Bending of smooth muscle myosin rod

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Abstract
Electron microscopy of mammalian smooth muscle myosin rods showed them to be 153 ± 7nm (SD) long, and to bend sharply (> 90°) but infrequently, and pH independently (range 6.5–9.5), at a single site 45 ± 4 nm from one end of the molecule. Light meromyosin (LMM) preparations were 99 ± 10 nm long, and showed no bends. Intrinsic viscosity vs temperature plots for rods and LMM indicated that neither fragment changed in flexibility in the range 4–40° C. Peptide mapping in the presence and absence of SDS established that the proteolytic susceptibility of the hinge at the N terminus of LMM reflects the presence of locally different structure, and not simply a clustering of susceptible residues. The isolated smooth muscle myosin rod thus contains only a single hinge, having significant stiffness, and lacks the second bend seen under certain conditions in the intact molecule.