Switching Gears with Myosin Binding Protein-C

Abstract

Myosin binding protein-C (MyBP-C) is a thick-filament protein in vertebrate sarcomeres that limits cross-bridge cycling kinetics and reduces myocyte power output. However, the mechanisms by which MyBP-C influences cross-bridge kinetics are not well understood. The goal of the present study was to investigate the ability of the first 4 N-terminal domains (C0-C1-motif-C2) of cardiac (c) MyBP-C to affect actomyosin interactions and interact with actin. Here we show that recombinant proteins containing the C1 and motif domains increased Ca2+ sensitivity of tension and increased rates of tension redevelopment (ktr) at submaximal [Ca2+] in permeabilized rat trabeculae. Proteins containing these domains also biphasically activated then inhibited Ca2+-activated ATPase rates of heavy meromyosin and myosin S1 in solution. Cosedimentation binding assays demonstrated saturable binding of the 4 N-terminal domains to F-actin at a 1:1 molar ratio (Kd ∼ 10μM). However, more than one interaction site was indicated by turbidity and electron microscope analyses that showed actin bundling in the presence of recombinant proteins. Phosphorylation of the motif or increasing pH reduced binding to a 1:2 molar ratio and abolished actin bundling. Phosphorylation reduced but did not eliminate effects of recombinant proteins to increase Ca2+ sensitivity of tension and ktr at submaximal [Ca2+] in permeabilized trabeculae. Together these results suggest that the N-terminus of cMyBP-C interacts with F-actin through multiple distinct sites, at least one site is modulated by electrostatic charge interactions, and that functional effects of the N-terminus of MyBP-C are mediated in part by phosphorylation independent mechanisms. Supported by NIH HL080367.