Thymosin β 4

:  The intracellular function of thymosin β 4 is not limited to simple sequestration of globular actin. Our recent studies revealed that thymosin β 4 affects actin critical concentration and forms a ternary complex with actin and profilin. The consequences of this complex formation can be very significant. Our new data demonstrate that it is likely that profilin affects binding of thymosin β 4 to actin in the ternary complex through allosteric changes in actin rather than through competition for the binding site. The N‐ and C‐terminal thymosin β 4 helices are known to be unstructured in aqueous solution and to adopt helical conformation in organic solvents or upon binding to actin. Osmolytes stabilize protein structure, and TMAO (trimethylamine N‐oxide) specifically stabilizes hydrogen bonds. This increases affinity of intact thymosin β 4 to actin significantly, but the increase is much less for thymosin β 4 sulfoxide. Our data show that oxidation does not alter binding of profilin to form a ternary complex, and therefore it is very likely that there is no direct steric interference by methionine 6 of thymosin β 4 . Rather, since TMAO has little effect on thymosin β 4 sulfoxide, this observation is consistent with the hypothesis that methionine oxidation prevents helix transition. The experiment with truncated versions of thymosin β 4 also supports this hypothesis. Oxidation and formation of the helices are important for both intra‐ and extracellular properties of thymosin β 4 . We found that actin and, in lesser extent, profilin–actin complex protect thymosin β 4 from oxidation.