Molecular mechanics of mouse cardiac myosin isoforms

Abstract

Two myosin isoforms are expressed in myocardium, alphaalpha-homodimers (V(1)) and betabeta-homodimers (V(3)). V(1) exhibits higher velocities and myofibrillar ATPase activities compared with V(3). We also observed this for cardiac myosin from normal (V(1)) and propylthiouracil-treated (V(3)) mice. Actin velocity in a motility assay (V(actin)) over V(1) myosin was twice that of V(3) as was the myofibrillar ATPase. Myosin's average force (F(avg)) was similar for V(1) and V(3). Comparing V(actin) and F(avg) across species for both V(1) and V(3), our laboratory showed previously (VanBuren P, Harris DE, Alpert NR, and Warshaw DM. Circ Res 77: 439-444, 1995) that mouse V(1) has greater V(actin) and F(avg) compared with rabbit V(1). Mouse V(3) V(actin) was twice that of rabbit V(actin). To understand myosin's molecular structure and function, we compared alpha- and beta-cardiac myosin sequences from rodents and rabbits. The rabbit alpha- and beta-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.

Factors controlling cardiac myosin-isoform shift during hypertrophy and heart failure

“…Single molecule studies have also demonstrated that the higher actin sliding velocity of the αMHC isoform is related to a shorter attachment time of myosin after the power stroke and not to the displacement generated by the myosin power stroke [29,30]. These studies have suggested that the kinetics (attachment time) rather than the mechanics (unitary displacement) of the myosin molecule accounts for the difference in the actin-sliding velocity of the α and βMHC isoforms [23,[28][29][30]. Based on these observations, it is generally believed that hearts expressing primarily αMHC isoform have a significantly higher velocity of muscle shortening; whereas hearts expressing mostly βMHC, which has low ATPase activity, have the ability to generate force with greater economy.…”

“…The mechanical properties of the heart are well correlated with the type of MHC isoform expressed [23]. Studies conducted on myofibrillar ATPase activity have documented that the speed with which muscle shortens (the velocity of shortening) is correlated with the ATPhydrolyzing capacity of the myosin molecule [24].…”

“…Many studies performed on single myocytes and isolated papillary muscle preparations, where the MHC ratio was altered by thyroid manipulation, have demonstrated that, indeed, a linear correlation exists between the maximal speed of shortening of unloaded cardiac muscle and the ratio of the α to βMHC isoform expression [25][26][27]. More tangible evidence for this finding came from in vitro motility assays where purified α and β MHC isoforms were mixed in different ratios and tested for their ability to slide fluorescence labeled actin [23,28]. Warshaw's group has shown in an in vitro motility assay, that the actin sliding velocity (V actin) of the myosin mixture was highest at 100% αMHC and gradually decreased as the proportion of the βMHC isoform was raised, reaching the lowest level at 100% βMHC isoform, thus confirming the linear correlation between V actin and α:βMHC isoforms ratio.…”

“…The rat and mouse α and βMHC isoforms have been shown to generate the same amount of average force (isometric peak tension) [23,28]. In contrast, the rabbit βMHC isoform has been shown to generate higher average force time integrals, compared to the rabbit αMHC isoform [23].…”

“…The rat and mouse α and βMHC isoforms have been shown to generate the same amount of average force (isometric peak tension) [23,28]. In contrast, the rabbit βMHC isoform has been shown to generate higher average force time integrals, compared to the rabbit αMHC isoform [23]. This difference is again explained on the basis of variations in kinetics (the fraction of time myosin is attached to actin during the entire cross-bridge cycle, also referred as duty ratio) of the power stroke generated by two MHC isoforms, and not by the unitary force, which is equal for both MHC isoforms of rabbits [23].…”