Alteration of cross-bridge kinetics by myosin light chain phosphorylation in rabbit skeletal muscle: implications for regulation of actin-myosin interaction.

Abstract

Myosin light chain phosphorylation in permeable skeletal muscle fibers increases isometric force and the rate of force production at submaximal levels of calcium activation; myosin light chain phosphorylation may underlie the increased rate and extent of force production associated with isometric twitch potentiation in intact fibers. To understand the mechanism by which myosin light chain phosphorylation manifests these effects, we have measured isometric force, isometric stiffness, rate of isometric force redevelopment after isotonic shortening, and isometric ATPase activity in permeabilized rabbit psoas muscle fibers. These measurements were made in the presence and absence of myosin light chain phosphorylation over a range of calcium concentrations that caused various levels of activation. The results were analyzed with a two-state cross-bridge cycle model as suggested by Brenner [Brenner, B. (1988) Proc. Natd. Acad. Sci. USA 85, [3265][3266][3267][3268][3269]. The results indicate that myosin light chain phosphorylation exerts its effect on force generation and the isometric rate of force redevelopment in striated muscle through a single mechanism, namely, by increasing the rate constant describing the transition from non-force-generating crossbridges to force-generating states (fapp). gappq the reverse rate constant, is unaffected by phosphorylation as are the number of cycling cross-bridges. Since both calcium and myosin light chain phosphorylation increase fapp, the possibility is considered that modulation Of fpp may represent a general mechanism for regulating force in actin-myosin systems.Ca2l-dependent myosin light chain phosphorylation was first discovered in skeletal muscle in 1972 (1). Since that discovery, myosin light chain phosphorylation has been shown to be a primary mechanism for initiating contraction in smooth muscle and in some nonmuscle systems (2-4). There has been much confusion surrounding its role in skeletal muscle. Early work demonstrated that decreases in shortening velocity and oxygen consumption correlated with myosin light chain phosphorylation (5, 6). However, subsequent work in skinned and intact skeletal muscle fibers did not support a causal relationship (7-9). The only known physiological correlation to myosin light chain phosphorylation is an increase in the rate of force development and potentiation of isometric twitch tension in fast-twitch skeletal muscle fibers (10-13). Myosin light chain phosphorylation causes an increase in force production at low levels of calcium activation (14,15) and also causes an increased rate of force development (16) over a wide range of activation levels in permeable skeletal muscle fibers. These observations support the proposal that the physiological correlation between myosin light chain phosphorylation to increased twitch force and rate of force development is causal. However, a thorough understanding of how myosin light chain phosphorylation modulates the actin-myosin interaction in fibers is needed to provide insights into the...

Coexistence of potentiation and fatigue in skeletal muscle

“…Evidence supporting this conclusion includes the following: muscle stiffness increases in proportion to the increase in active force; myosin ATPase activity increases in proportion to the increase in active force, and the rate of relaxation does not decrease (10). The proportional increase in stiffness indicates that the enhanced force is associated with an increased number of attached cross-bridges, and the proportional increase in ATPase activity suggests that the time-dependent crossbridge turnover is not affected by RLC phosphorylation.…”

Myosin light chain-2 mutation affects flight, wing beat frequency, and indirect flight muscle contraction kinetics in Drosophila.

“…Therefore, we conclude that the structural defects observed in the IFM of Mlc2 ~8 heterozygotes result from the absence of some key structural role MLC-2 plays during myofilament lattice formarion. MLC-2 is associated with the globular head of the myosin heavy chain and is believed to modulate the actin-activatedmyosin-linked Mg2 § (Sweeney and Stull, 1990, and references therein). Because myosin molecules assembly into thick filaments via interactions between the rod portion of the myosin molecule, it is not surprising that a reduction in MLC-2 stoichiometry has no effect on thick filament assembly.…”

“…The myosin cross-bridge projects out from the thick (myosin) filament and attaches to an adjacent thin (actin) filament, activating an actomyosin Mg2 § which provides the chemical energy required for muscle contraction (Adelstein and Eisenberg, 1980). The cyclic making and breaking of these cross-bridges, together with a conformational change within the myosin molecule, causes the actin and myosin filaments to slide past each other enabling the muscle to shorten against an external load (Huxley, 1969 Two independent systems regulate the actomyosin ATPase cycle and contraction: a thin filament control system regulated by the troponin-tropomyosin complex, and a thick filament control system modulated by MLC-2 (Lehman and Szent-Gyorgyi, 1975;Sweeney and Stull, 1990, and references therein). The sophisticated molecular and genetic manipulations possible in Drosophila provides a powerful approach with which to investigate the structure-funodon relationships of these regulatory proteins (Peckham et al, 1990;Fyrberg and Beall, 1990).…”

“…Two independent systems regulate the actomyosin ATPase cycle and contraction: a thin filament control system regulated by the troponin-tropomyosin complex, and a thick filament control system modulated by MLC-2 (Lehman and Szent-Gyorgyi, 1975;Sweeney and Stull, 1990, and references therein). The sophisticated molecular and genetic manipulations possible in Drosophila provides a powerful approach with which to investigate the structure-funodon relationships of these regulatory proteins (Peckham et al, 1990;Fyrberg and Beall, 1990).…”

Essential “ankle” in the myosin lever arm

“…1B). The RLC phosphorylation modulates the contractile performance in striated skeletal muscle by promoting tension development at low calcium levels (19,20). These effects are due to an increase in the apparent cross-bridge attachment rate.…”