The binding interaction between myosin subfragment 1 isozymes with immobilized nucleotide, where they show differential behavior, has been examined. By employing subfragment 1 hybrids formed by crosses between heavy and alkali light chains, it is possible to demonstrate that the differential behavior is modulated by the alkali light chain component of the protein and not by differences in the heavy chain subunits in these isozymes resulting from the proteolytic treatment used in their formation. The fact that the free alkali light chains show weak diffential binding under these conditions suggests that the binding in the case of the subfragment 1 isozymes may occur at a site distinct from the ATPase site. This was substantiated by examining the behavior of subfragment 1 containing ['4C]MgADP noncovalently trapped in the ATPase site by the bifunctional reagent N,N'-p-phenylenedimaleimide, on agarose-ATP. The data suggest that different ATP binding domains may be operating in myosin depending on the ionic conditions being employed.Skeletal muscle myosin is an oligomeric protein composed of two heavy polypeptide chains and two pairs of light polypeptide chains [1,2]. In its native state the two heavy chains are coiled together for half their mass into a relatively rigid r-helical, coiled-coil cable [3,4]. The two chains then separate and fold into two separate globular regions known as the myosin 'heads' or subfragment 1 moieties. One pair of light chains, which are not essential for ATPase activity, is appareiitly associated with the heavy chains at or near the junction between the x-helical rod and the two head regions [5,6]. The other pair of light chains, known as the alkali light chains, is apparently essential for activity [7-91 and each head contains one alkali light chain [lo]. Therefore, the myosin molecule is a duplex molecule in that each inyosin contains two distinct globular domains, each possessing ATPase activity. The alkali light chain pair exists as two chemically related but distinct types known as A1 and A2 [Ill. Myosin has been separated into isozymic variants differing in the A1 and A2 contents in that homodimers and heterodiiners with respect to these chains have been isolated [12,13]. Since there is substantial evidence that heavy chain heterogeneity also exists [14-161, the possibility that isozymes which are homodimeric iii alkali light chain may be heterodimers in heavy chain is quite likely. The question that immediately comes to mind is whether these distinct isoenzymes have particular physiological functions. To date, this question has not been satisfactorily answered. Weeds and Taylor [lo] showed that the individual isozyrnic forms of the myosin heads, S-l(A1) and S-1 (A2), differed in their actin-activated MgATPase but more recent data [17,18] indicate that this difference is only observable at very low ionic strengths and appears to be abolished at physiological levels of ionic strength. Another difference between the isozymes is their interaction with immobilized nucleotide such as...
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