Thick filament-associated protein located in the crossbridge region of vertebrate striated muscle a bands. Slow skeletal protein that binds to both myosin and actin. In vitro, binds to native thin filaments and modifies the activity of actin-activated myosin ATPase. May modulate muscle contraction or may play a more structural role. Interacts with USP25 (isoform USP25m only); the interaction prevents proteasomal degradation of MYBPC1. Belongs to the immunoglobulin superfamily. MyBP family.