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Biophys J, January 2001, p. 415-426, Vol. 80, No. 1
and
*Department of Physiology, Jefferson Medical College, Thomas
Jefferson University, Philadelphia, Pennsylvania 19107, and
Muscle Biology Group, University of Arizona, Tucson,
Arizona 85721 USA
The anterior byssus retractor muscle of Mytilus
edulis was used to characterize the myosin cross-bridge during
catch, a state of tonic force maintenance with a very low rate of
energy utilization. Addition of MgATP to permeabilized muscles in high
force rigor at pCa > 8 results in a rapid loss of some force
followed by a very slow rate of relaxation that is characteristic of
catch. The fast component is slowed 3-4-fold in the presence of 1 mM MgADP, but the distribution between the fast and slow (catch) components is not dependent on [MgADP]. Phosphorylation of twitchin results in loss of the catch component. Fewer than 4% of the myosin heads have ADP bound in rigor, and the time course (0.2-10 s) of ADP
formation following release of ATP from caged ATP is similar whether or
not twitchin is phosphorylated. This suggests that MgATP binding to the
cross-bridge and subsequent splitting are independent of twitchin
phosphorylation, but detachment occurs only if twitchin is
phosphorylated. A similar dependence of detachment on twitchin
phosphorylation is seen with AMP-PNP and ATP
S. Single turnover
experiments on bound ADP suggest an increase in the rate of release of
ADP from the cross-bridge when catch is released by phosphorylation of
twitchin. Low [Ca2+] and unphosphorylated twitchin appear
to cause catch by 1) markedly slowing ADP release from attached
cross-bridges and 2) preventing detachment following ATP binding to the
rigor cross-bridge.
Biophys J, January 2001, p. 415-426, Vol. 80, No. 1
© 2001 by the Biophysical Society 0006-3495/01/01/415/12 $2.00
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