Abstract

The beta thymosins are a highly conserved family of strongly polar 5 kDa polypeptides that are widely distributed among vertebrate classes; most are now known to bind to monomeric actin and inhibit its polymerization. One beta thymosin, beta four, (Tβ4) is the predominant form in mammalian cells, present at up to 0.5 mM. Many species are known to produce at least two beta thymosin isoforms, in some cases in the same cell. Their expression can be separately regulated. When present outside the cell, the N‐terminal tetrapeptide of beta four appears to affect cell cycle regulation; beta thymosins or smaller fragments derived from them may have additional regulatory functions. We suggest that many developmental changes in beta thymosin levels within cells and tissues may be related to changes in G‐actin pool size.