Orienting the precise axis of cell polarity and the plane of asymmetric cell division are essential for various cellular processes, including embryonic development and stem cell generation (Neumuller and Knoblich, 2009). Cells of the budding yeast Saccharomyces cerevisiae establish the axis of cell polarity directed by cell-type specific spatial cues. The small GTPase Cdc42, a highly conserved member of the Rho family, regulates the organization of the actin cytoskeleton and secretory apparatus in order to direct cell polarization toward the presumptive bud site. The coordination of GTPase cascades, including the Rsr1 GTPase and Cdc42 GTPase module, largely governs the patterns of yeast budding in all cell types.
Although several studies have provided significant insights into the mechanisms of polarity-organizing systems lacking spatial cues, the link between spatial cues and polarity-establishment components remains to be fully understood. Interestingly, Cdc42-GAP Rga1 has an exclusive role in positioning a correct polarity axis that is not shared by any other Cdc42-GAPs (Chen et al., 1996; Stevenson et al., 1995; Smith et al., 2002; Tong et al., 2007). In this study, the regulation of Cdc42 polarization by spatial cues and a Cdc42-GAP Rga1 was thoroughly explored.
Remarkably, two distinct phases of Cdc42 polarization were found in the G1 phase, dependent on Bud3 and Cdc24, respectively (Kang et al., 2014). Interestingly, haploid daughter cells exhibit temporal changes of the position of the Cdc42-GTP peak surrounding the axial landmark during early G1, whereas the dynamic Cdc42-GTP cluster is not observed in cells with depleted Rga1 activity. A stable polarization site is determined in mid G1, which is concurrent with the beginning of the T2 phase of G1 (Di Talia et al., 2007) in both wild-type and rsr1Δ cells. The robust Cdc42 polarization in early G1 is dependent on Bud3 GEF activity and the Rsr1 GTPase. Remarkably, the dynamic changes of Rga1 localization to the bud-neck in addition to Rga1 localization at the adjacent site to the bud-neck during M to the next G1 phase are crucial for selection of a proper bud site in haploid cells.
Moreover, Rga1 is necessary for directing Cdc42-GTP polarization to the pole opposite to the division site in diploid a/α daughter cells. Rga1 activity that inhibits Cdc42 activation at the cytokinesis site is critical for choosing a single bud site despite the presence of both pole markers in diploid daughter cells. Surprisingly, a high level of Cdc42-GTP persistently polarized to the distal tip of daughter cells in more than 50% of rga1Δ diploid daughter cells during M and to the next G1 phase, implicating the important role of Rga1 in Cdc42-GTP polarization during an apical to isotropic switch.
These results provide the mechanisms of cue-dependent Cdc42 polarization and the unique role of Cdc42-GAP Rga1 in determining the proper axis of cell polarization in yeast. As the intracellular players and pathways involved in orienting the cell polarity are remarkably conserved, this study will help to identify the detailed mechanisms of polarity establishment in higher eukaryotes.