Bnr1

Bnr1 is a formin protein in the budding yeast Saccharomyces cerevisiae, encoded by the BNR1 gene. It belongs to the diaphanous-related formin (DRF) family and functions as an actin nucleator and elongation factor, promoting the assembly of linear, unbranched actin filaments that form cytoskeletal cables. Bnr1 operates alongside another yeast formin, Bni1, to organize distinct pools of actin cables that support polarized growth and vesicle trafficking during the cell cycle.

Localization and function of Bnr1 are primarily at the mother cell cortex and the bud neck, where it nucleates and elongates actin cables toward the growing bud. These cables serve as tracks for myosin-driven transport of secretory vesicles and organelles, contributing to bud formation and the maintenance of cell polarity. Bnr1 is thought to anchor actin cables at specific cortical sites and to coordinate with other polarity factors such as Bud6, an actin-regulatory protein that stimulates nucleation in collaboration with formins.

Regulation of Bnr1 involves Rho-family GTPases and interactions with actin-regulatory proteins. The typical formin architecture includes FH1 and FH2 domains essential for actin assembly and profilin-mediated elongation, and Bnr1 is subject to regulatory inputs that modulate its activity. The BNR1 gene is nonessential under standard laboratory conditions, but loss of BNR1 disrupts actin cable organization and polarized growth, illustrating its role in cytoskeletal dynamics.

Evolutionarily, Bnr1 is one of two formins in S. cerevisiae (the other being Bni1) and has orthologs in related yeast species. Studying Bnr1 helps illuminate how cells coordinate cytoskeletal architecture with vesicle trafficking and polarity in eukaryotic cells.