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 increase in F-actin is correlated with a corresponding decrease in the complex of G-actin with T beta 4. Within 5 seconds after stimulation, nucleation sites for pyrene actin polymerization increase 1.5 times in Triton-lysed supernatants. Cytochalasin D, known to inhibit the increase in F-actin after thrombin, also inhibits the fall in T beta 4-actin complex and the increase in nucleation sites. Phosphorylation of T beta 4 could not be detected in either control or activated cells. Increased T beta 4 corresponding to that lost from the T beta 4-actin complex is present in lysates from activated platelets and retains the ability to complex with actin. The data, taken together with previous estimates for the dissociation constant of the T beta 4-actin complex, show that actin polymerization following platelet activation could be controlled primarily by the increased availability of free barbed ends of actin filaments which have a higher affinity for G-actin than does T beta 4 and suggest that the increased free T beta 4 may serve to limit the degree of polymerization.