Elevated viscosity Slows In Vitro Motility Of Actin Filament

P.B. Chase and K. Kulin

Depts. of Radiology, Physiology & Biophysics, and Zoology, University of Washington, Seattle, WA 98195 USA.

In maximally Ca²⁺-activated skinned fibers, elevated solution viscosity (/₀) decreased shortening velocity (VUS, Vmax) and the rate of isometric tension redevelopment (kTR) (Chase et al., 1996. Biophys J 70:A41); significantly, both kinetic parameters varied proportional to (/₀)-1 suggesting that some aspect(s) of each is diffusion limited. To directly investigate whether /₀acts at the molecular level, we studied motion of fluorescently-labeled F-actin in an in vitro motility assay and varied </₀. Assays used chymotryptic-HMM on nitrocellulose-coated surfaces (30°C); care was taken to minimize effects of ATP-insensitive HMM (Regnier et al., 1996. Biophys J 71:2786). Viscosity of actin buffer (AB) was varied by adding either sucrose or fructose. Speed in control assays was 7.2 mm s^-1 and, in concert with the result in skinnedfibers, speed varied in proportion to (/₀)-1. This effect was not due to osmotic forces since the relationship between speed and (/₀)-1 was the same for mono- and di-saccharides. At /₀= 6, speed was slowest (by 35%) for long filaments although this length dependence was not observed at /₀= 2 - 3 where speed was 40 - 50% of control. Taken together,these results implicate a diffusional-limitation to actin filament sliding at the molecular level. (Supported by NIH grant HL52558)

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