Polymerization of actin from the thymosin β4 complex initiated by the addition of actin nuclei, nuclei stabilizing agents or myosin S1

Thymosin β4 forms a 1:1 complex with actin and thereby prevents polymerization. Rapid formation of filaments from this complex was observed, however, when actin trimers were added. Polymerization can likewise be initiated by the addition of one equivalent of phalloidin or, less effectively, cytochalasin B. Since both toxins, which reportedly support nucleation, have similar effects as the covalently linked actin trimers, it appears that the formation of filaments from the actin—thymosin β4 complex depends on the availability of stable actin nuclei. Remarkably, rapid polymerization was also observed if small amounts of myosin S1 were added, suggesting that also myosin, a protein functionally connected with polymeric actin, can serve as a nucleation center. Considering the existence of thymosin β4 and related peptides in numerous mammalian tissues, our data suggest that spontaneous formation of microfilaments in non-muscle cells may be regulated at the level of nucleation. Uncontrolled polymerization induced by the formation of phalloidin-stabilized nuclei may explain the acute toxic effects of phalloidin in hepatocytes