My research focuses on how muscles contract. The structural events by which myosin heads projecting from the surface of thick filaments interact with actin of the thin filaments to generate force is being dramatically illuminated as the atomic structures of new myosin conformations are solved [fig 1]. Moreover, the kinetics of the steps of the cross-bridge cycle (attachment, power stroke, detachment and recovery) are known at least in solution. However, it is crucial to determine whether such a cycle can account for the mechanical properties of contracting muscle (its force, power output, ATP usage, velocity of shortening and efficiency). I have been testing this by developing mechano-kinetic computer models of the cross-bridge cycle. The effect of strain on the steps are specified using transition-state theory incorporating new understanding of how force dissociates protein complexes. The models are able to account for the experimental steady-state force-velocity relation of skeletal muscle and I am currently exploring their ability to explain tension transients resulting from the T-jumps or length ramps conducted by my colleagues Drs Ranatunga and Pinniger. The models challenge existing assumptions, offer new insights and predict behaviour which can be experimentally tested.
Complementing such studies I have a long-standing interest in the structure of the myosin molecule, the conformations it adopts, and how it packs to form the thick filament with other components such as the C-protein and H-protein which I discovered. For example, I have recently investigated what causes the hydrolysis of ATP to prime a cross-bridge in the "closed" conformation, ready for the power stroke (fig 2). In collaboration with Dr Yu (NIH) and Dr White (East Virginia Medical School) I have shown that the disorder=helical order transition of the thick filament also requires a change to the closed conformation. I have also been investigating how insertions in the otherwise regular heptad repeat of amino acid residues along the myosin tail cause deviations from the strict Crick coiled-coil structure (Fig 3) and what the consequences are for packing. Using my knowledge of coiled-coil structure, I proposed with Dr Knight (Leeds) a model of how the two heads of a myosin molecule were joined to the tail (fig 4); we successfully applied this structure in collaboration with Dr Pádrón (IVIC, Venezuela) to model the myosin heads on the highly ordered thick filaments of tarantula muscle (fig 5).
