Budding yeast do the Cdc42 two-step

Two different proteins sequentially activate Cdc42 to ensure yeast form a bud at the right place, Kang et al. reveal.

Haploid S. cerevisiae cells form new buds immediately adjacent to the division site from the previous cell cycle. The new bud’s position is determined by a protein complex called the axial landmark, which assembles at the mother–bud neck of yeast cells during mitosis. An axial landmark protein called Bud3 is required for bud site selection, but whether this function involves the protein’s Dbl homology (DH) domain, a module commonly found in nucleotide exchange factors that activate Rho family GTPases, was unknown.

Kang et al. found that yeast expressing a Bud3 variant lacking part of its DH domain failed to assemble the axial landmark and formed buds in the wrong position. A mutant allele of the GTPase Cdc42 produced an identical phenotype, which could be rescued by Bud3 overexpression. Bud3 might therefore control bud site selection by activating Cdc42. However, the only known Cdc42 exchange factor is Cdc24, which stimulates the GTPase downstream of the axial landmark to promote cell polarization and bud formation.

Bud3 stimulated nucleotide release from Cdc42 in vitro and activated Cdc42 in Cdc24-deficient cells. Live imaging of wild-type cells revealed that Cdc42 normally undergoes two waves of activation: a Bud3-dependent wave in late mitosis/early G1 followed by a Cdc24-dependent activation phase in late G1. Senior author Hay-Oak Park thinks that this biphasic activation is necessary for spatial cue–directed polarity establishment in haploid budding yeast.