Required for the assembly of F-actin structures, such as actin cables and stress fibers. Nucleates actin filaments. Binds to the barbed end of the actin filament and acts as leaky capper, slowing both polymerization and depolymerization. Protects the growing actin fiber from tight capping proteins and so increases the time of elongation and the total amount of F-actin. May organize microtubules by mediating spindle positioning and movement in the budding process. Required for the maintenance of polarized hyphal growth. BNI1-mediated actin cables are necessary for positioning the Golgi complex to a putative site of germ tube emergence and for coordinating the transport and deposition of membrane and cell wall material to a growing hypha. Plays a key role in virulence. Homodimer, and possibly also homotetramer (By similarity). Interacts with actin via the FH2 domain (By similarity). Interacts with IQG1. Localizes to the presumptive budding site in G1 cells, the bud tip of small to medium-budded cells and the bud neck of large-budded cells. During hyphal growth at least a fraction persistently localizes at the tips of germ tubes and hyphae independently of cell cycle progression. Localizes at the Spitzenkorper, a structure found in fungal hyphae which is the organizing center for hyphal growth and morphogenesis. The DAD domain regulates activation via by an autoinhibitory interaction with the GBD/FH3 domain. This autoinhibition is released upon competitive binding of an activated GTPase. The release of DAD allows the FH2 domain to then nucleate and elongate nonbranched actin filaments (By similarity). Leads to swollen cells, increased random budding pattern, and severe defect in cytokinesis with enlarged bud necks. Belongs to the formin homology family. BNI1 subfamily.