MULTIPLE VARIANTS OF MYOFIBRILLAR PROTEINS IN SINGLE FIBERS OF LOBSTER CLAW MUSCLES: EVIDENCE FOR TWO TYPES OF SLOW FIBERS IN THE CUTTER CLOSER MUSCLE.

SDS-polyacrylamide gel electrophoresis of myofibrillar proteins in single fibers of lobster claw closer muscles distinguished three types of fibers: one fast and two slow. The fibers differed both qualitatively and quantitatively in the variants of paramyosin and troponin present. There were four proteins unique to fast fibers (paramyosin, a 75-kD protein, troponin-I, and -I) and two proteins unique to slow fibers (troponin-I and -C). Fast fibers were found only in the cutter claw. The major type of slow fibers (S) appeared to account for the entire muscle mass in the crusher claw as well as ~85% of the slow fibers in the cutter claw. Another type (S) comprised 10-15% of the slow fibers in the cutter claw. The S and S fibers differed in the variants of troponin-I and -T. The S fibers contained troponin-T, and I as the major variant of troponin-I; S fibers lacked T and contained I as the major isoform. These data indicate that the heterogeneity of myofibrillar proteins observed in actomyosins extracted from whole muscle (Mykles, 1985) is due to three populations of fibers, each containing its own assemblage of regulatory and contractile isoforms. More than one variant of a myofibrillar protein can be expressed in a single fiber, forming unique assemblages by which subgroups can be discriminated within the broader categories of fast and slow fibers.