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J Biol Chem, Vol. 273, Issue 47, 31289-31296, November 20, 1998
From the Department of Molecular Physiology and Biological Physics,
University of Virginia, Charlottesville, Virginia 22906-0011 and the
The differential effects of essential light chain
isoforms (LC17a and LC17b) on the
mechanical properties of smooth muscle were determined by exchanging
recombinant for endogenous LC17 in permeabilized smooth
muscle treated with trifluoperazine (TFP). Co-precipitation with
endogenous myosin heavy chain verified that 40-60% of endogenous
LC17a could be exchanged for recombinant LC17a
or LC17b. Upon addition of MgATP in Ca2+-free
solution, recombinant LC17 exchange induced slow
contractions unaccompanied by regulatory light chain (RLC)
phosphorylation only in TFP-treated, but not in untreated,
permeabilized smooth muscle; the shortening velocity and rate of force
development were approximately 1.5 and 2 times faster, respectively, in
response to LC17a than LC17b. Additional
incubation with recombinant, thiophosphorylated RLC increased the
shortening velocity, independent of the LC17 isoform
exchanged. The LC17-induced contractions of TFP-treated muscles were abolished by prior addition of nonphosphorylated RLC. We
suggest that LC17 stiffens the lever arm of myosin and, in
the absence of regulation by RLC, permits cross-bridge cycling without
requiring RLC phosphorylation. Our results are compatible with
nonphosphorylated RLC acting as a repressor and with LC17 isoforms modulating the MgADP affinity and, consequently, rate of
cooperative cycling of nonphosphorylated cross-bridges.
Myosin Essential Light Chain Isoforms Modulate the Velocity of
Shortening Propelled by Nonphosphorylated Cross-bridges
, Department of Molecular Physiology, University of
Vermont, Burlington, Vermont 05405-0068
Copyright © 1998 by The American Society for Biochemistry and Molecular Biology, Inc.
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