Factors affecting filament velocity in in vitro motility assays and their relation to unloaded shortening velocity in muscle fibers.
The measurement of fluorescently labeled actin filament movement driven by mechanoenzymes (e.g. myosin) is an important methodology for the study of molecular motors. It is assumed that the filament velocity, Vf, is analogous to the unloaded shortening velocity, Vu, seen in muscle fibers. To evaluate this assumption we compared Vf to literature values for Vu with regard to the effects of [ATP], [ADP], [Pi], pH, ionic strength (10-150 mM) and temperature (15-30 degrees C). Vf and Vu are quantitatively similar with respect to the effects of substrate and product concentrations and temperatures > 20 degrees C. However, Vf is more sensitive to decreases in pH and temperatures < 20 degrees C than is Vu. At ionic strengths of 50-150 mM, Vf and Vu exhibit similar ionic strength dependencies (decreasing with ionic strength). At ionic strengths < 50 mM, Vf is markedly reduced. Thus while Vf is a good analogue for Vu under certain conditions (elevated ionic strength and temperatures > 20 degrees C), under others it is not. The results of motility assays must be cautiously interpreted.
Homsher, E; Wang, F; Sellers, J
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