M. Sata et al., DYNAMIC INTERACTION BETWEEN CARDIAC MYOSIN ISOFORMS MODIFIES VELOCITYOF ACTOMYOSIN SLIDING IN-VITRO, Circulation research, 73(4), 1993, pp. 696-704
To study the functional significance of cardiac isomyosin heterogeneit
y, active sliding of actin-myosin was studied using two different type
s of in vitro motility assay systems: (1) a sliding actin filament ass
ay, in which fluorescently labeled actin filaments were made to slide
on a myosin layer attached to a glass coverslip, and (2) a myosin-coat
ed bead assay, in which myosin-coated latex beads were made to slide o
n actin cables of an alga. Two different isomyosins were obtained from
3-week-old (VI) and hypothyroid (V3) rat hearts and were mixed to for
m solutions with various mixing ratios [V1/(V1 + V3)]. For these myosi
n mixtures, both ATPase activity and sliding velocity of actin-myosin
were determined. As the relative content of V1 increased, both ATPase
activity and velocity increased. However, in contrast to the linear re
lation between the mixing ratio and ATPase activity, the relation betw
een the mixing ratio and sliding velocity was sigmoid, suggesting the
existence of mechanical interaction between different isomyosins. To c
larify the nature of this interaction, sliding velocity was measured f
or mixtures of V1 and p-N,N'-phenylene-dimaleimide-treated V1 myosin (
pPDM-M). A convex relation was observed between the relative content o
f pPDM-M and velocity. Because pPDM-M is known to form a noncycling an
d weakly bound crossbridge with actin, it is expected to exert a const
ant internal load on V1, in contrast to the actively cycling V3. In co
nclusion, in actomyosin sliding, different isomyosins mechanically int
eract when they coexist. The interaction may be a dynamic one that can
not be explained by a simple load effect.