Actin filaments power multiple cellular processes such as motility, morphogenesis, and polarity, but the mechanisms controlling their dynamics is poorly understood. Michelot et al. investigated the dynamics of actin building, bundling, and breakage in vitro by covering polystyrene beads with an actin filament–promoting protein called formin and then adding fluorescently labeled actin monomers.
The formin and actin monomers were enough to induce continuous actin polymerization at the surface of the bead. When an actin-severing factor called cofilin was added to the mix, filaments began to switch rapidly between elongation and shortening. The shortening always occurred from the filament's “older” end (the one further from the bead). According to Blanchoin, this end preference results from the gradual conversion of each actin monomer's ATP to ADP after its incorporation into the filament, as only the ADP form is a suitable cofilin substrate.
As filaments grew out from the bead, neighboring filaments often “zipped” together to form thicker multifilament cables. These cables were considerably more resistant to cofilin severing than were individual filaments, probably because cofilin cuts one filament at a time and thus would only nick the cable rather than chop right through.
The in vitro system used by the authors involved a minimal set of proteins and yet show that actin filaments can generate their own stability simply by grouping together. The team now plans to add other actin associated factors, one by one, to observe how increasing complexity affects actin dynamics.