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A Model for Force Generation by Microtubule End-Binding Proteins

Luzelena Caro, University of Florida, Dept. of Chemical Engineering, PO Box 116005, Gainesville, FL 32611-6005, Richard B. Dickinson, Chemical Engineering, Biomedical Engineering, University of Florida, Dept. of Chemical Engineering, PO Box 116005, Gainesville, FL 32611-6005, and Daniel L. Purich, Biochemistry & Molecular Biology, University of Florida, Dept. of Biochemistry & Molecular Biology, R3126 ARB, Gainesville, FL 32611-6005.

Ciliary and flagellar assembly as well as chromosomal movement appears to require forces that are generated by microtubule (+)-end elongation, the process whereby tubulin dimers add to the fast growing ends of microtubules (MTs). The end-binding protein EB1 was previously demonstrated to bind specifically to the GTP-rich polymerizing microtubule plus-end, where the MT is tightly bound, suggesting a possible role in force generation at these sites. Current Brownian Ratchet models once thought to explain the protrusive forces caused by elongating microtubules cannot easily account for the magnitude of force generated or the strong attachment of an MT to the surface during rapid elongation. We recently proposed (Dickinson, R. B, Caro, L and Purich, D. L. (2004) Biophys. J. 87, 2838-2854) that the end-binding proteins, specifically EB1, are end-tracking cytoskeletal motors (operating akin to actoclampin), and suggested that these motors convert the energy of MT-GTP hydrolysis to work, thereby providing the significant forces required for cell motility and strong attachment between a motile object and polymerizing MTs. We now report that in the presence of force, such mechanisms exhibit the capacity of affinity-modulated end-tracking motors to achieve higher stall forces than predicted with the Brownian Ratchet system, while maintaining a strong, persistent attachment to the motile object. MT end-trackers enjoy kinetic and thermodynamic advantages over thermal ratchets, especially when elongation is energetically coupled to GTP hydrolysis. Such features are likely to be vitally important in high-fidelity translocation of chromosomes during cell division (mitosis) and gametogenesis (meiosis), where failure to maintain chromosome number can have devastating consequence for cell viability and proliferation.