Thymosin β 4 Is Not Always the Main β‐Thymosin in Mammalian Platelets

:  β‐thymosins constitute a family of highly conserved 5‐kDa polypeptides. Thymosin β 4 , 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 β 4 is released from human platelets and cross‐linked to fibrin after activation of platelets with thrombin. While in most mammalian tissues thymosin β 4 is accompanied by a second member of this peptide family, in human platelets only thymosin β 4 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 β 4 could be detected, whereas in bovine platelets thymosin β 9 , which is normally the minor β‐thymosin in bovine tissues, was identified as the main β‐thymosin. In rabbit platelets, thymosin β 4 is not simply replaced by the most homologous thymosin β 4 Ala , as might be expected from sequence homology. Thymosin β 4 Ala and thymosin β 10 were found, but thymosin β 10 is present in about 2.5‐fold higher amounts. Because thymosin β 4 Ala possesses about threefold higher affinity to G‐actin, compared to thymosin β 4 , β 10 , and β 9 , we suggest that expression of β‐thymosins is triggered by functional requirements and not sequence homology.