Orientational disorder and motion of weakly attached cross-bridges

In a relaxed muscle fiber at low ionic strength, the cross-bridges may well be in states comparable to the one that precedes the cross-bridge power stroke (Schoenberg, M. 1988. Adv. Exp. Med. Biol. 226:189-202). Using electron paramagnetic resonance (EPR) and (saturation transfer) electron paramagnetic resonance (ST-EPR) techniques on fibers labeled with maleimide spin label, under low ionic strength conditions designed to produce a majority of weakly-attached heads, we have established that (a) relaxed labeled fibers show a speed dependence of chord stiffness identical to that of unlabeled, relaxed fibers, suggesting similar rapid dissociation and reassociation of cross-bridges; (b) the attached relaxed heads at low ionic strength are nearly as disordered as in relaxation at physiological ionic strength where most of the heads are detached from actin; and (c) the microsecond rotational mobility of the relaxed heads was only slightly restricted compared to normal ionic strength, implying great motional freedom despite attachment. The differences in head mobility between low and normal ionic strength scale with filament overlap and are thus due to acto-myosin interactions. The spectra can be modeled in terms of two populations: one identical to relaxed heads at normal ionic strength (83%), the other representing a more oriented population of heads (17%). The spectrum of the latter is centered at approximately the same angle as the spectrum in rigor but exhibits larger (40 degrees) axial probe disorder with respect to the fiber axis. Alternatively, assuming that the chord stiffness is proportional to the fraction of attached crossbridges, the attached fraction must be even more disordered than 400, with rotational mobility nearly as great as for detached cross-bridges.