Published online 7 September 2004. doi:10.1083/jcb.200405061
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 166, Number 6, 889-900
Robust cell polarity is a dynamic state established by coupling transport and GTPase signaling
Roland Wedlich-Soldner,
Stephanie C. Wai,
Thomas Schmidt, and
Rong Li
Department of Cell Biology, Harvard Medical School, Boston, MA 02115
Address correspondence to Rong Li, Dept. of Cell Biology, Harvard Medical School, 240 Longwood Ave., Boston, MA 02115. Tel.: (617) 432-0640. Fax: (617) 432-4153. email: Rli{at}hms.harvard.edu
Yeast cells can initiate bud formation at the G1/S transition in a cue-independent manner. Here, we investigate the dynamic nature of the polar cap and the regulation of the GTPase Cdc42 in the establishment of cell polarity. Using analysis of fluorescence recovery after photobleaching, we found that Cdc42 exchanged rapidly between the polar caps and cytosol and that this rapid exchange required its GTPase cycle. A previously proposed positive feedback loop involving actomyosin-based transport of the Cdc42 GTPase is required for the generation of robust cell polarity during bud formation in yeast. Inhibition of actin-based transport resulted in unstable Cdc42 polar caps. Unstable polarity was also observed in mutants lacking Bem1, a protein previously implicated in a feedback loop for Cdc42 activation through a signaling pathway. When Bem1 and actin were both inhibited, polarization completely failed. These results suggest that cell polarity is established through coupling of transport and signaling pathways and maintained actively by balance of flux.
Key Words: feedback; cell polarity; actin; Cdc42; dynamic
R. Wedlich-Soldner and S.C. Wai contributed equally.
Abbreviations used in this paper: GEF, guanine nucleotide exchange factor; LatA, latrunculin A; MG, GFP-myc6.

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