Asymmetrically localized Bud8p and Bud9p proteins control yeast cell polarity and development

Diploid strains of the budding yeast Saccharomyces cerevisiae change the pa ttern of cell division from bipolar to unipolar when switching growth from the unicellular yeast form (YF) to filamentous, pseudohyphal (PH) cells in response to nitrogen starvation. The functions of two transmembrane protein s, Bud8p and Bud9p, in regulating YF and PH cell polarity were investigated . Bud8p is highly concentrated at the distal pole of both YF and PH cells, where it directs initiation of cell division. Asymmetric localization of Bu d8p is independent of the Rsr1p/Bud1p GTPase, rsr1/bud1 mutations are epist atic to buds mutations, placing Rsr1p/Bud1p downstream of Bud8p. In YF cell s, Bud9p is also localized at the distal pole, yet deletion of BUD9 favours distal bud initiation. In PH cells, nutritional starvation for nitrogen ef ficiently prevents distal localization of Bud9p. Because Bud8p and Bud9p pr oteins associate in vivo, we propose Bud8p as a landmark for bud initiation at the distal cell pole, where Bud9p acts as inhibitor. In response to nit rogen starvation, asymmetric localization of Bud9p is averted, favouring Bu d8p-mediated cell division at the distal pole.