UC Berkeley Library

The necessary biophysical conditions for the formation of tubular membrane protrusions by polymerizing actin filament bundles have not yet been fully understood. For this reason we introduce a novel grand canonical simulation model that describes stochastic polymerization of filaments against a fluctuating fluid membrane, while only considering a minimum set of biological proteins. Although still relatively simple and highly tractable, our model explicitly accounts for thermal fluctuations of membrane and filaments, stochastic and quantized polymerization dynamics at the filament tip, cooperativity of multiple filaments, and steric interactions between all model constituents in a physically realistic way. This approach enables us to go well beyond previous static zero-temperature theoretical considerations to filament bundling and explore the physical origins of membrane tube formation dynamics on length and time scales that are currently inaccessible to both experiments and atomistically detailed simulations. Our results suggest a membrane mediated dynamical transition from single filaments to cooperatively growing bundles as an important dynamical bottleneck to tubular protrusion.