The influence of regulatory light chains on structural organization of cardiac myosin heads interacting with actin and ATP.

The susceptibility of papain cleavage sites on the cardiac myosin essential light chain (LC1) was studied at low and high Ca2+ concentration in cardiac myosin filaments alone and complexed with pure skeletal actin or cardiac regulated actin in the absence or presence of ATP. Enzymatic properties of cardiac myosin containing papain cleaved LC1 were compared to those of intact myosin. It was found that the kind of divalent cations (Mg2+, Ca2+) saturating the regulatory light chains influences the susceptibility of essential light chains to papain cleavage. The cardiac myosin having shortened essential light chains showed increased affinity for skeletal pure actin and a significant decrease of Ca2+ sensitivity of Mg2+ activated ATPase activity. This was observed both in the case of cardiac myosin complexed with cardiac regulated actin and skeletal actin complexed with cardiac regulated proteins.