Angles of fluorescently labelled myosin heads and actin monomers in contracting and rigor stained muscle fiber.

Abstract

The measurement of polarized fluorescence from a fluorescent ATP or ADP analog, epsilon-ATP or epsilon-ADP (1, N6-Etheno-ATP or ADP) bound to myosin heads in a glycerinated muscle fiber revealed that during isometric contraction at low concentrations of Mg-epsilon-ATP the bound nucleotides are highly oriented with respect to the fiber axis, and their mean angle is almost the same as that in rigor. Furthermore, the polarization of tryptophan fluorescence did not change when the fiber was transferred from a rigor state to an active state at low ATP concentrations. Thus, if a rotation of myosin heads occurs during contraction, it seems to be very limited. The angle of bound epsilon-ADP is not changed by passive stretching of the fiber in the presence of epsilon-ADP. By using the same technique, it was found that orientation of a phalloidin-FITC complex (Ph-FITC) specifically bound to F-actin in a glycerinated muscle fiber is significantly changed when the fiber is activated from relaxation or rigor to contraction. No change in orientation of Ph-FITC bound to F-actin is induced by simple addition of Mg-ADP or by passive stretching of the fiber in the absence of ATP. These results suggest that a rotation or a distortion of actin monomers occurs during contraction, which is involved in the process of active tension development.