Scaffold-mediated symmetry breaking by Cdc42p

Abstract

Cell polarization generally occurs along a single well-defined axis that is frequently determined by environmental cues such as chemoattractant gradients or cell-cell contacts, but polarization can also occur spontaneously in the apparent absence of such cues, through a process called symmetry breaking. In Saccharomyces cerevisiae, cells are born with positional landmarks that mark the poles of the cell and guide subsequent polarization and bud emergence to those sites, but cells lacking such landmarks polarize towards a random cortical site and proliferate normally. The landmarks employ a Ras-family GTPase, Rsr1p, to communicate with the conserved Rho-family GTPase Cdc42p, which is itself polarized and essential for cytoskeletal polarization. We found that yeast Cdc42p was effectively polarized to a single random cortical site even in the combined absence of landmarks, microtubules and microfilaments. Among a panel of Cdc42p effectors and interacting proteins, we found that the scaffold protein Bem1p was uniquely required for this symmetry-breaking behaviour. Moreover, polarization was dependent on GTP hydrolysis by Cdc42p, suggesting that assembly of a polarization site involves cycling of Cdc42p between GTP- and GDP-bound forms, rather than functioning as a simple on/off switch.

Rho GTPase activity zones and transient contractile arrays

“…Visualization of Cdc42 localization during bud formation in yeast indicates that the bud site represents a zone of tightly localized, semi-stable Cdc42 activity. (27)(28)(29) Other examples may be found during exocytosis, endocytosis, phagocytosis, and cell locomotion, all of which entail semi-stable, spatially constrained regions of Rho GTPase activity. (30)(31)(32) These observations indicate that the Rho zones represent a widespread signaling mechanism harnessed in diverse cell types to assemble actomyosin-based arrays, differing in the overall geometric organization of the zones (Fig.…”

“…Moreover, precedent for such GTPase flux comes from yeast budding, where normal maintenance of Cdc42 at the incipient bud site is critically dependent on GTP hydrolysis. (29) GDIs and GAPs provide the obvious means to direct GTPase flux in concert with elevated GEF activity. At least two different (but emphatically non-exclusive) scenarios can be envisioned: Rho GTPases are inactivated by local GAPactivity and then bound by GDIs, which drive them into a soluble pool, or active GTPases are directly bound by GDIs.…”

“…Self-amplification has been described for Cdc42 activation during yeast bud site selection and may involve both direct modulation of Cdc42 by GEFs and GAPs or indirect modulation via actin-based transport. (28,29,42,61,62) Another plausible positive feedback mechanism can be proposed for the cytokinetic Rho zones. Accumulation of active Rho might activate PIP kinase, (63) thereby promoting the observed accumulation of active PIP2 in the furrow.…”

Yeast and fungal morphogenesis from an evolutionary perspective

“…Recent work demonstrates that the resulting polarized state is dynamic and its maintenance requires active recycling through processes such as endocytosis [70]. The second mechanism for polarization does not require actin but appears to simply involve the Cdc42 GTPase module and a protein called Bem1 [67,69]. Bem1 homologs have been found in many fungal species but not yet in animal organisms.…”

“…Work in S. cerevisiae during the past several years has indeed revealed at least two intrinsic mechanisms for cell polarization without recourse to external signals [67][68][69] (Figure 1). One of these mechanisms is constituted by cyclic interactions between three core elements: the Cdc42 GTPase module, the actin-based transport system and exocytosis.…”