Interaction between bud-site selection and polarity-establishment machineries in budding yeast

Abstract

Saccharomyces cerevisiae yeast cells polarize in order to form a single bud in each cell cycle. Distinct patterns of bud-site selection are observed in haploid and diploid cells. Genetic approaches have identified the molecular machinery responsible for positioning the bud site: during bud formation, specific locations are marked with immobile landmark proteins. In the next cell cycle, landmarks act through the Ras-family GTPase Rsr1 to promote local activation of the conserved Rho-family GTPase, Cdc42. Additional Cdc42 accumulates by positive feedback, creating a concentrated patch of GTP-Cdc42, which polarizes the cytoskeleton to promote bud emergence. Using time-lapse imaging and mathematical modelling, we examined the process of bud-site establishment. Imaging reveals unexpected effects of the bud-site-selection system on the dynamics of polarity establishment, raising new questions about how that system may operate. We found that polarity factors sometimes accumulate at more than one site among the landmark-specified locations, and we suggest that competition between clusters of polarity factors determines the final location of the Cdc42 cluster. Modelling indicated that temporally constant landmark-localized Rsr1 would weaken or block competition, yielding more than one polarity site. Instead, we suggest that polarity factors recruit Rsr1, effectively sequestering it from other locations and thereby terminating landmark activity.

Sequential Logic of Polarity Determination during the Haploid-to-Diploid Transition in Saccharomyces cerevisiae

“…In this regard, these zygotes lack accurate spatial memory. They may constitute a normally occurring variant of the stochastic "wandering" of polarity guidance proteins in cells exposed to low doses of pheromone and in cells with lesions in signal transmission to actin (19,52,63). We suggest that a prerequisite for this increased consistency is that progeny cells emerge from a bud neck that provides a fixed spatial point of reference.…”

“…The activated form of Bud1 then interacts with the GEF (Cdc24) of a second GTPase, Cdc42, that guides actin in conjunction with the formin Bni1. Deletion of Bud1 randomizes successive budding in cycling cells (6,7,(18)(19)(20)(21). Related events linking surface determinants to actin are characteristic of many eukaryotes (22).…”

Regulation of Cdc42 polarization by the Rsr1 GTPase and Rga1, a Cdc42 GTPase-activating protein, in budding yeast

“…The second feedback could be the same as that of the Bem1-mediated signaling network, which includes Cdc24, as proposed for symmetry breaking (Irazoqui et al, 2003;Wedlich-Soldner et al, 2004). A recent model has also suggested that, in addition to the Bem1-mediated feedback, transient or weak activity of a Cdc42 GEF is required for Cdc42 polarization in wild-type haploid cells (Wu et al, 2013).…”

Cell Polarity in Yeast

“…How does that position get picked from all the possible locations? As during symmetry-breaking polarization, cells often generate more than one initial cluster of Cdc42 at the poles and then eliminate supernumerary clusters (Wu et al 2013). Thus, the GEF recruited by Rsr1 may provide a small stimulus that biases where positive feedback takes off, with the final polarity site growing and competing as it does during symmetry breaking.…”