Editors' ChoiceCell Biology

Septins restrict polar growth

Sci. Signal.  10 Feb 2015:
Vol. 8, Issue 363, pp. ec31
DOI: 10.1126/scisignal.aaa8620

In a weak gradient of mating phermone, yeast cells polarize and extend a mating projection towards the pheromone source (chemotropic growth). Pheromone activates the G protein–coupled receptor (GPCR) Ste2, which results in localized exocytosis and actin polymerization at a structure called the polar cap. The polar cap moves to track the pheromone gradient and maintains chemotropic growth. Sst2, a member of the regulator of G protein signaling (RGS) family, binds to Ste2 and acts as the GTPase-activating protein (GAP) for Gpa1, the α subunit of the large G protein downstream of Ste2. By inactivating Gpa1, Sst2 attenuates pheromone signaling to reduce noise and improve signaling fidelity. Kelley et al. found that Sst2 also organizes septins to properly position the polar cap in Saccharomyces cerevisiae. When cells were placed in a pheromone gradient, Sst2 and septins colocalized to the base of the mating projection. The GAP activity of Sst2, but not its ability to interact with Ste2, was required for proper movement of the polar cap as cells tracked a pheromone gradient and for the septin structure to form near, but separate from, the polar cap. Exocytosis delivers proteins to the polar cap, and endocytosis removes proteins from the edge of the cap. The GAP activity of Sst2 was also important for limiting the area in which exocytosis occurred and for excluding septin complexes from areas of active endocytosis. Movement of the septin complex preceded and predicted the movement of the polar cap in cells orienting to a pheromone gradient, and Ste2 and septins accumulated together as cells reoriented in response to a change in the phermone gradient. These results imply a model in which pheromone signaling drives the localized activation of Sst2, which promotes the formation of a septin complex that limits the movements of the polar cap, thus directing chemotropic growth.

J. B. Kelley, G. Dixit, J. B. Sheetz, S. P. Venkatapurapu, T. C. Elston, H. G. Dohlman, RGS proteins and septins cooperate to promote chemotropism by regulating polar cap mobility. Curr. Biol. 25, 275–285 (2015). [PubMed]