2,3-Butanedione monoxime (BDM) reversibly inhibits force production in muscle. At least part of its action appears to be directly on the contractile apparatus. To understand better its mechanism of action, we studied the effect of BDM on the steps of myosin subfragment 1 Mg(2+)-ATPase in 0.1 M potassium acetate, pH 7.4. Because of the rapidity of certain processes, we experimented at 4 degrees C and our main technique was the rapid flow quench method. By varying the experimental conditions (relative concentrations of reagents, time scale, quenching agent), it was possible to study selectively the different steps of the S1 Mg(2+)-ATPase: [formula: see text] At saturation (20 mM), BDM had two major effects on the ATPase. First, it increased the equilibrium constant of the cleavage step (K3) from 2 to > 10. Second, it slowed the kinetics of the release of Pi by an order of magnitude (k4; from 0.054 to 0.004 s-1). By contrast, the kinetics of the binding of ATP (k) and the release of ADP (k6) were little affected by BDM. Thus, the oxime appears to interact specifically with M**.ADP.Pi, and it is a rare example of an uncompetitive inhibitor. Its effect is to reduce the steady-state concentration of the "strong" actin binding state M*.ADP and to increase that of the "weak" binding state, M**.ADP.Pi. The effect of BDM on the initial ATPase of Ca2+ activated myofibrils was very similar to that on S1 ATPase. Thus, with myofibrils too BDM seems to exert its main effect subsequent to the initial binding and cleavage steps.(ABSTRACT TRUNCATED AT 250 WORDS)

Myosin II is the major component of the muscle thick filament. It consists of two N-terminal S1 subfragments (''heads'') connected to a long dimeric coiled-coil rod. The rod is in itself twofold symmetric, but in the filament, the two heads point away from the filament surface and are therefore not equivalent. This breaking of symmetry requires the initial section of the rod, subfragment 2 (S2), to be relatively flexible. S2 is an important functional element, involved in various mechanisms by which the activity of smooth and striated muscle is regulated. We have determined crystal structures of the 126 N-terminal residues of S2 from human cardiac ␤-myosin II (S2-⌬), of both WT and the disease-associated E924K mutant. S2-⌬ is a straight parallel dimeric coiled coil, but the N terminus of one chain is disordered in WT-S2-⌬ due to crystal contacts, indicative of unstable local structure. Bulky noncanonical side chains pack into a͞d positions of S2-⌬'s N terminus, leading to defined local asymmetry and axial stagger, which could induce nonequivalence of the S1 subfragments. Additionally, S2 possesses a conserved charge distribution with three prominent rings of negative potential within S2-⌬, the first of which may provide a binding interface for the ''blocked head'' of smooth muscle myosin in the OFF state. The observation that many disease-associated mutations affect the second negatively charged ring further suggests that charge interactions play an important role in regulation of cardiac muscle activity through myosin-binding protein C.coiled coil ͉ muscle ͉ crystallography ͉ regulation ͉ familial hypertrophic cardiomyopathy M uscle contraction results from the ATP-dependent cyclic interaction of the proteins myosin II and actin, assembled in thick and thin filaments, respectively. Myosin II consists of two heavy chains, each of which binds one essential and one regulatory light chain (Fig.

New X-ray data suggest how myosin rods, themselves alpha-helical coiled coils, form the thick filament backbone of crustacean muscles by additional supercoiling. Natural transformations of this structure may describe the myosin backbone in many other animals also.

X-ray diffraction patterns from relaxed invertebrate muscles reveal the thick filament symmetries and cross-bridge configurations. The cross bridges are substantially angled to the filament axes. The results on symmetry are generally consistent with Squire's model.