Diffusion rate limitations in actin-based propulsion of hard and deformable particles

¹ University of Florida
² University of Florida College of Medicine

^* To whom correspondence should be addressed. E-mail: dickinso{at}che.ufl.edu.

Submitted on January 31, 2006
Revised on April 4, 2006
Accepted on 15 May 2006

	Abstract

The mechanism by which actin polymerization propels intracellular vesicles and invasive microorganisms remains controversial. Several recent quantitative studies have examined propulsion of biomimetic particles such as polystyrene microspheres, phospholipid vesicles and oil droplets. In addition to allowing quantitative measurement of parameters such as the dependence of particle speed on its size, these systems have also revealed characteristic behaviors such a saltatory motion of hard particles and oscillatory deformation of soft particles. Such measurements and observed behaviors provide tests for proposed mechanisms of actin-based motility. In the "actoclampin" filament end-tracking motor model, particle-surface-bound filament end-tracking proteins are involved in load-insensitive processive insertion of actin subunits onto elongating filament plus-ends that are persistently tethered to the surface. In contrast, the "Tethered Ratchet" model assumes working filaments are untethered and grow as thermal ratchets in a load-sensitive manner. This paper presents a model for the diffusion and consumption of actin monomers during actin-based particle propulsion in order to predict the monomer concentration field around the motile particle. The results demonstrate that the various behaviors of biomimetic particles, including dynamic saltatory motion of hard particles and oscillatory vesicle deformations, can be quantitatively and self-consistently explained by load-insensitive, diffusion-limited elongation of (+)-end-tethered actin filaments, consistent with predictions of the 'actoclampin' end-tracking model.

Key Words: Brownian Ratchet, Cell Motility, Force Generation, Molecular Motors, Polymerization Motors

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