Regulatory properties of single-headed fragments of scallop myosin.

Calcium control was studied in single-headed myosin and subfragment-1 (S1) preparations obtained by papain digestion of scallop myosin. Single-headed myosin, containing light chains in stoichiometric amounts, was calcium regulated; in contrast, the actin-activated Mg-ATPase of all S1 species lacked calcium sensitivity. Both regulatory and essential light chains were retained by S1 and single-headed myosin preparations provided divalent cations were present during papain digestion, although a peptide amounting to 10% of the mass was removed from regulatory light chains. The modified regulatory light chain retained its ability to confer calcium binding and restore calcium sensitivity to the ATPase of desensitized myofibrils. Regulatory light chains protected the essential light chains from fragmentation by papain. S1 bound regulatory light chains with a uniformly high affinity and appeared to consist of a single species. The results demonstrate that head to head interactions are not obligatory for calcium control, although they may occur in the intact myosin molecule, and suggest a role for the subfragment-2 region in calcium regulation of myosin.