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Vol. 18, Issue 9, 3323-3339, September 2007

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Distinct Domains of Yeast Cortical Tag Proteins Bud8p and Bud9p Confer Polar Localization and Functionality

Anne-Brit Krappmann^*, Naimeh Taheri^,, Melanie Heinrich^*, and Hans-Ulrich Mösch^*

*Department of Genetics, Philipps-University, D-35032 Marburg, Germany; and Institute for Microbiology and Genetics, Georg-August University, D-37077 Göttingen, Germany

Submitted October 6, 2006; Revised June 5, 2007; Accepted June 12, 2007
Monitoring Editor: Daniel Lew

In Saccharomyces cerevisiae, diploid yeast cells follow a bipolar budding program, which depends on the two transmembrane glycoproteins Bud8p and Bud9p that potentially act as cortical tags to mark the cell poles. Here, we have performed systematic structure-function analyses of Bud8p and Bud9p to identify functional domains. We find that polar transport of Bud8p and Bud9p does not depend on N-terminal sequences but instead on sequences in the median part of the proteins and on the C-terminal parts that contain the transmembrane domains. We show that the guanosine diphosphate (GDP)/guanosine triphosphate (GTP) exchange factor Bud5p, which is essential for bud site selection and physically interacts with Bud8p, also interacts with Bud9p. Regions of Bud8p and Bud9p predicted to reside in the extracellular space are likely to confer interaction with the N-terminal region of Bud5p, implicating indirect interactions between the cortical tags and the GDP/GTP exchange factor. Finally, we have identified regions of Bud8p and Bud9p that are required for interaction with the cortical tag protein Rax1p. In summary, our study suggests that Bud8p and Bud9p carry distinct domains for delivery of the proteins to the cell poles, for interaction with the general budding machinery and for association with other cortical tag proteins.

The online version of this article contains supplemental material at MBC Online (http://www.molbiolcell.org).

Present address: Department of Molecular Genetics, The Ohio State University, Columbus, OH 43210-1292.

Address correspondence to: Hans-Ulrich Mösch (moesch{at}staff.uni-marburg.de).