Chymotryptic heavy meromyosin from gizzard myosin: A proteolytic fragment with the regulatory properties of the intact myosin

Proteolytic cleavage of gizzard myosin using α-chymotrypsin under conditions which produce HMM when applied to skeletal muscle myosin yields an enzymatically active fragment which behaves like the original myosin in the its ATPase activity is activated by actin but only with the participation of Ca 2+ and additional regulatory proteins. In contrast, papain-subfragment-1 from gizzard myosin has an actin-activated ATPase that is dependent neither on Ca 2+ nor on regulatory proteins. The chymotryptic fragment can be phosphorylated by a partially purified preparation of light chain kinase, and contains intact 20,000 and 17,000-dalton light chains and a heavy chain with an apparent molecular weight of approximately 120,000, based on gel electrophoresis in sodium dodecyl sulfate.