The Journal of Cell Biology, Vol 105, 2225-2234, Copyright © 1987 by The Rockefeller University Press
Rigor crossbridges are double-headed in fast muscle from crayfish
F Bard, C Franzini-Armstrong and W Ip
Department of Biology, University of Pennsylvania, Philadelphia 19104- 6018.
The structure of rigor crossbridges was examined by comparing rigor
crossbridges in fast muscle fibers from glycerol-extracted abdominal flexor
muscle of crayfish with those in "natively decorated" thin filaments from
the same muscle. Natively decorated thin filaments were obtained by
dissociating the backbone of the myosin filaments of rigor myofibrils in
0.6 M KCl. Intact fibers were freeze-fractured, deep- etched, and rotary
shadowed; isolated filaments were either negatively stained or freeze dried
and rotary shadowed. The crossbridges on the natively decorated actin
maintain the original spacing and the disposition in chevrons and double
chevrons for several hours, indicating that no rearrangement of the
actomyosin interactions occurs. Thus the crossbridges of the natively
decorated filaments were formed within the geometrical constraints of the
intact myofibril. The majority of crossbridges in the intact muscle have a
triangular shape indicative of double-headed crossbridge. The triangular
shape is maintained in the isolated filaments and negative staining
resolves two heads in a single crossbridge. In the isolated filaments,
crossbridges are attached at uniform acute angles. Unlike those in insect
flight muscle (Taylor et al., 1984), lead and rear elements of the double
chevron may be both double-headed. Deep-etched images reveal a twisted
arrangement of subfilaments in the backbone of the thick filament.
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