THYMOSIN BETA-4 (T-BETA(4)) IN ACTIVATED PLATELETS

When resting human blood platelets are stimulated with thrombin, 50 to 60% of the G-actin polymerizes to F-actin within 60 seconds. The incr ease in F-actin is correlated with a corresponding decrease in the com plex of G-actin with Tbeta4. Within 5 seconds after stimulation, nucle ation sites for pyrene actin polymerization increase 1.5 times in Trit on-lysed supernatants. Cytochalasin D, known to inhibit the increase i n F-actin after thrombin, also inhibits the fall in Tbeta4-actin compl ex and the increase in nucleation sites. Phosphorylation of Tbeta4 cou ld not be detected in either control or activated cells. Increased Tbe ta4 corresponding to that lost from the Tbeta4-actin complex is presen t in lysates from activated platelets and retains the ability to compl ex with actin. The data, taken together with previous estimates for th e dissociation constant of the Tbeta4-actin complex, show that actin p olymerization following platelet activation could be controlled primar ily by the increased availability of free barbed ends of actin filamen ts which have a higher affinity for G-actin than does Tbeta4 and sugge st that the increased free Tbeta4 may serve to limit the degree of pol ymerization.