Conformational changes in subdomain-2 of G-actin upon polymerization into F-actin and upon binding myosin subfragment-1.

The susceptibility of subdomain-2 of actin to different proteases has been examined, for G-actin, F-actin, G-actin-S1(A2) and F-actin-S1(A2) complexes on a comparative basis. The sites of subtilisin, alpha-chymotrypsin and trypsin attack, exposed on G-actin, are protected in F-actin, F-actin-S1(A2) as well as in the G-actin-S1(A2) complex. In contrast, a new cleavage site (Arg39-His40) for ArgC protease, which is protected in G-actin, is exposed in G-actin-S1(A2) as well as in F-actin and F-actin-S1(A2). These results are consistent with the previously proposed structural analogy between the ternary (G-actin)2S1 and the F-actin-S1 complexes, and provide information on the mechanism of S1-induced polymerization of G-actin.