  Filopodia are ubiquitous fingerlike protrusions, spawned by many eukaryotic
cells, to probe and interact with their environments. Polymerization dynamics
of actin filaments, comprising the structural core of filopodia, largely
determine their instantaneous lengths and overall lifetimes. The polymerization
reactions at the filopodial tip require transport of G-actin, which enter the
filopodial tube from the filopodial base and diffuse toward the filament barbed
ends near the tip. Actin filaments are mechanically coupled into a tight bundle
by cross-linker proteins. Interestingly, many of these proteins are relatively
short, restricting the free diffusion of cytosolic G-actin throughout the
bundle and, in particular, its penetration into the bundle core. To investigate
the effect of steric restrictions on G-actin diffusion by the porous structure
of filopodial actin filament bundle, we used a particle-based stochastic
simulation approach. We discovered that excluded volume interactions result in
partial and then full collapse of central filaments in the bundle, leading to a
hollowed-out structure. The latter may further collapse radially due to the
activity of cross-linking proteins, hence producing conical-shaped filament
bundles. Interestingly, electron microscopy experiments on mature filopodia
indeed frequently reveal actin bundles that are narrow at the tip and wider at
the base. Overall, our work demonstrates that excluded volume effects in the
context of reaction-diffusion processes in porous networks may lead to
unexpected geometric growth patterns and complicated, history-dependent
dynamics of intermediate metastable configurations.
