The strength of binding of the weakly-binding crossbridge created by sulfhydryl modification has very low calcium sensitivity

Cross-bridge cooperativity during isometric contraction and unloaded shortening of skeletal muscle

Whether the two heads of skeletal muscle myosin work independently or cooperatively remains an open question in muscle biophysics. While individual myosin heads are sufficient for ATPase activity (Reisler (1980) J Mol Biol 138: 93-107) and force production (Harada et al. (1987) Nature 326: 805-808), it has also been reported that in situ, the two heads of a myosin molecule work cooperatively (Chaen et al. (1986) J Biol Chem 261(29): 13,632-13,636). To examine the role of cross-bridge cooperativity on isometric contraction and unloaded shortening we progressively inactivated myosin cross-bridges via titration with para-phenylenedimaleimide. The resting fiber ATPase was measured to provide an estimate of the fraction of active cross-bridges remaining during the titration. Isometric force and unloaded shortening velocity decline more rapidly than the resting ATPase as the titration proceeds. This is inconsistent with models for independent force generation and suggests cooperative action of myosin cross-bridges when muscle is isometrically contracting or shortening under zero load. However the degree of cooperativity depends on the type of muscle activity. While isometric force declines in a manner consistent with pair-wise cooperative action of myosin heads, unloaded shortening velocity declines more rapidly (greater cooperativity). Therefore, myosin cross-bridges in situ may be capable of at least two types of cooperative interactions, pair-wise cooperativity (when isometric) and another form of cooperativity that is sensitive to longer range interactions transmitted from other cross-bridges in the ensemble (during unloaded shortening).

A model of cross-bridge attachment to actin in the A*M*ATP state based on x-ray diffraction from permeabilized rabbit psoas muscle

A model of cross-bridges binding to actin in the weak binding A*M*ATP state is presented. The modeling was based on the x-ray diffraction patterns from the relaxed skinned rabbit psoas muscle fibers where ATP hydrolysis was inhibited by N-phenylmaleimide treatment (S. Xu, J. Gu, G. Melvin, L. C. Yu. 2002. Biophys. J. 82:2111-2122). Calculations included both the myosin filaments and the actin filaments of the muscle cells, and the binding to actin was assumed to be single headed. To achieve a good fit, considerable flexibility in the orientation of the myosin head and the position of the S1-S2 junction is necessary, such that the myosin head can bind to a nearby actin whereas the tail end was kept in the proximity of the helical track of the myosin filament. Hence, the best-fit model shows that the head binds to actin in a wide range of orientations, and the tail end deviates substantially from its lattice position in the radial direction (approximately 60 A). Surprisingly, the best fit model reveals that the detached head, whose location thus far has remained undetected, seems to be located close to the surface of the myosin filament. Another significant requirement of the best-fit model is that the binding site on actin is near the N terminus of the actin subunit, a position distinct from the putative rigor-binding site. The results support the idea that the essential role played by the weak binding states M*ATP <--> A*M*ATP for force generation lies in its flexibility, because the probability of attachment is greatly increased, compared with the weak binding M*ADP*P(i) <--> A*M*ADP*P(i) states.

Structural characterization of weakly attached cross-bridges in the A*M*ATP state in permeabilized rabbit psoas muscle

It is well established that in a skeletal muscle under relaxing conditions, cross-bridges exist in a mixture of four weak binding states in equilibrium (A*M*ATP, A*M*ADP*P(i), M*ATP, and M*ADP*P(i)). It has been shown that these four weak binding states are in the pathway to force generation. In the past their structural, biochemical, and mechanical properties have been characterized as a group. However, it was shown that the myosin heads in the M*ATP state exhibited a disordered distribution along the thick filament, while in the M*ADP*P(i) state they were well ordered. It follows that the structures of the weakly attached states of A*M*ATP and A*M*ADP*P(i) could well be different. Individual structures of the two attached states could not be assigned because protocol for isolating the two states has not been available until recently. In the present study, muscle fibers are reacted with N-phenylmaleimide such that ATP hydrolysis is inhibited, i.e., the cross-bridge population under relaxing conditions is distributed only between the two states of M*ATP and A*M*ATP. Two-dimensional x-ray diffraction was applied to determine the structural characteristics of the attached A*M*ATP state. Because the detached state of M*ATP is disordered and does not contribute to layer line intensities, changes as a result of increasing attachment in the A*M*ATP state are attributable to that state alone. The equilibrium toward the attached state was achieved by lowering the ionic strength. The results show that upon attachment, both the myosin and the first actin associated layer lines increased intensities, while the sixth actin layer line was not significantly affected. However, the intensities remain weak despite substantial attachment. The results, together with modeling (see J. Gu, S. Xu and L. C. Yu, 2002, Biophys. J. 82:2123-2133), suggest that there is a wide range of orientation of the attached A*M*ATP cross-bridges while the myosin heads maintain some degree of helical distribution on the thick filament, suggesting a high degree of flexibility in the actomyosin complex. Furthermore, the lack of sensitivity of the sixth actin layer line suggests that the binding site on actin differs from the putative site for rigor binding. The significance of the flexibility in the A*M*ATP complex in the process of force generation is discussed.

Reversible inactivation of myosin subfragment 1 activity by mechanical immobilization

The Mg-ATPase activity of skeletal muscle myosin subfragment 1 (S1) is reversibly eliminated when it is aggregated by the force of osmotic pressure dehydration using polyethylene glycol (PEG). Several experiments indicate nucleotides bind aggregated S1, but the effects of binding are attenuated. Compared with S1 in solution, epsilonADP binds aggregated S1 with reduced affinity, and the bound epsilonADP fluorescence intensity is more effectively quenched by acrylamide. When ATP binds aggregated S1, the tryptophan intensity increases to only 50% of the solution level. Chemical cross-linking of cys-707 to cys-697 by p-phenylenedimaleimide is less efficient for aggregated S1 x MgADP. The data are consistent with aggregated S1 being able to bind nucleotide but not being able to complete the usual conformation change(s) in response to binding. If S1 is kept from aggregating by increasing the ionic strength at the same osmotic pressure, its Mg-ATPase activity and ATP-induced tryptophan fluorescence intensity increase are normal. The combined data are consistent with an ATP hydrolysis mechanism in which S1 segmental motion is coupled to its enzymatic activity. In this model, segmental motion is mechanically constrained by aggregation; the constrained S1 can bind ATP, but it cannot complete the hydrolysis mechanism.