The ionic requirements for regulation by molluscan thin filaments.

Initial studies on molluscan muscle regulation indicated that thin filaments do not confer Ca2+-dependence on vertebrate myosin ATPase, and hence that molluscan muscles do not possess thin filament-linked regulatory systems. Subsequently it was shown that molluscan thin filaments do, in fact, impart Ca2+-sensitivity but only at Mg2+ concentrations greater than those used in the earlier studies. In the present study it is shown that Mg2+ prevents significant dissociation of tropomyosin and troponin subunits from thin filaments at the low monovalent ion concentrations typically employed to assay actomyosin ATPase; as a result Mg2+ allows expression of the molluscan thin filament regulatory system under these conditions.