Calcium-sensitive binding of heavy meromyosin to regulated actin requires light chain 2 and the head-tail junction

Sedimentation in a preparative ultracentrifuge was used to determine the affinity of heavy meromyosin, HMM, for regulated actin, F-actin plus troponin-tropomyosin, in the presence of MgATP at pH 7.0, 20 degrees C, and mu = 18 mM. HMM was prepared from vertebrate striated muscle myosin by a mild chymotryptic digestion. This HMM contained 85-90% intact 19 000-dalton light chains, LC2. In the presence of calcium, 90% of the HMM bound to regulated actin with an association constant of (2-4) X 10(4) M-1. In the absence of calcium, while one-third of the HMM bound with an affinity similar to that observed in the presence of calcium, the rest bound much more weakly. It was not possible to accurately determine the association constant for this weakly binding HMM, but a 20-fold reduction in affinity is consistent with the binding data. The binding of single-headed heavy meromyosin to regulated actin was similarly sensitive to the calcium concentration. Since removal of calcium inhibits the regulated actin-activated ATPase of HMM greater than 20-fold, troponin-tropomyosin must be capable of inhibiting both the binding of HMM to regulated actin and a step which occurs after binding but prior to product release. Removal of LC2 increased the fraction of HMM with calcium-insensitive binding, and adding LC2 back to this depleted HMM restored most of the calcium sensitivity. Chymotryptic cleavage of LC2 to a 17 000-dalton fragment destroyed the calcium-sensitive binding of HMM to regulated actin. Phosphorylation of LC2, however, had no detectable effect on this binding. Thus, the calcium-sensitive binding of HMM to regulated actin requires that both the head-tail junction and the N-terminal part of LC2 be intact. Binding studies with cross-linked regulated actins and kinetic measurements of the rates of change in turbidity demonstrate that this calcium sensitivity is due to calcium binding to troponin and not to LC2.