Poster abstracts

Poster number 30 submitted by Kristi Miller

Regulation of cell polarity by Rga1, a Cdc42 GTPase-activating protein

Kristi E. Miller (MCDB), Wing-Cheong Lo (Department of Mathematics, City University of Hong Kong, Kowloon, Hong Kong), Mid Eum Lee (MCDB *Present address Department of Biochemistry and Cell Biology, The Geisel School of Medicine at Dartmouth, Hanover, NH 03755, USA), Hay-Oak Park (Department of Molecular Genetics, The Ohio State University, Columbus, OH 43210, USA)

Abstract:
Establishment of cell polarity is critical for cell movement, cell growth, and division. Disruption of cell polarity is linked to diseases such as cancer. The Rho GTPase Cdc42 plays a central role in polarity development in species ranging from yeast to humans. Budding yeast cells choose a growth site (i.e. bud site) in the G1 phase, directing the proper orientation of the polarity axis. Yeast cells never bud within previous cell division sites, yet how yeast cells choose a single new site for polarization is not well understood. Here we show that Rga1, a Cdc42 GTPase-activating protein (GAP), is highly dynamic at both current and old division sites and that these transient localization patterns are likely to be involved in inhibiting Cdc42 repolarization to any previously used site. Rga1 associates with the proteins Nis1 and Nba1, whose localization depends on the persistent markers of old division sites, Rax1 and Rax2. A series of time-lapse analyses indicate that Rga1 localizes to the division site until late G1 in wild-type cells. In contrast, Rga1 delocalizes before mid G1 (i.e. prior to bud-site selection) in daughter cells of rax1 or nba1 mutants, leading to Cdc42 repolarization within the division site. This defect is minor in mother cells of these mutants, likely because the G1 phase is shorter and thus bud-site selection occurs before Rga1 delocalization. We previously proposed a biphasic model for Cdc42 polarization involving two temporal activation steps of Cdc42 in G1. By implementing temporal changes of Rga1 localization, this model can explain the different bud-site selection defects in mother vs. daughter cells of rax1 and nba1 mutants. Indeed, the model predicts different orientation of Cdc42 polarization axis when various lengths of the first temporal step of G1 are implemented. Together, our findings indicate that timely distribution of Rga1 to the current and old cell division sites is critical for establishing the proper orientation of Cdc42 polarization. This study thus highlights that polarity establishment is tightly linked to regulation of a Cdc42 GAP in the cell cycle. Given the universal role of Cdc42 in polarity development in eukaryotes, these mechanisms of Cdc42 polarization in yeast may be applicable to higher eukaryotes.

References:
Tong Z, Gao X-D, Howell AS, Bose I, Lew DJ, Bi E (2007) Adjacent positioning of cellular structures enabled by a Cdc42 GTPase-activating protein mediated zone of inhibition. J Cell Biol 179:1375–1384.
Lee ME, Lo W-C, Miller KE, Chou C-S, Park H-O (2015) Regulation of Cdc42 polarization by the Rsr1 GTPase and Rga1, a Cdc42 GTPase-activating protein, in budding yeast. J Cell Sci 128:2106–2117.
Miller KE, Lo W-C, Lee ME, Park H-O (2017) Dynamic localization of Rga1, a Cdc42 GTPase-activating protein, is involved in orientating the polarity axis. (Submitted).

Keywords: cell polarity, Rho GTPase, GTPase-activating protein