Negative feedback equalizes polarity sites in a multi-budding yeast

Morphogenesis in fungi and animals is directed by the polarization of the small GTPases Cdc42 and Rac. In the budding yeast Saccharomyces cerevisiae, competition between polarity patches results in one polarized patch and the growth of a single bud. Here, we describe cell polarity in the yeast Aureobasidium pullulans, which establishes multiple coexisting polarity sites yielding multiple buds during a single cell-division cycle. Polarity machinery components oscillate in their abundance in these coexisting sites but do so independently of one another, pointing to a lack of global coupling between sites. Previous theoretical work has demonstrated that negative feedback in a polarity circuit could promote the coexistence of multiple polarity sites, and time-delayed negative feedback is known to cause oscillations. We show that both these features of negative feedback depend on a protein we identified as Pak1 and that Pak1 requires Rac1 but not Cdc42 for its localization. This work shows how conserved signaling networks can be modulated for distinct morphogenic programs even within the constraints of fungal budding.