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A Mechanistic Model of the Actin Cycle

M. Bindschadler ^*, E. A. Osborn , C. F. Dewey, Jr.  and J. L. McGrath ^*

^* Department of Biomedical Engineering, University of Rochester, Rochester, New York 14642; and Fluid Mechanics Laboratory, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

Correspondence: Address reprint requests to Dr. James L. McGrath, Dept. of Biomedical Engineering, University of Rochester, 601 Elmwood Ave., PO Box 639, Rochester, NY 14642. Tel.: 585-253-5489; Fax: 585-273-4746; E-mail: jmcgrath{at}bme.rochester.edu.

We have derived a broad, deterministic model of the steady-state actin cycle that includes its major regulatory mechanisms. Ours is the first model to solve the complete nucleotide profile within filaments, a feature that determines the dynamics and geometry of actin networks at the leading edges of motile cells, and one that has challenged investigators developing models to interpret steady-state experiments. We arrived at the nucleotide profile through analytic and numerical approaches that completely agree. Our model reproduces behaviors seen in numerous experiments with purified proteins, but allows a detailed inspection of the concentrations and fluxes that might exist in these experiments. These inspections provide new insight into the mechanisms that determine the rate of actin filament treadmilling. Specifically, we find that mechanisms for enhancing Pi release from the ADP·Pi intermediate on filaments, for increasing the off rate of ADP-bound subunits at pointed ends, and the multiple, simultaneous functions of profilin, make unique and essential contributions to increased treadmilling. In combination, these mechanisms have a theoretical capacity to increase treadmilling to levels limited only by the amount of available actin. This limitation arises because as the cycle becomes more dynamic, it tends toward the unpolymerized state.

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