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Am J Physiol Cell Physiol (December 11, 2002). doi:10.1152/ajpcell.00474.2002
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Articles in PresS, published online ahead of print December 11, 2002
Am J Physiol Cell Physiol, 10.1152/ajpcell.00474.2002
Submitted on October 10, 2002
Accepted on December 6, 2002

The Converter Domain Modulates the Kinetic Properties of Drosophila Myosin

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

1 Biology, San Diego State University, San Diego, CA, USA
2 Chemistry, San Diego State University, San Diego, CA, USA
3 Molecular Physiology and Biophysics, University of Vermont, Burlington, VT, USA

* To whom correspondence should be addressed. E-mail: kplittle{at}scripps.edu.

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. Presently, we investigate 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, mean ±SEM). 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's. 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.




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