene expression to promote cellular homeostasis. It is hypothesized that cellular fitness requires the coevolved mitonuclear genomes to coordinate adaptive responses using gene‐encoded factors that cross‐regulate the opposite genome. This suggests that cellular gene expression requires the bipartite split genomes to operate as a unified system, rather than the nucleus being the sole master regulator. (shrink)

Microsomal glucose‐6‐phosphatase catalyses the last step in liver glucose production. Glucose‐6‐phosphatase deficiency, now termed type 1 glycogen storage disease, was first described almost 40 years ago but until recently very little was known about the molecular basis of the various type 1 glycogen storage diseases. Recently we have shown that at least six different proteins are needed for normal glucose‐6‐phosphatase activity in liver. Four of the proteins have been purified and three cloned. Study of the type 1 glycogen storage diseases (...) has stimulated investigations of the mechanisms of small molecule transport across the endoplasmic reticulum membrane and demonstrated the existence of novel endoplasmic reticulum transport proteins for glucose and phosphate. (shrink)

Most metazoan organisms have evolved a mildly acidified and calcium diminished sorting hub in the early secretory pathway commonly referred to as the Endoplasmic Reticulum‐Golgi intermediate compartment (ERGIC). These membranous vesicular‐tubular clusters are found tightly juxtaposed to ER subdomains that are competent for the production of COPII‐coated transport carriers. In contrast to many unicellular systems, metazoan COPII carriers largely transit just a few hundred nanometers to the ERGIC, prior to COPI‐dependent transport on to the cis‐Golgi. The mechanisms underlying (...) formation and maintenance of ERGIC membranes are poorly defined. However, recent evidence suggests an important role for Trk‐fused gene (TFG) in regulating the integrity of the ER/ERGIC interface. Moreover, in the absence of cytoskeletal elements to scaffold tracks on which COPII carriers might move, TFG appears to promote anterograde cargo transport by locally tethering COPII carriers adjacent to ERGIC membranes. This action, regulated in part by the intrinsically disordered domain of TFG, provides sufficient time for COPII coat disassembly prior to heterotypic membrane fusion and cargo delivery to the ERGIC. -/- . (shrink)

In recent years, membrane contact sites (MCS), which mediate interactions between virtually all subcellular organelles, have been extensively characterized and shown to be essential for intracellular communication. In this review essay, we focus on an emerging topic: the regulation of MCS. Focusing on the tether proteins themselves, we discuss some of the known mechanisms which can control organelle tethering events and identify apparent common regulatory hubs, such as the VAP interface at the endoplasmic reticulum (ER). We also highlight (...) several currently hypothetical concepts, including the idea of tether oligomerization and redox regulation playing a role in MCS formation. We identify gaps in our current understanding, such as the identity of the majority of kinases/phosphatases involved in tether modification and conclude that a holistic approach—incorporating the formation of multiple MCS, regulated by interconnected regulatory modulators—may be required to fully appreciate the true complexity of these fascinating intracellular communication systems. (shrink)

The secretory system of plant cells sorts a large number of soluble proteins that either are secreted or accumulate in vacuoles. Secretion is a bulk‐flow process that requires no information beyond the presence of a signal peptide necessary to enter the endoplasmic reticulum. Many vacuolar proteins are glycoproteins and the glycans are often modified as the proteins pass through the Golgi complex. Vacuolar targeting information is not contained in glycans as it is in animal cells; rather, targeting information (...) is in polypeptide domains as it is in yeast cells. Several such domains have now been identified, but these show little or no amino acid sequence homology. We discuss the possibilities that targeting of protein to plant vacuoles may involve receptors as well as aggregation of protein at low pH. (shrink)

At first glance the three eukaryotic protein translocation machineries – the ER‐associated degradation (ERAD) transport apparatus of the endoplasmic reticulum, the peroxisomal importomer and SELMA, the pre‐protein translocator of complex plastids – appear quite different. However, mechanistic comparisons and phylogenetic analyses presented here suggest that all three translocation machineries share a common ancestral origin, which highlights the recycling of pre‐existing components as an effective evolutionary driving force.Editor's suggested further reading in BioEssays ERAD ubiquitin ligases Abstract.

Volatile compounds, such as nitric oxide and ethylene gas, play a vital role as signaling molecules in organisms. Ethylene is a plant hormone that regulates a wide range of plant growth, development, and responses to stress and is perceived by a family of ethylene receptors that localize in the endoplasmic reticulum. Constitutive Triple Response 1 (CTR1), a Raf‐like protein kinase and a key negative regulator for ethylene responses, tethers to the ethylene receptors, but undergoes nuclear translocation upon activation (...) of ethylene signaling. This ER‐to‐nucleus trafficking transforms CTR1 into a positive regulator for ethylene responses, significantly enhancing stress resilience to drought and salinity. The nuclear trafficking of CTR1 demonstrates that the spatiotemporal control of ethylene signaling is essential for stress adaptation. Understanding the mechanisms governing the spatiotemporal control of ethylene signaling elements is crucial for unraveling the system‐level regulatory mechanisms that collectively fine‐tune ethylene responses to optimize plant growth, development, and stress adaptation. (shrink)

The origin of the eukaryotic cell nucleus and the selective forces that drove its evolution remain unknown and are a matter of controversy. Autogenous models state that both the nucleus and endoplasmic reticulum (ER) derived from the invagination of the plasma membrane, but most of them do not advance clear selective forces for this process. Alternative models proposing an endosymbiotic origin of the nucleus fail to provide a pathway fully compatible with our knowledge of cell biology. We propose (...) here an evolutionary scenario that reconciles both an ancestral endosymbiotic origin of the eukaryotic nucleus (endosymbiosis of a methanogenic archaeon within a fermentative myxobacterium) with an autogenous generation of the contemporary nuclear membrane and ER from the bacterial membrane. We specifically state two selective forces that operated sequentially during its evolution: (1) metabolic compartmentation to avoid deleterious co‐existence of anabolic (autotrophic synthesis by the methanogen) and catabolic (fermentation by the myxobacterium) pathways in the cell, and (2) avoidance of aberrant protein synthesis due to intron spreading in the ancient archaeal genome following mitochondrial acquisition and loss of methanogenesis. BioEssays 28: 525–533, 2006. © 2006 Wiley Periodicals, Inc. (shrink)

Autophagy, an essential cellular process for maintaining cellular homeostasis and eliminating harmful cytoplasmic objects, involves the de novo formation of double‐membraned autophagosomes that engulf and degrade cellular debris, protein aggregates, damaged organelles, and pathogens. Central to this process is the phagophore, which forms from donor membranes rich in lipids synthesized at various cellular sites, including the endoplasmic reticulum (ER), which has emerged as a primary source. The ER‐associated omegasomes, characterized by their distinctive omega‐shaped structure and accumulation of phosphatidylinositol (...) 3‐phosphate (PI3P), play a pivotal role in autophagosome formation. Omegasomes are thought to serve as platforms for phagophore assembly by recruiting essential proteins such as DFCP1/ZFYVE1 and facilitating lipid transfer to expand the phagophore. Despite the critical importance of phagophore biogenesis, many aspects remain poorly understood, particularly the complete range of proteins involved in omegasome dynamics, and the detailed mechanisms of lipid transfer and membrane contact site formation. (shrink)

This essay provides an introduction to the terminology, concepts, methods, and challenges of image‐based modeling in biology. Image‐based modeling and simulation aims at using systematic, quantitative image data to build predictive models of biological systems that can be simulated with a computer. This allows one to disentangle molecular mechanisms from effects of shape and geometry. Questions like “what is the functional role of shape” or “how are biological shapes generated and regulated” can be addressed in the framework of image‐based systems (...) biology. The combination of image quantification, model building, and computer simulation is illustrated here using the example of diffusion in the endoplasmic reticulum. (shrink)

A variety of intracellular motile processes involve the directed movement of particles along microtubules, including organelle transport, endoplasmic reticulum extension, and movements in mitosis. Recently, a microtubule‐dependent motor protein, kinesin, was purified and was found to be present in a soluble form in a wide variety of organisms and tissues. Because microtubules provide polar pathways over long distances within cells, kinesin and the motors which move in the opposite direction to kinesin on microtubules provide a mechanism for directed (...) communications within cells. The possible roles of kinesin and other soluble microtubule‐dependent motors in intracellular motile functions are discussed in the light of recent studies of the reconstitution of organelle motility with isolated components. (shrink)

Protein disulfide isomerase (PDI) is one of the most abundant and critical protein folding catalysts in the endoplasmic reticulum of eukaryotic cells. PDI consists of four thioredoxin domains and interacts with a wide range of substrate and partner proteins due to its intrinsic conformational flexibility. PDI plays multifunctional roles in a variety of pathophysiological events, both as an oxidoreductase and a molecular chaperone. Recent studies have revealed that the conformation and activity of PDI can be regulated in multiple (...) ways, including posttranslational modification and substrate/ligand binding. Here, we summarize recent advances in understanding the function and regulation of PDI in different pathological and physiological events. We propose that the multifunctional roles of PDI are regulated by multiple mechanisms. Furthermore, we discuss future directions for the study of PDI, emphasizing how different regulatory modes are linked to the conformational changes and biological functions of PDI in the context of diverse pathophysiologies. (shrink)

Endosomes shuttle select cargoes between cellular compartments and, in doing so, maintain intracellular homeostasis and enable interactions with the extracellular space. Directionality of endosomal transport critically impinges on cargo fate, as retrograde (microtubule minus‐end directed) traffic delivers vesicle contents to the lysosome for proteolysis, while the opposing anterograde (plus‐end directed) movement promotes recycling and secretion. Intriguingly, the endoplasmic reticulum (ER) is emerging as a key player in spatiotemporal control of late endosome and lysosome transport, through the establishment of (...) physical contacts with these organelles. Earlier studies have described how minus‐end‐directed motor proteins become discharged from vesicles engaged at such contact sites. Now, Raiborg et al. implicate ER‐mediated interactions, induced by protrudin, in loading plus‐end‐directed motor kinesin‐1 onto endosomes, thereby stimulating their transport toward the cell's periphery. In this review, we recast the prevailing concepts on bidirectional late endosome transport and discuss the emerging paradigm of inter‐compartmental regulation from the ER‐endosome interface viewpoint. (shrink)

Progressive supranuclear palsy is a movement disorder with prominent tau neuropathology. Brain diseases with abnormal tau deposits are called tauopathies, the most common of which is Alzheimer's disease. Environmental causes of tauopathies include repetitive head trauma associated with some sports. To identify common genetic variation contributing to risk for tauopathies, we carried out a genome-wide association study of 1,114 individuals with PSP and 3,247 controls followed by a second stage in which we genotyped 1,051 cases and 3,560 controls for the (...) stage 1 SNPs that yielded P ≤ 10-3. We found significant previously unidentified signals associated with PSP risk at STX6, EIF2AK3 and MOBP. We confirmed two independent variants in MAPT affecting risk for PSP, one of which influences MAPT brain expression. The genes implicated encode proteins for vesicle-membrane fusion at the Golgi-endosomal interface, for the endoplasmic reticulum unfolded protein response and for a myelin structural component. © 2011 Nature America, Inc. All rights reserved. (shrink)

Much progress has been made in recent years regarding the mechanisms of targeting of secretory proteins to, and across, the endoplasmic reticulum (ER) membrane. Many of the cellular components involved in mediating translocation across this bilayer have been identified and characterized. Polypeptide domains of secretory proteins, termed signal peptides, have been shown to be necessary, and in most cases sufficient, for entry of preproteins into the lumen of the ER. These NH2‐ terminal segments appear to serve multiple roles (...) in targeting and translocation. The structural features which mediate their multiple functions are currently the subject of intense study. (shrink)

Molecular and biochemical analysis of the biogenesis of peroxisomes has made rapid progress in recent years. Research on the mechanism of targeting of peroxisomal proteins has revealed that many, but not all, peroxisomal proteins have a conserved tripeptide motif in their carboxy‐terminal portions which is required for entry into peroxisomes; the topogenic signal mechanism thus differs in these instances from those employed in mitochondria and endoplasmic reticulum. Other factors involved in peroxisome biogenesis are also coming to light.

Organelles transported along microtubules are normally moved to precise locations within cells. For example, synaptic vesiceles are transported to the neruronal synapse, the Golgi apparatus is generally found in a perinuclear location, and the membranes of the endoplasmic reticulum are actively extended to the cell periphery. The correct positioning of these organelles depends on microtubules and microtubule motors. Melanophores provide an extreme example of organized organelle transport. These cells are specialized to transport pigment granules, which are coordinately moved (...) towards or away from the cell center, and result in the cell appearing alternately light or dark. Melanophores have proved to be an ideal system for studying the mechanisms by which the cell controls the direction of its organelle transport. Pigment granule dispersion (the movement away from the cell center) requires protein phosphorylation, while pigment aggregation (the movement towards the cell center) requires protein dephosphorylation. The target of this phosphorylation and dephosphorylation event is a protein that interacts with the microtubule motor protein, kinesin. Thus, the direction of organelle transport along microtubules may be regulated by controlling the activity of a microtubule motor. (shrink)

Proteins of the exocytotic (secretory) pathway are initially targeted to the endoplasmic reticulum (ER) and then translocated across and/or inserted into the membrane of the ER. During their anterograde transport with the bulk of the membrane flow along the exocytotic pathway, some proteins are selectively retained in various intracellular compartments, while others are sorted to different branches of the pathway. The signals or structural motifs that are involved in these selective targeting processes are being revealed and investigations into (...) the mechanistic nature of these processes are actively underway. (shrink)

In animal cells, the Golgi complex undergoes reversible disassembly during mitosis. The disassembly/reassembly process has been intensively studied in order to understand the mechanisms that govern organelle assembly and inheritance during cell division. A long‐standing controversy in the field has been whether formation of Golgi structure is template‐mediated or self‐organizes from components of the endoplasmic reticulum. A recent study1 however, has demonstrated that a subset of proteins that form a putative Golgi matrix can be inherited during cell division (...) in the absence of membrane input from the endoplasmic reticulum. The outcome of this study suggests that a templating mechanism for the formation of Golgi structure may exist. This study has important implications for understanding mechanisms that govern Golgi biogenesis. BioEssays 24:584–587, 2002. © 2002 Wiley Periodicals, Inc. (shrink)

An increasing number of studies indicate that changes in cytosolic free Ca2+ ([Ca2+]c) mediate specific types of signal transduction in plant cells. Modulation of [Ca2+]c is likely to be achieved through changes in the activity of Ca2+ channels, which catalyse passive influx of Ca2+ to the cytosol from extracellular and intracellular compartments. Voltage‐sensitive Ca2+ channels have been detected in the plasma membranes of algae, where they control membrane electrical properties and cell turgor. These channels are sensitive to 1,4‐dihydropyridines, which in (...) animal cells specifically affect one class of voltage‐regulated plasma membrane Ca2+ channel. Ca2+‐permeable channels with different pharmacological properties have been found in the plasma membrane of higher plants. Recent evidence suggests the existence of two discrete classes of Ca2+ channel co‐resident in the vacuolar membrane (tonoplast) of higher plants. The first is gated by inositol 1,4,5‐trisphosphate, and bears a number of similarities to its animal counterpart which is located in the endoplasmic reticulum (ER). The second tonoplast Ca2+ channel is voltage‐operated. However, the specific roles of these tonoplast channels in signal transduction have yet to be elucidated. (shrink)

We suggest a new view of secretory and membrane protein folding that emphasizes the role of pathways of biogenesis in generating functional and conformational heterogeneity. In this view, heterogeneity results from action of accessory factors either directly binding specific sequences of the nascent chain, or indirectly, changing the environment in which a particular domain is synthesized. Entrained by signaling pathways, these variables create a combinatorial set of necessary‐but‐not‐sufficient conditions that enhance synthesis and folding of particular alternate, functional, conformational forms. We (...) therefore propose that protein conformation is productively regulated by the cell during translocation across the endoplasmic reticulum (ER), a concept that may account for currently poorly understood aspects of physiological function, natural selection, and disease pathogenesis. BioEssays 24:741–748, 2002. © 2002 Wiley Periodicals, Inc. (shrink)

Secretory proteins and membranes move in transfer vesicles from the rough endoplasmic reticulum through the transitional region to the outer saccule of the Golgi complex. In both arthropod and vertebrate cells, the GC beads are a characteristic structural component of the transitional region. The beads are particles about half the size of ribosomes arranged equidistantly from one another and the smooth face of the ER. In an active GC, the beads are in rings through which the ER membrane (...) emerges to form transfer vesicles. The beads may be part of the energy‐dependent step required for the movement of proteins along the secretory pathway, since they lose their ring arrangement under conditions that lower cellular ATP. The beads are organizers for Golgi complexes in the sense that they are the first recognizable components of new GCs as they arise from ER. Arthropod GC beads, but not those of vertebrates, can be visualized through their reaction with bismuth in vivo and in fixed tissue. Useful paradigms for traffic between the ER and the GC need to combine structural and biochemical information. Insect fat body, with its readily resolvable bismuth‐stained beads and easily fractionated cell components may have particular value for this problem. (shrink)

Presenilin‐1 (PS1) is thought to regulate cell differentiation and survival by modulating the Notch signaling pathway. Mutations in PS1 have been shown to cause early‐onset inherited forms of Alzheimer's disease (AD) by a gain‐of‐function mechanism that alters proteolytic processing of the amyloid precursor protein (APP) resulting in increased production of neurotoxic forms of amyloid β‐peptide. The present article considers a second pathogenic mode of action of PS1 mutations, a defect in cellular calcium signaling characterized by overfilling of endoplasmic (...) class='Hi'>reticulum (ER) calcium stores and altered capacitive calcium entry; this abnormality may impair synaptic plasticity and sensitize neurons to apoptosis and excitotoxicity. The calcium signaling defect has also been documented in lymphocytes, suggesting a contribution of immune dysfunction to the pathogenesis of AD. A better understanding of the calcium signaling defect resulting from PS1 mutations may lead to the development of novel preventative and therapeutic strategies for disorders of the nervous and immune systems. BioEssays 23:733–744, 2001. © 2001 John Wiley & Sons, Inc. (shrink)

Both prokaryotic and eukaryotic cells share the basic mechanisms of secretory protein synthesis. However, unlike prokaryotes, eukaryotic cells posses a system of compartments, the so-called endomembrane system, which are involved in the synthesis process. A comparison of the prokaryotic and eukaryotic protein synthesis processes and particularly the observation of the functional and structural similarity between the prokaryotic cell membrane (the interface to the cell exterior) and the membrane of the eukaryotic endoplasmic reticulum (one of the compartments within the (...) endomembrane system) inspire a description that refers to either the eukaryotic endoplasmic reticulum or its membrane or the endomembrane system altogether as a representation of the extracellular medium. However, unless the terms in such a statement are carefully analysed and refined the description would be just a colloquial usage of the concept of “representation” rather than a biosemiotic statement. Another problem associated with employing the concept of “representation” in a biosemiotic context is due to the fact that it may evoke the impression of a conscious mind as the “owner” of the representation. In this paper theories about the emergence of the eukaryotic endomembrane system, as well as its functionality within secretory protein synthesis will be analysed in order to specify in what sense the concept of “representation” can be employed in this context without implying consciousness. Such a study is expected not only to provide an insight into the conditions and assumptions under which this concept can be used for lower level biological processes but also to shed some light on how representations emerge in general. (shrink)

The biological membranes of eukaryotic cells harbor sensitive surveillance systems to establish, sense, and maintain characteristic physicochemical properties that ultimately define organelle identity. They are fundamentally important for membrane homeostasis and play active roles in cellular signaling, protein sorting, and the formation of vesicular carriers. Here, we compare the molecular mechanisms of Mga2 and Ire1, two sensors involved in the regulation of fatty acid desaturation and the response to unfolded proteins and lipid bilayer stress in order to identify their commonalities (...) and specializations. We will speculate on the cellular significance of membrane property sensors in other organelles and discuss their putative mechanisms. Based on these findings, we propose membrane property sensors as an emerging class of proteins with wide implications for organelle communication and function. (shrink)

An entirely different mechanism and localization were recently proposed for the COPII coat complex, challenging its well‐accepted function to select and concentrate cargo into small COPII‐coated spherical transport vesicles. Instead, the COPII complex is suggested to form a dynamic yet stationary collar that forms a boundary between the ER and the ER export membrane domain. This membrane domain, the ER exit site (ERES), is the site of COPII‐mediated sorting and concentration of transport competent proteins. Subsequently, the ERES is implicated to (...) mature and bud to form a sizeable pleiomorphic transport carrier that translocate on microtubules to fuse with the Golgi apparatus. Despite this drastic mechanistic dogma shift, most of the underlying protein‐protein and protein‐membrane interactions remain unchanged. Here, we attempt to provide a detailed description of the newly proposed model of how ER to Golgi transport works by describing the role of several essential proteins of the transport machinery. (shrink)

Although many aspects of the physiological and pathophysiological mechanisms remain unknown, recent advances in our knowledge suggest that the CREC proteins are promising disease biomarkers or targets for therapeutic intervention in a variety of diseases. The CREC family of low affinity, Ca2+‐binding, multiple EF‐hand proteins are encoded by five genes,RCN1,RCN2,RCN3,SDF4, andCALU, resulting in reticulocalbin, ER Ca2+‐binding protein of 55 kDa (ERC‐55), reticulocalbin‐3, Ca2+‐binding protein of 45 kDa (Cab45), and calumenin. Alternative splicing increases the number of gene products. The proteins are (...) localized in the cytosol, in various parts of the secretory pathway, secreted to the extracellular space or localized on the cell surface. The emerging functions appear to be highly diverse. The proteins interact with several different ligands. Rather well‐described functions are attached to calumenin with the inhibition of several proteins in the endoplasmic or sarcoplasmic reticulum membrane, the vitamin K12,3‐epoxide reductase, the γ‐carboxylase, the ryanodine receptor, and the Ca2+‐transporting ATPase. Other functions concern participation in the secretory process, chaperone activity, signal transduction as well as participation in a large variety of disease processes. (shrink)

Parvalbumins (PVs) are acidic, intracellular Ca2+‐binding proteins of low molecular weight. They are associated with several Ca2+‐mediated cellular activities and physiological processes. It has been suggested that PV might function as a “Ca2+ shuttle” transporting Ca2+ from troponin‐C (TnC) to the sarcoplasmic reticulum (SR) Ca2+ pump during muscle relaxation. Thus, PV may contribute to the performance of rapid, phasic movements by accelerating the contraction–relaxation cycle of fast‐twitch muscle fibers. Interestingly, PVs promote the generation of power stroke in fish by (...) speeding up the rate of relaxation and thus provide impetus to attain maximal sustainable speeds. However, immunological monitoring of diverse tissues demonstrated that PVs are also present in non‐muscle cells. The axoplasmic transport and various intracellular secretory mechanisms including the endocrine secretions seem to be controlled by the Ca2+ regulation machinery. Any defect in the Ca2+ handling apparatus may cause several clinical problems; for instance, PV deficiency alters the neuronal activity, a key mechanism leading to epileptic seizures. Moreover, atypical relaxation of the heart results in diastolic dysfunction, which is a major cause of heart failure predominantly among the aged people. PV may offer a unique potential to correct defective relaxation in energetically compromised failing hearts through PV gene transfer. Consequently, PV gene transfer may present a new therapeutic approach to correct cellular disturbances in Ca2+ signaling pathways of diseased organs. Hence, PVs appear to be amazingly useful candidate proteins regulating a variety of cellular functions through action on Ca2+ flux management. (shrink)

Ca2+ transients in myocardial cells are modulated by cyclic AMP‐dependent phosphorylation of a protein in the sarcoplasmic reticulum. This protein, termed phospholamban, serves to regulate the Ca2+ pump ATPase of this membrane, thus altering the mode of Ca2+ transients and the myocardial contractile response. Elucidating the structure of phospholamban and its intimate interaction with the Ca2+ pump ATPase should provide the basis for understanding, at the molecular level, how the cAMP system contributes to excitation‐contraction coupling in muscle cells.

Mitochondria can change their shape from discrete isolated organelles to a large continuous reticulum. The cellular advantages underlying these fused networks are still incompletely understood. In this paper, we describe and compare hypotheses regarding the function of mitochondrial networks. We use mathematical and physical tools both to investigate existing hypotheses and to generate new ones, and we suggest experimental and modelling strategies. Among the novel insights we underline from this work are the possibilities that (i) selective mitophagy is not (...) required for quality control because selective fusion is sufficient; (ii) increased connectivity may have non‐linear effects on the diffusion rate of proteins; and (iii) fused networks can act to dampen biochemical fluctuations. We hope to convey to the reader that quantitative approaches can drive advances in the understanding of the physiological advantage of these morphological changes. (shrink)

As endosymbionts, the mitochondria are unique among organelles. This review provides insights into mitochondrial behavior and introduces the idea of a unified collective, an interconnected reticulum reminiscent of the Borg, a fictional humanoid species from the Star Trek television series whereby decisions are made within their network (or “hive”), linked to signaling cascades that coordinate the cross‐talk between mitochondrial and cellular processes (“subspace domain”). Similarly, mitochondrial dynamics are determined by two distinct processes, namely the local regulation of fission/fusion and (...) the global control of their behavior through cellular signaling pathways. Indeed, decisions within the hive provide each mitochondrial unit with autonomous control of their own degradation, whereby mitochondrial fusion is inactivated and they become substrates for autophagy. Decisions within the subspace domain couple signaling pathways involved in the functional integration of mitochondria with complex cellular transitions, including developmental cues, mitosis, and apoptosis. (shrink)

The immune surveillance theory postulates that spontaneous tumors are normally rejected by the immune system and appear only when they override host‐immune recognition and rejection mechanisms. The present mini‐review describes a spontaneous tumor system, the reticulum cell sarcomas (RCS) in SJL/J mice, that is dependent on host tumor‐specific immune lymphocytes for growth. This continuous tumor‐specific response results in tumor progression and death of the host. This tumor system contradicts the basic concept of immune surveillance. We propose as an explanation (...) that some highly antigenic tumors, like the RCS, may have evolved in a non‐autonomous fashion but, nevertheless, have lost regulatory controls of cell proliferation. In the RCS system, the tumor expresses Class II MHC I‐E like specificities that are not expressed on the host cells and which selectively stimulate a subpopulation of I‐E specific T cells, the Vβ17a+ clonotype, leading to their expansion and continuous nurturing of the tumor via secreted lymphokines. This neoantigenic stimulation bypasses the tumor regulatory response that might have resulted if the tumor had not expressed neo‐antigens. Furthermore, passive administration of anti‐clonotypic antibody to tumor‐bearing mice results in tumor regression and long‐term survival through removal of the tumor reactive T cells. Thus, in this tumor system, immunosuppressive treatments are the prescription for tumor rejection. (shrink)