Molecular mechanics of mouse cardiac myosin isoforms

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Abstract
Two myosin isoforms are expressed in myocardium, αα-homodimers (V1) and ββ-homodimers (V3). V1exhibits higher velocities and myofibrillar ATPase activities compared with V3. We also observed this for cardiac myosin from normal (V1) and propylthiouracil-treated (V3) mice. Actin velocity in a motility assay ( Vactin) over V1myosin was twice that of V3as was the myofibrillar ATPase. Myosin's average force (Favg) was similar for V1and V3. Comparing Vactinand Favgacross species for both V1and V3, our laboratory showed previously (VanBuren P, Harris DE, Alpert NR, and Warshaw DM. Circ Res 77: 439–444, 1995) that mouse V1has greater Vactinand Favgcompared with rabbit V1. Mouse V3Vactinwas twice that of rabbit Vactin. To understand myosin's molecular structure and function, we compared α- and β-cardiac myosin sequences from rodents and rabbits. The rabbit α- and β-cardiac myosin differed by eight and four amino acids, respectively, compared with rodents. These residues are localized to both the motor domain and the rod. These differences in sequence and mechanical performance may be an evolutionary attempt to match a myosin's mechanical behavior to the heart's power requirements.