Cloning and sequencing of cDNA for fish slow skeletal muscle myosin light chains

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Various species of fish have two types of skeletal muscle, the ordinary (white) and dark (red) muscles. It has been suggested that ordinary muscle corresponds to fast muscle of higher vertebrates, while dark muscle corresponds to slow muscle. Both skeletal muscle myosins consist of two heavy chains of approximately 200kDa and four light chains of approximately 20kDa as in the case of higher-vertebrate counterparts. The fish fast skeletal muscle myosin has been shown to have three kinds of light chains, which are classified into two groups, one pair of alkali light chains consisting of two heterologous components (A1 and A2 in the order of decreasing molecular weight) and another pair of 5,5'-dithio-bis-2-nitrobenzoic acid (DTNB) light chains. On the other hand, it is well-known that fish slow muscle myosin differs from fast muscle myosin in light chain composition. The former myosin generally has only two kinds of light chain (D1 and D2) as does cardiac myosin. However, information on structural characterization of fish slow skeletal muscle myosin light chains is not available at all. This study described cloning, sequencing, and characterization of cDNA clones encoding D1 and D2 light chains of striped jack Caranx delicatissimus and horse mackerel Trachurus japonicus slow skeletal muscles, respectively. D1 light chain was found to have a part of the difference peptide portion in the N-terminal region, as was detected in A1 light chains. This result strongly suggests that D1 light chain in slow skeletal muscle myosin possess the same function as A1 light chain in fast skeletal muscle myosin. D2 light chain was also found to have serine residue, showing phosphorylation as was detected in DTNA light chain of fast muscle. The results in this study were the first known reports on the complete primary structure of fish slow muscle myosin light chains.