The Rho GTPase Cdc42 is important for the establishment of eukaryotic cell polarity and is regulated by guanine-nucleotide exchange factors (GEFs). In the fission yeast Schizosaccharomyces pombe, there are two Cdc42 GEFs, Scd1 and Gef1, which both localise to the cell tips during interphase, but it is unclear how their function is coordinated during polarised cell growth. In this work (Tay et al., 2018), Andrew Goryachev, Kenneth Sawin and co-workers now address this question by using scd1Δ cells that have previously shown to be polarised during interphase, despite having a wide and round shape. They show here that these cells are able to grow in a polarised manner, and, interestingly, this depended on interphase microtubules (MTs), which are dispensable for polarised growth in wild-type cells. Furthermore, in cells with impaired scd1, the landmark polarity proteins Tea1 and Tea4 are also involved in polarised growth, as their mutation gives rise to an isotropic-like growth, similar to that obtained upon MT disruption. Interestingly, the authors also find that Gef1is normally cytosolic and acts as a ‘global’ GEF, whereas its recruitment to cell tips, demonstrated here by artificial tethering, results in the local activation of Cdc42 to promote polarised growth. On the basis of these data, the authors propose a model for the regulation of polarised growth of yeast that involves the coordination of both globally and locally acting GEFs, which is mediated by MTs and the landmark polarity proteins.
