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Am J Physiol Cell Physiol 284: C1031-C1038, 2003. First published December 11, 2002; doi:10.1152/ajpcell.00474.2002
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Vol. 284, Issue 4, C1031-C1038, April 2003

The converter domain modulates kinetic properties of Drosophila myosin

Kimberly Palmiter Littlefield1, Douglas M. Swank1, Becky M. Sanchez1, Aileen F. Knowles2, David M. Warshaw3, and Sanford I. Bernstein1

Departments of 1 Biology and 2 Chemistry, Molecular Biology Institute and Heart Institute, San Diego State University, San Diego, California 92182-4614; and 3 Department of Molecular Physiology and Biophysics, University of Vermont, Burlington, Vermont 05405

Recently the converter domain, an integral part of the "mechanical element" common to all molecular motors, was proposed to modulate the kinetic properties of Drosophila chimeric myosin isoforms. Here we investigated the molecular basis of actin filament velocity (Vactin) changes previously observed with the chimeric EMB-IC and IFI-EC myosin proteins [the embryonic body wall muscle (EMB) and indirect flight muscle isoforms (IFI) with genetic substitution of the IFI and EMB converter domains, respectively]. In the laser trap assay the IFI and IFI-EC myosins generate the same unitary step displacement (IFI = 7.3 ± 1.0 nm, IFI-EC = 5.8 ± 0.9 nm; means ± SE). Thus converter-mediated differences in the kinetics of strong actin-myosin binding, rather than the mechanical capabilities of the protein, must account for the observed Vactin values. Basal and actin-activated ATPase assays and skinned fiber mechanical experiments definitively support a role for the converter domain in modulating the kinetic properties of the myosin protein. We propose that the converter domain kinetically couples the Pi and ADP release steps that occur during the cross-bridge cycle.

actin-activated adenosine 5'-triphosphatase activity; unitary step displacement; skinned fiber preparations; cross-bridge cycle; chemomechanical coupling


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