An immunological approach to myosin light-chain function in thick filament linked regulation. 2. Effects of anti-scallop myosin light-chain antibodies. Possible regulatory role for the essential light chain

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Abstract
Specific antibodies directed against the regulatory L chains (R-LC) or essential L chains (SH-LC) of scallop myosin abolished Ca regulation in myofibrils, myosin and heavy meromyosin by elevating the actin-activated Mg2+-ATPase activity in the absence of Ca. Ca dependence was completely eliminated at molar ratios of 2.5-3 antibodies bound per myosin. Monovalent anti-R-LC Fab and anti-SH-LC Fab fragments desensitized myofibrils fully. High Ca2+-ATPase activity remained unaffected by the antibodies. Anti-SH-LC IgG reduced to about 1/2 the actin-activated Mg2+-ATPase in the presence of Ca and the K-activated EDTA-ATPase activities. Anti-SH-LC Fab desensitized without inhibiting the actin-activated Mg2+-ATPase. The desensitizing effect of both antibodies was abolished by prior absorption with the homologous myosin L chain. Ca binding and R-LC content remained unaffected by anti-R-LC and anti-SH-LC IgG and by anti-SH-LC Fab. The anti-R-LC Fab fragment induced a significant (70%) dissociation of R-LC from myofibrils and myosins with concomitant losses in Ca binding. Anti-R-LC IgG prevented the dissociation of R-LC from myosin by EDTA. Binding of anti-R-LC IgG to myofibrils was proportional to their R-LC content. Increased amounts of anti-SH-LC IgG were bound by myofibrils devoid of R-LC. Bound anti-SH-LC antibody significantly inhibited the reuptake of R-LC by EDTA-treated myofibrils and the full binding of anti-R-LC antibody. Certain rabbits produced a population of anti-SH-LC antibodies which were specific for this L chain and bound extensively to myosin but failed to desensitize it (nondesensitizing anti-SH-LC antibody). The desensitizing and nondesensitizing anti-SH-LC IgG populations bound to different regions of the SH-LC on the myosin, and the binding of the 2 types of antibody to the SH-LC was nearly additive. The nondesensitizing SH-antibody inhibited the reuptake of R-LC less, and its binding to myofibrils was not influenced by the absence of R-LC. These studies indicate a direct or indirect involvement of the SH-LC in myosin-linked regulation, raise the possibility of an interaction between the R-LC and SH-LC, and confirm the regulatory function of the scallop R-LC. A model for a relative location of the 2 types of L chains and the involvement of the subfragment-2 region of myosin in myosin linked regulation is discussed.