Regulation of processive actin filament elongation mediated by formin.

Abstract

The formin family of proteins directs the assembly of unbranched actin filaments that are incorporated into a diverse set of higher-order cytoskeletal structures, including cytokinetic rings, filopodia and stress fibers. Formins mediate polymerization by binding the barbed end with their dimeric formin homology 2 (FH2) domains, which processively step onto incoming actin monomers to incorporate them into the filament. By remaining bound to filaments over thousands of cycles of subunit addition, formins specify changes in filament lengths while protecting the barbed ends from capping. Formin isoforms have been shown to possess a range of processive properties, which are readily modulated by changes in ionic strength, the concentrations of actin and the actin-binding protein profilin, and the application of force. However, despite its critical importance to filament assembly, the relationship between a formin's polymerization activity and its processivity remains poorly understood. To address this question, we investigated the mechanism of processive elongation mediated by the budding yeast formin Bni1p. Using in vitro assays to reconstitute polymerization under a range of conditions, we found that processive association of Bni1p with filament ends is regulated by both the rate and the mechanism by which actin monomers are added to the barbed end. As a result, the rate at which Bni1p dissociates from barbed ends and the lengths of the filaments it polymerizes are regulated differently by the polymerization conditions. This suggests that formin-mediated elongation and processivity can be independently tuned to generate networks of actin filaments with defined lengths.