Differential binding of rabbit fast muscle myosin light chain isoenzymes to regulated actin

The direct binding of S1(A1) and S1(A2) to regulated actin has been investigated by centrifugation. Binding was measured in the presence of either Mg X AdoPP[NH]P or Mg X ADP at 24 degrees C at various ionic strengths. At low ionic strength, in either the presence or absence of Ca2+, the binding of S1(A1) to regulated actin was always stronger than for S1(A2). As the ionic strength was increased the differential binding between S1(A1) and S1(A2) was still maintained in the presence of Ca2+ but not in its absence. These data are discussed in terms of a modifying role for the N-terminal region of the A1 light chain in regulation of the contractile process.