Support for a trimeric collar of myosin binding protein C in cardiac and fast, but not slow, skeletal muscle

Abstract

Myosin binding protein C (MyBPC) has been shown to have both a structural and regulatory function in the striated muscle sarcomere. We have previously proposed a quaternary structure for this molecule within the sarcomere in which MyBPC takes on a trimeric collar arrangement around the thick filament backbone, based on interactions between domains C5 and C8. We set out to determine whether the C5:C8 interaction is directed by the cardiac-specific insertion in domain C5, and, if not, whether C5:C8 interactions also occur in skeletal MyBPC isoforms, using qualitative and quantitative yeast two-hybrid protein–protein interaction assays. Findings were confirmed by surface plasmon resonance. We demonstrate enhanced interaction between cardiac domain C8 and a cardiac domain C5 from which the insertion loop had been deleted (Ka=1.3×105), as compared to interaction of domain C8 with native cardiac C5 (Ka=8.2×104). Furthermore, we find evidence for a C5:C8 interaction in the fast skeletal isoform (Ka=5.7×105), but not in the slow skeletal isoform. These results indicate that the cardiac-specific insertion does not drive the C5:C8 interaction first demonstrated for the cardiac isoform. Moreover, the finding that the interaction holds true for fast, but not slow, skeletal muscle, may indicate that the trimeric collar arrangement of MyBPC is significant for the regulation of high rates of contraction and relaxation in fast skeletal and cardiac muscle. Further studies are required to explore the quaternary structure of MyBPC in slow skeletal muscle.