ABSTRACT
The small GTPase Cdc42, a conserved regulator of cell polarity in eukaryotes, is activated by two GEFs, Gef1 and Scd1, in fission yeast. Gef1 and Scd1 localize sequentially to the division site to activate Cdc42 for efficient cytokinesis. The significance of multiple Cdc42 GEFS is not well understood. Here we report a novel interplay between Gef1 and Scd1 that fine-tunes Cdc42 activation during two cellular programs: cytokinesis and polarized growth. We find that Gef1 promotes Scd1 localization to the division site during cytokinesis. During polarized growth, Gef1 is required for bipolar Scd1 localization. Gef1 recruits Scd1 through the recruitment of the scaffold Scd2; we propose this facilitates polarized cell growth at a second site. In turn, Scd1 restricts Gef1 localization to the division site and to the cell cortex, thus maintaining polarity. Our results suggest that crosstalk between GEFs is a conserved mechanism that orchestrates Cdc42 activation during complex processes.