The statistical mechanics of filaments with transient cross linkers: Casimir interactions, the bundling transition, and topological defects

Alex Levine (Hosted by Nussinov), Departments of Physics & Astronomy, Chemistry & Biochemistry, and Computational Medicine, UCLA

The interior of our cells is pervaded by a low-density network of stiff, proteinaceous filaments bound into complex networks and filament bundles by a panoply of cross-linking proteins.  These cytoskeletal networks are the locus of the cell’s mechanical rigidity, the structure by which it exerts forces on its surroundings, and the principal organizer of the cell’s shape.  It is also a remarkable system for studying statistical physics.  In this talk, I will discuss recent theoretical, simulation, and experimental work on the formation of filament bundles and bundle networks. Specifically, I will begin with an analysis of thermal Casimir interactions between cross-linkers in a bundle, which enables a discontinuous (first-order) bundling transition.  I will discuss the role of topologically-protected defects, such as braids, in bundles, their long-time dynamics, and their effect on bundle shape and mechanics.  Finally, I will comment on the collective mechanics of networks of bundles, as are often found in the cytoskeleton of living cells. 

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