Active Site Control of Myosin Cross-Bridge Zeta Potential

The electrical properties of contractile proteins contribute to muscle structure and perhaps function but have not been characterized adequately. Electrophoretic mobility, μe, is sensitive to the net electric charge and hydrodynamic size of a molecule in solution. Zeta potential, ζ, particle charge, Q e, and particle charge-to-mass ratio are proportional to μe. We measured μe for nucleotide complexes of skeletal muscle heavy meromyosin (HMM) and subfragment 1 (S1). The results indicate that μe for HMM changes depending on the ligand bound in the active site. The changes in electric charge appear to occur mainly on the S1 moieties. For HMM(MgATPγS)2 and HMM(MgADP·Pi)2 the values of μe are −0.077 and −0.17 (μm/s)/(V/cm), respectively. For these complexes, μe is independent of [ATP], [ADP], and [Pi]. When Pi dissociates from HMM(MgADP·Pi)2 to form HMM(MgADP)2, μe decreases to −0.61 (μm/s)/(V/cm). This large decrease in μe is independent of free [ADP] or [ATP]. Increasing [Pi], on the other hand, increases μe for HMM(MgADP)2 to values near those observed for the steady-state intermediate. For HMM, μe = −0.34 and is independent of Pi. MgADP binding to HMM decreases μe to −0.57 (μm/s)/(V/cm), and the dissociation constant is 9 μM. Taken together, these data indicate that μe and, thus, ζ are controlled by ligand binding to the active site. The magnitudes of the particle charge-to-mass ratios for the HMM complexes are all in a range that falls within published values determined for a variety of other proteins. Possible roles that the observed nucleotide-dependent changes in cross-bridge electric charge might have in the contractile cycle in muscle are considered.