Bem1 complexes and the complexity of yeast cell polarization

Summary A paper in a recent issue of Current Biology from Kozubowski et al. [1] concluded that spontaneous symmetry breaking in yeast can be explained by the formation of a Bem1-mediated complex between Cdc24 (the guanine nucleotide exchange factor (GEF) for Cdc42) and the p21-activated kinase (PAK) Cla4. A previous finding, on which this paper was based, is that a rsr1Δ bem1Δ double mutant is inviable. Because Rsr1 is required for polarization in response to the position of the previous bud scar [2], a process known as bud site selection, it was assumed that this synthetic lethality reflects an essential role for Bem1 in spontaneous cell polarization in the absence of the bud scar cue (referred to as symmetry-breaking polarization by Kozubowski et al. [1]). On the basis of this assumption, a central aim of the present study was to test whether Bem1's role was simply to tether Cdc24 to Cla4, which then phosphorylates Cdc24 [3,4]. To this end, the authors generated a construct that artificially linked Cdc24 and Cla4 and found that this construct could rescue the lethality of the rsr1Δ bem1Δ double mutant. In the light of this result and the assumption that phosphorylation by Cla4 positively regulates Cdc24, they proposed a model in which formation of a complex between Cdc24 and Cla4 creates a feedback loop for Cdc42 activation and is sufficient to drive symmetry breaking in yeast.