Distribution of light chains in fast skeletal myosin

Abstract

Myosin is composed of two heavy chains of molecular weight 200,000 and four light chains of approximate molecular weight 20,000. The heavy chains are organised into a two-headed structure of which each head is completed by the presence of two light chains1,2. In fast skeletal myosin the light chains can be divided into two classes which are chemically and functionally distinct. One member of each class is associated with each head. The class known as the alkali light chain contains two related variants: alkali 1 (A1) and alkali 2 (A2). These light chains are identical in amino acid sequence over their C-terminal 141 residues, but differ in their N-terminal sequences3. The non integral stoichiometry of light chains in rabbit skeletal myosin4,5 first suggested that at least some myosins must be homodimeric (a molecule with the same light chain on each head), but it was not clear to what extent heterodimers (molecules containing both alkali 1 and alkali 2 light chains) also occurred. The most direct approach to determining the distribution of light chains in myosin is to isolate the different myosin isoenzymes. Although ion-exchange chromatography has been successful in separating the single-headed sub-fragments of myosin6, it has not been possible to fractionate double-headed species except by some type of affinity chromatography7,8. We have shown that homodimers of myosin can be isolated from chicken pectoralis myosin by immunological methods9,10. By comparing the electrophoretic patterns of homodimeric molecules with whole myosin in non-denaturing gels, it has been possible to demonstrate conclusively the existence of a light chain heterodimer.