Cdc42 couples septin recruitment to the axial landmark assembly via Axl2 in budding yeast

Cell polarization generally occurs along a single axis that is directed by a spatial cue. Cells of the budding yeast Saccharomyces cerevisiae undergo polarized growth and oriented cell division in a spatial pattern by selecting a specific bud site. Haploid a or α cells bud in the axial pattern in response to a transient landmark that includes Bud3, Bud4, Axl1, and Axl2. Septins, a family of filament-forming GTP-binding proteins, are also involved in axial budding and recruited to an incipient bud site, but the mechanism of recruitment remains unclear. Here, we show that Axl2 interacts with Bud3 and the Cdc42 GTPase in its GTP-bound state. Axl2 also interacts with Cdc10, a septin subunit, promoting efficient recruitment of septins near the cell division site. Furthermore, a cdc42 mutant defective in the axial budding pattern at a semi-permissive temperature had a reduced interaction with Axl2 and compromised septin recruitment in the G1 phase. We thus propose that active Cdc42 brings Axl2 to the Bud3-Bud4 complex and that Axl2 then interacts with Cdc10, linking septin recruitment to the axial landmark.