INTERACTION OF THYMOSIN-BETA-4 WITH MUSCLE AND PLATELET ACTIN - IMPLICATIONS FOR ACTIN SEQUESTRATION IN RESTING PLATELETS

Quantitative measurements of the interactions of T-beta-4 with muscle actin suggest that its only physiological role is monomer sequestration. T-beta-4 forms a 1:1 complex with monomeric actin under physiological salt conditions. Its K(d) for actin is not affected by calcium. T-beta-4 binds only to actin monomers and not to filament ends or alongside the filament. T-beta-4-actin complexes do not elongate actin filaments at either the barbed or the pointed end, and, unlike actobindin, T-beta-4 does not specifically suppress the nucleation of polymerization. We assessed the fraction of monomeric actin that can be sequestered by T-beta-4 in resting platelets. This was done on the basis of (a) its K(d) of 0.4-0.7-mu-M for platelet actin, which had been prepared by a newly devised simpler method, and (b) the values for the concentrations of monomeric actin and of T-beta-4 which we measured as 280 and 560-mu-M, respectively. Using the higher K(d) value of 0.7-mu-M, the T-beta-4-complexed actin is calculated to be between 70 and 240-mu-M, depending on the steady-state free G-actin concentration. This may vary from 0.1 to 0.5-mu-M, the critical concentrations for uncapped and for fully barbed-end-capped actin filaments. If the K(d) in the platelet is the same as in vitro, most of the sequestered actin would be bound to T-beta-4 if more than 95% of the actin filaments are capped at their barbed ends in resting platelets.