Mutations in beta-myosin S2 that cause familial hypertrophic cardiomyopathy (FHC) abolish the interaction with the regulatory domain of myosin-binding protein-C

The myosin filaments of striated muscle contain a family of enigmatic myosi n-binding proteins (MyBP), MyBP-C and MyBP-H. These modular proteins of the intracellular immunoglobulin superfamily contain unique domains near their N termini. The N-terminal domain of cardiac MyBP-C, the MyBP-C motif, cont ains additional phosphorylation sites and may regulate contraction in a pho sphorylation dependent way. In contrast to the C terminus, which binds to t he light meromyosin portion of the myosin rod, the interactions of this dom ain are unknown. We demonstrate that fragments of MyBP-C containing the MyB P-C motif localise to the sarcomeric A-band in cardiomyocytes and isolated myofibrils, without affecting sarcomere structure. The binding site for the MyBP-C motif resides in the N-terminal 126 residues of the S2 segment of t he myosin rod. In this region, several mutations in beta-myosin are associa ted with FHC; however, their molecular implications remained unclear. We sh ow that two representative FHC mutations in beta-myosin S2, R870H and E924K , drastically reduce MyBP-C binding (K-d approximate to 60 mu M for R870H c ompared with a K-d of approximate to 5 mu M for the wild-type) down to unde tectable levels (E924K). These mutations do not affect the coiled-coil stru cture of myosin. We suggest that the regulatory function of MyBP-C is media ted by the interaction with S2, and that mutations in beta-myosin S2 may ac t by altering the interactions with MyBP-C. (C) 1999 Academic Press.