Myosin-linked calcium regulation in squid mantle muscle. Light-chain components of squid myosin

As reported by Kendrick-Jones et al. (1976), myosin from squid mantle muscle contains two types of light-chain components, different in size but similar in net charge. We were able to separate the two types of light chains by a five-step procedure, yielding LC-1 (17,000 daltons) and LC-2 (15,000 daltons). It was found that squid mantle LC-1 and LC-2 function exactly like SH-light chains and EDTA-light chains of scallop adductor myosin, respectively. In functional tests, we used "desensitized" myosin of scallop adductor muscle, simply because "EDTA washing" removed neither LC-1 nor LC-2 from squid mantle myosin. The removal and recombination of light chains were examined by gel electrophoresis, and Ca or Sr sensitivity was determined by measuring the Mg-ATPase activity of skeletal acto-scallop or squid myosin. It was found that EDTA washing readily released the EDTA-light chains of scallop myosin completely, and that the EDTA-washed scallop myosin was capable of regaining its full content of EDTA-LC as well as its full sensitivity to calcium. We also found that as regards combining with, and conferring calcium sensitivity on the EDTA-washed myosin of scallop adductor, squid mantle LC-2 could effectively replace scallop adductor EDTA-LC. In addition, calcium or strontium ions were found to induce changes in the UV absorption spectrum of scallop adductor EDTA-LC, although the apparent binding constants estimated from the difference spectrum were too low to account for the Ca or Sr sensitivity of scallop actomyosin-ATPase. The divalent cations also induced changes in the UV absorption spectrum of squid LC-2, and the apparent binding constants estimated from the difference spectrum were sufficiently high (1.5 X 10(5) M-1 for Ca binding, and 1.6 X 10(3) M-1 for Sr binding) to account for the Ca and Sr sensitivities of squid mantle myosin B-ATPase. The findings with scallop adductor myosin are in conflict with those reported by Kendrick-Jones et al., and must be accounted for in formulating the molecular mechanism of myosin-linked calcium regulation in molluscan muscles.