Thymosin β4 Is Not Always the Main β-Thymosin in Mammalian Platelets

: β‐thymosins constitute a family of highly conserved 5‐kDa polypeptides. Thymosin β4, the most abundant member of this family, is expressed in most mammalian cell types and is regarded as the main intracellular G‐actin sequestering peptide. In addition to this important intracellular function several other activities have been attributed to this peptide. Thymosin β4 is released from human platelets and cross‐linked to fibrin after activation of platelets with thrombin. While in most mammalian tissues thymosin β4 is accompanied by a second member of this peptide family, in human platelets only thymosin β4 is present. To elucidate if it is common to mammalian platelets that only one β‐thymosin is present, we analyzed platelets from several mammals for their β‐thymosin content. In human platelets only thymosin β4 could be detected, whereas in bovine platelets thymosin β9, which is normally the minor β‐thymosin in bovine tissues, was identified as the main β‐thymosin. In rabbit platelets, thymosin β4 is not simply replaced by the most homologous thymosin β4Ala, as might be expected from sequence homology. Thymosin β4Ala and thymosin β10 were found, but thymosin β10 is present in about 2.5‐fold higher amounts. Because thymosin β4Ala possesses about threefold higher affinity to G‐actin, compared to thymosin β4, β10, and β9, we suggest that expression of β‐thymosins is triggered by functional requirements and not sequence homology.