Both isoforms of skeletal muscle subfragment 1 (S1A1 and S1A2) can induce actin polymerization with equal speed in the absence of ATP.

: The ability of myosin subfragment 1 to induce actin polymerization was reinvestigated using the DNase I inhibition assay, by electron microscopy after negative staining, cosedimentation, measurement of viscosity and the fluorescence increase of pyrenyl-labeled actin. Using these techniques we demonstrate that rabbit skeletal muscle myosin subfragment 1 containing either the alkali light chain 1 (S1A1) or the alkali light chain 2 (S1A2) is able to promote actin polymerization even in the absence of divalent cations or salt. In the presence of ATP the rate of induction of actin polymerization by S1A2 is slower than by A1A1. In contrast, in the absence of free ATP, both subfragment 1 variants exhibit equal ability to induce actin polymerization. Evidence is given that the slower rate of induction of actin polymerization by S1A2 in the presence of free ATP is due to a slower rate of ATP-hydrolysis by S1A2 and thus to a slower rate of ATP depletion. We therefore assume that the formation of rigor type complexes involving the subfragment 1 heavy chain is necessary for the induction of actin polymerization. The ability of subfragment 1 to induce actin polymerization is retarded by a synthetic heavy chain mimetic peptide which inhibits its actin binding or after proteolytic cleavage of the subfragment 1 heavy chain by trypsin.