Skeletal regulatory proteins enhance thin filament sliding speed and force by skeletal HMM

At saturating calcium and nucleotide concentrations, troponin (Tn) and tropomyosin (Tm) enhance the in vitro motility speed of individual actin filaments, suggesting the roles of these thin filament proteins in regulating contraction may include a modulation of crossbridge kinetics. Using a homogeneous complement of fast rabbit skeletal proteins, we examined if Tn and Tm modify specific transitions in the crossbridge cycle by varying skeletal muscle crossbridge kinetics and measuring actin filament sliding speed and steady-state force using the in vitro motility and microneedle assays, respectively. Skeletal regulatory proteins increased the force and sliding speed of actin filaments sliding on skeletal HMM. Faster crossbridge cycling with increased temperature or with substitution of dATP as the contractile substrate resulted in both increased sliding speed and force of unregulated filaments, while the addition of regulatory proteins diminished or eliminated this increase. In contrast, regulatory proteins did not influence filament mechanics when crossbridge cycling was slowed with lowered ATP concentration. The results are most simply explained if addition of the Tn and Tm complex to actin enhances both the transition rate of the force-generating actomyosin isomerization (or the preceding transition) and the apparent crossbridge detachment rate, but that the relative influence of Tn and Tm is dependent on the external load.