Complex formation between regulatory and essential light chain on squid mantle myosin subfragment-1 revealed by thermal denaturation method.

Thermal treatment of squid myosin subfragment-1 (S-1) in the presence of EDTA results in a rapid inactivation of ATPase, a marked turbidity increase, and a dissociation of light chains. These effects were suppressed by addition of calcium ion. Different light chain binding in EDTA-medium from that in Ca-medium was demonstrated by the tryptic digestion of native squid S-1; the two types of light chain are both resistant to trypsinolysis in Ca-medium, whereas they are readily degraded in EDTA-medium. S-1 heavy chain was converted into three fragments with sizes of 27, 47, and 22 kDa in both media. However, trypsinolysis of S-1 inactivated in Ca-medium generated no such heavy chain fragments that survived, while the two types of light chain survived. These light chains were isolated as a complex lacking any heavy chain fragments, and the complex formation was Ca-sensitive. It is concluded that regulatory and essential light chains are present on S-1 as a complex whose formation is mediated by calcium ion, and this binding might alter the S-1 conformation so as to confer resistance to thermal treatment.