Analysis of myosins from lobster muscles: Fast and slow isozymes differ in heavy‐chain composition

The contractile speed of striated muscle is correlated with myosin adenosine triphosphatase (ATPase) activity. In crustaceans, ATPase activity in fast muscle is about four‐fold greater than that in slow muscle. Slow and fast myosins were purified from the crusher claw closer and the deep abdominal muscles, respectively, of teh American lobster, Homarus americanus , and analyzed by peptide mapping, chymotryptic digestion, and two‐dimensional polyacrylamide gel electrophoresis (PAGE). Peptide mapping of myosin heavy chains with Staphylococcus aureus V8 protease produced seven peptides unique to slow‐muscle myosin and 11 peptides unique to fastmuscle myosin. Heavy‐chain fragments produced by chymotryptic digestion of native myosin differed in molecular weight and isoelectric point between the two muscle types, suggesting possible differences in the flexible hinge region in the rod portion of the molecule. In contrast, twodimensional PAGE showed that the corresponding light chains of both fast and slow myosins were identical in size and net charge. Therefore, the molecular basis of the difference in ATPase activities between fast‐ and slow‐muscle myosins can be attributed solely to heavy‐chain isoform composition.