The Role of Macromolecular Crowding in Cytoskeletal Organization

The cytoskeleton is not only responsible for cell morphology and rheology, but is also involved in such functions as cell motility, intracellular transport and cell division. Key to all these roles is the suitable deployment of its labile parts, a process that occurs in a highly crowded environment. Here we consider the effects of crowding on the assembly and organization of cytoskeletal filaments, and how the cell exploits or evades these effects, as needed. Due to entropic trade-offs among different degrees of freedom, long protein filaments crowded by globular proteins spontaneously form segregated bundles. This behavior can be recruited to specific locations by proteins that nucleate filaments and can be adjusted by proteins that cross-link parallel filaments in a fashion that stabilizes polarity, spacing and register within the bundle. Alternatively, spontaneous bundling can be prevented by limiting filament growth with capping and severing proteins or by frustrating filament alignment with oblique cross-links or the formation of branches. Thus, building upon the effects of macromolecular crowding, a modest library of regulatory proteins is able to achieve versatile results.