SUMMARY PARAGRAPH for BNI1
The S. cerevisiae genome encodes two genes that are members of the formin family, BNI1 and BNR1, which assemble linear actin cables in the bud and bud neck, respectively (9). Formins are a conserved family of proteins that promote the assembly of actin filaments, a function that is necessary for remodeling of the actin cytoskeleton during such processes as budding, mating, cytokinesis, endocytosis, and in higher cells, cell adhesion and migration. The hallmark of formin proteins is the presence of two highly conserved FH (formin homology) domains: the FH1 domain, containing polyproline motifs that mediate binding to profilin, which in turn binds actin monomers; and the FH2 domain, which nucleates actin assembly (reviewed in 10 and 11). Bni1p and Bnr1p are classified in the diaphanous-related formin (DRF) group of metazoan formins, named for the founding Drosophila gene diaphanous (dia) (12). The FH2 domains of Bni1p and Bnr1p are distinct from those of the metazoan groups, containing a yeast-specific insert that is not found in other organisms (13). In addition to FH1 and FH2 domains, DRFs also contain a regulatory Rho-binding domain (RBD) and a Dia-autoregulatory-domain (DAD) (reviewed in 11). Null mutations in either BNI1 or BNR1 do not impair cell viability, but the double bni1 bnr1 mutant is inviable, indicating that formins play an essential role in S. cerevisiae (14).
A model for formin-mediated actin assembly proposes the following sequence of events (reviewed in 15). Activated Rho protein binds to the formin RBD and releases the formin from a conformation in which it is autoinhibited, due to interaction between its amino and carboxy termini, to a conformation that exposes the FH1 and FH2 domains. The FH1 domain interacts with profilin-bound actin monomers, "delivering" them to the FH2 domain, which is dimeric in structure and thus may interact with two actin monomers to stabilize a dimeric actin form, prior to polymerization to form actin cables. The FH2 domain remains associated with the growing end of the filament to protect it from interaction with capping proteins (an FH2 function termed "processive capping").
Consistent with the model, Bni1p is known to be a downstream target of the small GTPase Rho1p, which regulates reorganization of the actin cytoskeleton, and hence the process of bud formation (16, 17). In addition, Bni1p activation is regulated by Rho3p and Rho4p, which affect the inhibitory interaction between the RBD and the DAD domains in the formin, and the Rho protein Cdc42p is needed for proper cable assembly during initiation of bud growth (7). Bni1p autoinhibition is also disrupted by the action of Prk1p kinase, which phosphorylates Bni1p (18). Support for the model also comes from crystal structure studies of the Bni1p FH2 domain complexed with actin (19).
Bni1p functions as a component of the polarisome, a complex that also includes Spa2p, Bud6p, Pea2p, Msb3p, and Msb4p (20, 21, reviewed in 22). The polarisome is important for determining cell polarity; it acts as the focal point for polymerization of actin monomers into actin cables, and is required for the proper initiation of bud growth and the proper shape of vegetative buds and mating shmoos (reviewed in 22, 23).
Localization of Bni1p is dynamic; in small-budded cells Bni1p moves along the bud cortex, becoming more abundant at the bud tip and bud neck as growth proceeds (24, 25). In addition, Bni1p is found as abundant cytoplasmic speckles throughout the cell cycle; these speckles are associated with actin cables and are not visible upon loss of polymerized actin (25).
Although bni1 null mutants are viable, they exhibit a shorter replicative lifespan than wild type cells (26). They are partially deficient in cytokinesis, and homozygous diploids are predominantly round in shape, exhibit a random budding pattern, and are defective for filamentous growth in nitrogen starvation conditions (27, reviewed in 8). Null mutants also display reduced mating efficiencies in response to pheromone treatment, and detailed genetic studies indicate Bni1p plays a role in polarized recruitment of Ste5p and the consequent activation of Fus3p during mating response (28).
Last updated: 2010-04-23