Characterization Of The Regulation Of Formin Protein Bni1P In Saccharomyces Cerevisiae

Abstract

Formins are a family of conserved proteins that assemble unbranched actin filaments. In budding yeast, Saccharomyces cerevisiae, the formin isoforms Bni1p and Bnr1p are vital for nucleating and elongating the essential actin cables to guide polarized growth. Here I demonstrate that there are three localization regions in the Nterminal domain of Bni1p: one in the N-terminal 333 amino acids which requires dimerization, one in the region encompassed by amino acids 334-834 which covers the DID-DD-CC domains, and the third in the Spa2p binding domain. The three localization regions can each localize to the bud cortex and bud neck independently of endogenous Bni1p at the right stage of the cell cycle. Bni1p truncations with internal regions of the N-terminal half deleted confirm that the N-terminal domain is important for localization. Cells with delocalized Bni1p truncations have misoriented cables, defects in nuclear movement and spindle orientation, with the extent of these phenotypes varying in accordance with how much of the N-terminal region is truncated. Defects in nuclear movement and spindle orientation are due to the Bni1p truncations assembling actin cables at the wrong place. Thus, although the N-terminal localization regions of Bni1p are not needed for the viability of cells, they are needed for proper functions of formins.