High impact information on BNI1

• We used total internal reflection fluorescence microscopy to observe directly individual actin filament polymerization in the presence of two mammalian formins (mDia1 and mDia2) and two yeast formins (Bni1p and Cdc12p) [1].
• The Saccharomyces cerevisiae BNI1 gene product (Bni1p) is a member of the formin family of proteins, which participate in cell polarization, cytokinesis, and vertebrate limb formation [2].
• During mating pheromone response, bni1 mutants showed defects both in polarized morphogenesis and in reorganization of the underlying actin cytoskeleton [2].
• Pheromone-induced polarization is dependent on the Fus3p MAPK acting through the formin Bni1p [3].
• These data suggest a model wherein activated Fus3p is recruited back to the cortex, where it activates Bni1p to promote polarization and cell fusion [3].

Biological context of BNI1

• This phenotype is similar to that previously observed in cells lacking the kinesin Kip3p and, in fact, BNI1 and KIP3 were found to be in the same genetic pathway [4].
• Bni1p shares the FH1 and FH2 domains with proteins involved in cytokinesis or establishment of cell polarity [5].
• Bni1p fused with green fluorescent protein localized to the sites of cell growth throughout the cell cycle [6].
• A bni1-disrupted cell showed a defect in directed growth to the pre-bud site and to the bud tip (apical growth), causing its abnormally spherical cell shape and thick bud neck [6].
• Together, these results suggest that one function of Ste20p may be to activate the polarisome complex by phosphorylation of Bni1p [7].

Anatomical context of BNI1

• A Bni1p-binding protein was affinity purified from the yeast cytosol fraction and was identified to be Tef1p/Tef2p, translation elongation factor 1alpha (EF1alpha) [8].
• We demonstrate that either domain III of EF1alpha or the EF1alpha-binding site of Bni1p inhibits EF1alpha binding to beta-actin mRNA in vitro and causes delocalization of beta-actin mRNA in chicken embryo fibroblasts [9].
• A formin Bni1p nucleates actin to assemble actin cables, which guide the polarized transport of secretory vesicles in budding yeast [10].

Associations of BNI1 with chemical compounds

• EF1alpha bound to the 186 amino acids region of Bni1p, located between the FH1 domain, the proline-rich profilin-binding domain, and the FH2 domain, of which function is not known [8].
• Introduction of constitutively active ras1G13V in the bni1 strain or addition of cyclic AMP to the growth medium did not bypass bni1 hyphal growth defects [11].

Physical interactions of BNI1

• Moreover, yeast two-hybrid and biochemical studies demonstrate that Bni1p and Bnr1p interact directly with profilin at the FH1 domains [5].
• The BNI1 gene deleted in the EF1alpha-binding region did not suppress the bni1 bnr1 mutation in which the actin organization was impaired [8].
• We mapped Bud6 binding activity to specific residues in the carboxyl terminus of Bni1 that are adjacent to its diaphanous autoregulatory domain (DAD) [12].

Regulatory relationships of BNI1

• Bni1p stimulates actin polymerization for the formation of actin cables [13].
• Ras1-induced hyphal development in Candida albicans requires the formin Bni1 [11].
• We show that the essential function of Rho3p and Rho4p is to activate the formins Bni1p and Bnr1p, and that activated alleles of either formin are able to bypass the requirement for these Rho proteins [14].

Other interactions of BNI1

• Cells of the bni1 bnr1 mutant arrested at the restrictive temperature are deficient in bud emergence, exhibit a random distribution of cortical actin patches and often become multinucleate [5].
• Disruption of BNI1 or BNR1 does not show any deleterious effect on cell growth, but the bni1 bnr1 mutant shows a severe temperature-sensitive growth phenotype [5].
• We have recently shown that Bni1p is a potential target of Rho1p [5].
• Genetic analysis placed BNI1 in the KAR9 pathway for nuclear migration [15].
• BNI1 was originally identified on the basis of genetic interaction with CDC12, which encodes one of the neck-filament-associated septin proteins, suggesting that these proteins may be involved in positioning the bipolar signals [16].

Analytical, diagnostic and therapeutic context of BNI1

• Immunofluorescence microscopic analysis showed that Bni1p was localized at the bud tip in wild-type cells [17].
• Complementation tests defined three genes, PPF1, PPF2, and PPF3 (for pointed projection formation), necessary for pointed projection formation [18].

References