ADP-INDUCED CHANGES IN ORDERING OF SPIN-LABELED MYOSIN HEADS IN MUSCLE-FIBERS

Rotational dynamics and ordering of myosin heads in glycerinated skele tal muscle fibres were studied using an isothiocyanate-based spin labe l attached to the fast-reacting thiol sites of myosin and were compare d with data obtained for maleimide and iodoacetamide spin labels attac hed to the same sites. The ordering of probe molecules on the millisec ond time scale in the rigor state, at sarcomere length 2.2-2.3 +/- 0.1 mu m, was static. Isothiocyanate probe molecules showed greater mobil ity; the segment holding the label rotated in the microsecond time ran ge. In the saturation transfer EPR time domain, MgADP did not produce a significant change in the mobility of spin labels. The spectra of is othiocyanate spin-labelled fibres were analyzed in terms of two narrow distributions with mean angles of 75 degrees and 56 degrees. In the r igor state, the fractions represented approximately 76% and 24% of the total EPR absorbance. In the presence of MgADP, the conventional EPR spectra showed large changes in the ordering of isothiocyanate probe m olecules towards a new distribution, the population with a theta value of 56% increased from 24% to 71% at the expense of the 75% population with no change in the mean angles of the distributions. In the case o f maleimide and iodoacetamide spin-labelled Fibres, however, the effec t of MgADP on the probe angular distribution was small.