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© The Rockefeller University Press, 0021-9525/1999//1177 $5.00
The Journal of Cell Biology, Volume 145, Number 6, , 1999 1177-1188

Article

O-Glycosylation of Axl2/Bud10p by Pmt4p Is Required for Its Stability, Localization, and Function in Daughter Cells



Sylvia L. Sanders*, Martina Gentzsch{ddagger}, Widmar Tanner{ddagger}, and Ira Herskowitz*

* Department of Biochemistry and Biophysics, University of California, San Francisco, California 94143-0448; and {ddagger} Lehrstuhl für Zellbiologie und Pflanzenphysiologie, Universität Regensburg, D-93053 Regensburg, Germany

Cells of the yeast Saccharomyces cerevisiae choose bud sites in a manner that is dependent upon cell type: a and {alpha} cells select axial sites; a/{alpha} cells utilize bipolar sites. Mutants specifically defective in axial budding were isolated from an {alpha} strain using pseudohyphal growth as an assay. We found that a and {alpha} mutants defective in the previously identified PMT4 gene exhibit unipolar, rather than axial budding: mother cells choose axial bud sites, but daughter cells do not. PMT4 encodes a protein mannosyl transferase (pmt) required for O-linked glycosylation of some secretory and cell surface proteins (Immervoll, T., M. Gentzsch, and W. Tanner. 1995. Yeast. 11:1345–1351). We demonstrate that Axl2/Bud10p, which is required for the axial budding pattern, is an O-linked glycoprotein and is incompletely glycosylated, unstable, and mislocalized in cells lacking PMT4. Overexpression of AXL2 can partially restore proper bud-site selection to pmt4 mutants. These data indicate that Axl2/Bud10p is glycosylated by Pmt4p and that O-linked glycosylation increases Axl2/ Bud10p activity in daughter cells, apparently by enhancing its stability and promoting its localization to the plasma membrane.

Key Words: O-glycosylation • PMT4 • AXL2/BUD10 • budding • polarity

Abbreviations used in this paper: bud-site selection axial determinant; GFP, green fluorescent protein; HA, hemagglutinin epitope; HF, hydrogen fluoride; ORF, open reading frame; pmt, protein mannosyl transferase; PNGase, peptide N-glycosidase F; SLAHD, synthetic low ammonium histidine dextrose; WT, wild-type.

S.L. Sanders' present address is Department of Biology, Massachusetts Institute of Technology and Howard Hughes Medical Institute, 31 Ames Street, Cambridge, MA 02139.

M. Gentzsch's present address is Mayo Clinic Scottsdale, S.C. Johnson Medical Research Center, 13400 East Shea Boulevard, Scottsdale, AZ 85259.


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