Intracellular β-Thymosins

The β-thymosins are a family of highly conserved polar peptides consisting of 40 to 44 amino acid residues. All β-thymosins bind monomeric G-actin in a 1:1 complex. The dissociation constant of the complex is in the micromolar range and allows for fast binding and release of G-actin. Because of the high intracellular concentration of β-thymosins (up to 500 µM) in most vertebrate cells, β-thymosins are considered the main intracellular G-actin sequestering peptides. Thymosin β4 binds to G-actin in an extended conformation, and folds into a stable conformation upon binding. The N- and C-termini of thymosin β4 contact the barbed and pointed ends of the monomeric actin. Thymosin β4 is present in the nucleus as well as the cytoplasm and might be responsible for sequestering nuclear actin. Even minor cell damage might be responsible for the release of β-thymosins detectable in the extracellular fluids. Extracellular β-thymosins affect matrix metallo-proteinases, chemotaxis, angiogenesis and wound healing. However, only very little is known about the molecular mechanisms mediating the effects attributed to extracellular β-thymosins.