Letter | Published:

Separation of subfragment-1 isoenzymes from rabbit skeletal muscle myosin

Naturevolume 257pages5456 (1975) | Download Citation



RABBIT skeletal muscle myosin is composed of two heavy chains and four light chains1. Two classes of light chains can be distinguished both chemically and functionally. Within the class of essential light chains (those which cannot be removed without loss of ATPase activity), two chemically related but pheno-typically distinct proteins have been identified2, both of which are present in single fibres of fast-twitch muscles3. The existence of both phenotypes within a single psoas muscle cell indicates that either there is a single population of myosin molecules with a different alkali light chain on each subfragment-1 head, or there are at least two populations of myosin present. Densitometric and radiochemical methods have shown that there is an unequal distribution of these two light chains; this supports the hypothesis that myosin isoenzymes occur in histochemically homogeneous muscles3–5 which cannot be attributed to the presence of contaminating slow-twitch fibres. The presence of different heavy chains also is indicated by the observation of amino acid substitutions in certain peptide sequences6,7. Thus fast-twitch muscles contain myosin showing heterogeneity of both light and heavy chains, suggesting isoenzyme populations, but so far these have not been separated, nor have the proteolytic subfragments derived from them.

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  1. MRC Laboratory of Molecular Biology, Hills Road, Cambridge, CB2 2QH, UK

    • A. G. WEEDS
    •  & R. S. TAYLOR


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