Syunsuke Matsushita and Kuniyuki Hatori
Department of Biosystem Engineering,
Graduate School of Science and Engineering,
Yamagata University, Yonezawa, 992-8510 Japan
The sliding movement of actin filaments, consisting of heterogeneous
components, on skeletal muscle myosin molecules was examined to specifically
evaluate the effect of internal modulation of the actin filaments for force
transmission on the sliding movement. Inactive actin molecules were prepared by
conjugation with indocarbocyanine fluorescent dyes (IC3-OSu or Cy3-NHS) in
molar ratios greater than a 3-fold excess. IC3-OSu is
an analogue of Cy3-NHS, and it can bind to primary amino groups. IC3-conjugated
actin (IC3-actin) monomers were polymerized into the filaments
which led to complete impairment of both motile activity and
myosin-ATPase activation. Filaments of Cy3-conjugated actin (Cy3-actin)
exhibited a decrease in velocity to a third of the value (33%) observed for
intact actin filaments. In the absence of ATP, dissociation rates of IC3- and
Cy3-actin filaments from myosin molecules were greater than those of intact
actin filaments, indicating that IC3- and Cy3-actin act as smaller resistance
components against sliding movement compared to intact filaments. Subsequently,
two types of copolymer filaments were prepared. The first type of copolymer
were filaments copolymerized homogeneously with intact actin monomers and
IC3-actin monomers, while the second kind were block
copolymer filaments composed of two short filaments of intact actin and
IC3-actin. The sliding velocities of these copolymer filaments hyperbolically
decreased as the fraction of IC3-actin monomer increased. In practice, 75%
IC3-actin within homogeneous copolymer was required to reduce the velocity by
half. In the case of block copolymer 65% IC3-actin led to the same decrease in
velocity. For Cy3-actin copolymer filaments similar differences between
homogeneous and block copolymer filaments were also observed. Drag ratio
between IC3-actin (or Cy3-actin) and intact actin was estimated by
consideration of the force balance between the power force and the drag force
imposed on a filament during steady movement. Consequently, the drag ratios of
IC3-actin to intact actin were 0.31 (homogeneous copolymer) and 0.47 (block
copolymer), respectively. Thus, IC3-actin incorporated homogeneous copolymers
exhibits a smaller resistance to sliding movement than IC3-actin modified block
copolymers.