Fig. 1.

Internal cues and the mating-pheromone-response pathway converge on the activation of Cdc42. (A) The proximal and distal poles of a daughter cell are specified relative to the position of the mother cell. For budding, axial-budding cells utilize the protein Axl2, the axial landmark, which is positioned in every cell cycle adjacent to the last division site (the proximal pole in daughter cells) by the action of Bud3, Bud4, and Axl1. Bipolar budding cells use distal-pole Bud8 and proximal-pole Bud9 to choose the budding site. The Rax1-Rax2 complex participates in the localization of both Bud8 and Bud9. (B) In late G1, Axl2, Bud8, or Bud9 (depending on cell type and condition) can recruit Bud5, which activates Rsr1, which then brings Cdc24, the Cdc42 GEF, to the membrane to generate a concentrated pool of active Cdc42-GTP. During mating in MATa cells, α-factor binds to its receptor Ste2, and the activated receptor dissociates the G-protein trimer into Gα (not illustrated) and Gβ/Ste4. Ste4 recruits the scaffold protein Ste5 (not illustrated) and the adaptor protein Far1 in complex with Cdc24 to the side of the membrane exposed to the higher concentration of α-factor. As in budding, Cdc24 locally activates Cdc42. (C) Definition of the three angles measured in our studies: θ, the angle of alignment; ϕ, the neck angle; and ω, the angle to the proximal pole.