STRUCTURE OF MUSCLE IN THE RELAXED STATE AND RIGOR LABELLED WITH MYOSIN S-1: IMPLICATIONS ABOUT FORCE PRODUCTION. Harford, J. J., Hudson, L., Denny, R., Squire, I. M., Biophysics Section, Imperial College, Prince Consort Rd, London, UK

The purpose of this project is to solve the structure of the A-band unit cell in relaxed fish muscle and to use this to follow the molecular movements in active muscle regulation and force generation. The first frame, relaxed muscle, has now been solved. Time-resolved X-ray diffraction patterns from contracting plaice fin muscle have been recorded at 1ms or 5ms time-intervals (depending on the region being studied) and these are then processed using CCP13 software and modelled using the known myosin head shape and actin filament structure. The solution for the first frame has revealed the organisation of myosin heads around the actin filaments in the hexagonal A-band unit cell. This frame is now being used as a starting structure to model successive frames through the time-series, thus producing a `Movie' of the molecular processes involved in contraction. Already, from analysis of the resting and rigor muscle structures, it has been shown that myosin heads need to swing axially on actin by about 5 to 15nm in order to proceed from the resting to rigor states. It is presumed that the heads need to do this in active force generation as well. More recently we have recorded X-ray diffraction patterns to a resolution of about 1nm from chemically skinned plaice muscle in the rigor state labelled with exogenous myosin S-1 heads. Modelling of these patterns, now in progress, is revealing in much greater detail the changes in conformation of the myosin heads that accompany this transition from the relaxed to the rigor state.