Bni1

Bni1, or Bni1p, is a formin-family protein in the budding yeast Saccharomyces cerevisiae. The BNI1 gene encodes this actin-nucleating factor, which plays a key role in the assembly of actin cables required for polarized cell growth during bud formation. Bni1p promotes directional vesicle trafficking and secretion to the growing bud and helps establish and maintain cell polarity.

Protein features: Bni1 belongs to the diaphanous-related formin (DRF) family. It contains an N-terminal regulatory region that binds Rho-family GTPases, a proline-rich FH1 domain, and a conserved FH2 domain at the C-terminus that nucleates and elongates actin filaments. Like other DRFs, Bni1p is regulated by an intramolecular DID–DAD interaction that keeps the protein autoinhibited until Rho GTPase binding relieves this inhibition.

Localization and function: In living cells, Bni1p localizes to the growing bud cortex and is the primary nucleator of actin cables that emanate from the bud toward the mother cell, complementing Bnr1p, which nucleates cables at the bud neck. Through interaction with actin regulators such as Bud6 and profilin, Bni1p coordinates actin filament formation with vesicle delivery.

Regulation and outcomes: Activation of Bni1p is coupled to cell cycle cues and Rho GTPase signaling (including Cdc42). Proper operation of Bni1p is essential for efficient polarized growth, timely bud emergence, and normal cytokinesis. Deletion or impairment of BNI1 results in defects in actin cable organization, reduced polarity, and slowed growth.