ORIENTATION AND DYNAMICS OF MYOSIN HEADS IN ALUMINUM FLUORIDE-INDUCEDPRE-POWER STROKE STATES - AN EPR STUDY

We have determined the orientation and dynamics of the putative pre-po wer stroke crossbridges in skinned muscle fibers labeled with maleimid e spin-label at Cys-707 of myosin. Orientation was measured using elec tron paramagnetic resonance (EPR) and mobility by saturation transfer EPR. The crossbridges are trapped in the pre-power stroke conformation in the presence of aluminum fluoride, Ca, and ATP. In agreement with data published for unlabeled fibers (Chase et al., 1994), spin-labeled muscle fibers display 42.5% of rigor stiffness, without the generatio n of force. The trapped crossbridges are as disordered as the relaxed heads, but their microsecond dynamics are significantly restricted. Mo deling of the immobile fraction (35%), in terms of attached heads as e stimated from stiffness, suggests that the bound heads rotate with a c orrelation time tau(r) = 150-400 mu s, as compared to tau(r) = 3 mu s for the heads in relaxed fibers. These ''strongly'' attached myosin he ads, at orientations other than in rigor, are a candidate for the stat e from which head rotation generates force, as postulated by H. E. Hux ley (1969). Ordering of the heads may well be the structural event dri ving the generation of force.