Solution structure of myosin-ADP-MgFn ternary complex by fluorescent probes and small-angle synchrotron X-ray scattering

Muscle and nonmuscle myosins probed by a spin label at equivalent sites in the force-generating domain

We have engineered a mutant of Dictyostelium discoideum (Dicty) myosin II that contains the same fast-reacting "SH1" thiol as in muscle myosin, spin-labeled it, and performed electron paramagnetic resonance (EPR) to compare the structure of the force-generating region of the two myosins. Dicty myosin serves as a model system for muscle myosin because of greater ease of mutagenesis, expression, and crystallization. The catalytic domains of these myosins have nearly identical crystal structures in the apo state, but there are significant differences in ATPase kinetics, and there are no crystal structures of skeletal muscle myosin with bound nucleotides, so another structural technique is needed. Previous EPR studies, with a spin label attached to SH1 in muscle myosin, have resolved the key structural states of this region. Therefore, we have performed identical experiments on both myosins spin-labeled at equivalent sites. Spectra were identical for the two myosins in the apo and ADP-bound states. With bound ADP and phosphate analogs, (i) both proteins exhibit two resolved structural states (prepowerstroke, postpowerstroke) in a single biochemical state (defined by the bound nucleotide), and (ii) these structural states are essentially identical in the two myosins but (iii) are occupied to different extents as a function of the biochemical state. We conclude that (i) myosin structural and biochemical states do not have a one-to-one correspondence, and (ii) Dicty myosin can serve as a good analog for structural studies of muscle myosin only if differences in the coupling between biochemical and structural states are taken into account.

The asymmetric ATPase cycle of the thermosome: elucidation of the binding, hydrolysis and product-release steps

Using a combination of intrinsic fluorescence to report ATP-induced rearrangements, quenched-flow to measure ATP hydrolysis "on-enzyme" and optical methods to probe the kinetics of product release, we have begun to dissect the process of energy transduction in the thermosome, a type II chaperonin from Thermoplasma acidophilum. Stoichiometric measurements of ATP binding reveal the tight association of eight nucleotide molecules per hexa-decamer, implying the filling of only one ring owing to strong negative cooperativity. After binding, we show that these eight ATP molecules are hydrolysed over the next 50 s, after which hydrolysis slows down markedly during the establishment of the steady state in the ATPase reaction, demonstrating that the kinetic system is off-rate limited. Looking in more detail, this rapid first-turnover can be dissected into two phases; the first occurring with a half-time of 0.8 s, the second with a half-time of 14 s, possibly reflecting the differential behaviour of the four alpha and four beta subunits in a single thermosome ring. To investigate the post-hydrolytic events, we used two heat-stable enzyme-linked optical assays to measure the rate of evolution of ADP and of phosphate from the thermosome active site. Neither product showed a rapid dissociation phase prior to the establishment of the steady state, showing that both are released slowly at a rate that limits the cycle. These data highlight the importance of the highly populated thermosome/ADP/Pi complex in the molecular mechanism.