Thymosin beta 4 is a ubiquitous 43 amino acid, 5 kDa polypeptide that is an important mediator of cell proliferation, migration, and differentiation. Thymosin beta 4 is the most abundant member of the beta-thymosin family in mammalian tissue and is regarded as the main G-actin sequestering peptide. Thymosin beta 4 is angiogenic and can promote endothelial cell migration and adhesion, tubule formation, aortic ring sprouting, and angiogenesis.  Thymosin beta 4 also accelerates wound healing and reduces inflammation when applied in dermal wound-healing assays.

Thymosin beta 4 was originally identified in calf thymus. Although numerous activities have been attributed to thymosin beta 4, its physiological role remains elusive. Recently, thymosin beta 4 was found to bind actin in human platelet extracts and to inhibit actin polymerization in vitro, raising the possibility that it may be a physiological regulator of actin assembly.

Thymosin beta 4 sequesters actin, holding it in a form that is unable to polymerize. Due to its profusion in the cytosol and its ability to bind ATP G-actin but not F-actin, thymosin beta 4 is regarded as the principal actin-sequestering protein. Thymosin beta 4 binds ATP G- monomeric actin in a 1:1 complex where G-actin cannot polymerize.

Increase in cytosolic concentrations of thymosin beta 4 increases the concentration of sequestered actin subunits and correspondingly decreases F-actin due to actin filaments being in equilibrium with actin monomers.

Thymosin beta 4 and profilin serve complementary roles in regulating the polymerization of G-actin. (a) Profilin is bound to PIP2, a membrane lipid, at the cell membrane while the majority of G-actin is bound in a complex with thymosin beta 4 and thus unable to polymerize. (b) In response to an extracellular signal, such as chemotactic molecules that stimulate actin assembly, profilin is released from the membrane by the hydrolysis of PIP2. The released profilin displaces thymosin β4, forming profilin G-actin complexes that can assemble into filaments. (c) The profilin-actin complexes interact with proline-rich proteins in the membrane, where profilin adds actin monomers to the (+) end of actin filaments. (d) Eventually, the incorporation of monomers into filaments depletes the pools of profilin-actin and thymosin beta 4 actin complexes. ADP G-actin subunits that have dissociated from a filament are converted into ATP G-actin by profilin, thus helping to replenish the cytoplasmic pool of ATP G-actin.

Thymosin beta 4 accelerated skin wound healing in a rat model of a full thickness wound where the epithelial layer was destroyed. When thymosin beta 4 was applied topically to the wound or injected into the animal, epithelial layer restoration in the wound was increased 42% by day four and 61% by day seven, after treatment, compared to untreated. Furthermore, thymosin beta 4  stimulated collagen deposition in the wound and angiogenesis. Thymosin beta 4 accelerated keratinocyte migration, resulting in the wound contracting by more than 11%, compared to untreated wounds, to close the skin gap in the wound. An analysis of skin sections (histological observations) showed that the thymosin beta 4 treated wounds healed faster than the untreated. Proof of accelerated cell migration was also seen in vitro, where thymosin beta 4 increased keratinocyte migration two to threefold, within four to five hours after treatment, compared to untreated keratinocytes.