Distribution and Structure-Function Relationship of Myosin Heavy Chain Isoforms in the Adult Mouse Heart

Abstract The two cardiac myosin heavy chain isoforms, α and β, exhibit distinct functional characteristics and therefore may be distributed regionally within the heart to match the functional demands of a specific region. In adult mouse hearts, which predominantly express α-myosin heavy chain, we observed high concentrations of β-myosin in distinct areas such as at the tip of papillary muscles and at the base close to the valvular annulus. In light of these distinct distribution patterns of the myosin isoforms, we subsequently explored the isoform-specific structure-function relationships of the myosins. The α- and β-isoforms are 93% identical in amino acid sequence, but it remains unclear which of the nonidentical residues determines isoform functionality. We hypothesized that residues situated within or close to the actin-binding interface of the myosin head influence actin binding and thereby modulate actin-activated ATPase activity. A chimeric myosin was created containing β-sequence from amino acid 417 to 682 within the α-backbone. In mice, ∼70% of the endogenous cardiac protein was replaced with the chimeric myosin. Myofibrils containing chimeric myosin exhibited ATPase activities that were depressed to the levels observed in hearts expressing ∼70% β-myosin. In vitro motility assays showed that the actin filament sliding velocity generated by chimeric myosin was similar to that of α-myosin, almost twice the velocities observed with β-myosin. These data indicate that this large domain sequence switch conferred β-like actin-activated ATPase activities to the chimeric myosin, suggesting that this region is responsible for the distinct hydrolytic properties of these myosin isoforms.