Bni5p, a Septin-Interacting Protein, Is Required for Normal Septin Function and Cytokinesis in Saccharomyces cerevisiae

Abstract

In the budding yeast Saccharomyces cerevisiae, the Cdc3p, Cdc10p, Cdc11p, Cdc12p, and Sep7p/Shs1p septins assemble early in the cell cycle in a ring that marks the future cytokinetic site. The septins appear to be major structural components of a set of filaments at the mother-bud neck and function as a scaffold for recruiting proteins involved in cytokinesis and other processes. We isolated a novel gene, BNI5, as a dosage suppressor of the cdc12-6 growth defect. Overexpression of BNI5 also suppressed the growth defects of cdc10-1, cdc11-6, and sep7Δ strains. Loss of BNI5 resulted in a cytokinesis defect, as evidenced by the formation of connected cells with shared cytoplasms, and deletion of BNI5 in a cdc3-6, cdc10-1, cdc11-6, cdc12-6, or sep7Δ mutant strain resulted in enhanced defects in septin localization and cytokinesis. Bni5p localizes to the mother-bud neck in a septin-dependent manner shortly after bud emergence and disappears from the neck approximately 2 to 3 min before spindle disassembly. Two-hybrid, in vitro binding, and protein-localization studies suggest that Bni5p interacts with the N-terminal domain of Cdc11p, which also appears to be sufficient for the localization of Cdc11p, its interaction with other septins, and other critical aspects of its function. Our data suggest that the Bni5p-septin interaction is important for septin ring stability and function, which is in turn critical for normal cytokinesis.

We are grateful to Christine Field and Jeremy Thorner for valuable discussions; Yeon-Sun Seong, Young-Wook Cho, and Dan Ilkovitch for technical support; and Jason Chong for assistance in the initial identification of BNI5. We also thank Susan Garfield for helping with confocal microscopy and Jim McNally and Tatiana Karpova for processing confocal images obtained at the LRBGE Fluorescence Imaging Facility at NCI.

This work was supported in part by grants NIH GM59964 (R.L.) and NIH GM31006 (J.R.P.).

P.R.L., S.S., and H.-S.R. contributed equally to this work.