How cells determine the number of polarity sites

The diversity of cell morphologies arises, in part, through regulation of cell polarity by Rho-family GTPases. A poorly understood but fundamental question concerns the regulatory mechanisms by which different cells can generate different numbers of polarity sites. Theoretical models of polarity circuits develop multiple initial polarity sites, but then those sites engage in competition, leaving a single winner. The timescale of competition slows dramatically as GTPase concentrations at polarity sites approach a “saturation point”, allowing multiple sites to coexist. Here, we show that these principles hold in more complex mechanistic models of the Saccharomyces cerevisiae polarity machinery, and confirm model predictions in vivo . Further, we elucidate a novel design principle whereby cells can switch from competition to equalization among polarity sites. These findings provide insight into how cells determine the number of polarity sites.