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Biophys J, June 2002, p. 3150-3159, Vol. 82, No. 6
Department of Molecular Biology and Immunology, University of North Texas, Fort Worth, Texas 76107-2699 USA
Myosin head consists of a globular catalytic domain and a
long 
-helical regulatory domain. The catalytic domain is responsible for binding to actin and for setting the stage for the main
force-generating event, which is a "swing" of the regulatory
domain. The proximal end of the regulatory domain contains the
essential light chain 1 (LC1). This light chain can interact through
the N and C termini with actin and myosin heavy chain. The interactions
may inhibit the motion of the proximal end. In consequence the motion
of the distal end (containing regulatory light chain, RLC) may be
different from the motion of the proximal end. To test this
possibility, the angular motion of LC1 and RLC was measured
simultaneously during muscle contraction. Engineered LC1 and RLC were
labeled with red and green fluorescent probes, respectively, and
exchanged with native light chains of striated muscle. The confocal
microscope was modified to measure the anisotropy from 0.3 µm3 volume containing ~600 fluorescent cross-bridges.
Static measurements revealed that the magnitude of the angular change
associated with transition from rigor to relaxation was less than
5o for both light chains. Cross-bridges were activated by a
precise delivery of ATP from a caged precursor. The time course of the angular change consisted of a fast phase followed by a slow phase and
was the same for both light chains. These results suggest that the
interactions of LC1 do not inhibit the angular motion of the proximal
end of the regulatory domain and that the whole domain rotates as a
rigid body.
Biophys J, June 2002, p. 3150-3159, Vol. 82, No. 6
© 2002 by the Biophysical Society 0006-3495/02/06/3150/10 $2.00
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