page 99) define a new step in this polarization dance—a step that may link the extracellular yeast pheromone signal to the intracellular establishment of polarized actin cables.
The proposed link involves phosphorylation of Bni1p, a cable-generating formin protein, by Fus3p, a MAP kinase. Fus3p got its name because cells lacking this kinase could not fuse during mating. Sure enough, activated Fus3p was long ago found to lead to transcription of fusion genes. Then Fus3p was found to help arrest the cell cycle of mating cells. But replacement of both of these functions did not restore fusion to cells lacking Fus3p.
Now Matheos et al. find that Fus3p is also needed for polarization of the cell and localization of Bni1p to the tips of shmoos, or mating projections. Overexpression of Bni1p overcomes the need for Fus3p, but multiple actin-containing projections replace the normal single shmoo.
All of this activity is set in motion when pheromone binds its receptor, liberating two G protein activities: a βγ complex that turns on both Fus3p and Cdc42-mediated actin polymerization, and an α subunit that helps localize the Fus3p that activates Bni1p. The need for both pathways may be a failsafe mechanism. Or perhaps it is related to the very different types of actin networks that the pathways make. Cdc42 is good at making actin meshworks that push out the cell membrane, but Bni1p is better at making cables that facilitate transport to the site of mating. ▪