Thick filament-associated protein located in the crossbridge region of vertebrate striated muscle A bands. In vitro it binds MHC, F-actin and native thin filaments, and modifies the activity of actin-activated myosin ATPase. It may modulate muscle contraction or may play a more structural role. May be involved in the early phase of myofibrillogenesis. Expressed specifically in cardiac muscle among adult tissues, but is also expressed transiently in the skeletal muscle at early developmental stages. Isoform Type I is found in embryonic skeletal muscle and isoform Type II is found in both embryonic skeletal and cardiac muscle. Substrate for phosphorylation by PKA and PKC. Reversible phosphorylation appears to modulate contraction. Belongs to the immunoglobulin superfamily. MyBP family.