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MBC in Press, published online ahead of print September 27, 2006
Mol. Biol. Cell 10.1091/mbc.E05-08-0798

A more recent version of this article appeared on December 1, 2006
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Submitted on August 24, 2005
Revised on August 25, 2006
Accepted on September 18, 2006

The Yeast Tumor Suppressor Homologue Sro7p Is Required for Targeting of the Sodium Pumping ATPase to the Cell Surface

Ingrid Wadskog,*{dagger} Annabelle Forsmark,* Guendalina Rossi,{ddagger} Catherine Konopka,{sect} Mattias Öyen,|| Mattias Goksör,¶ Hans Ronne,||# Patrick Brennwald,{ddagger} and Lennart Adler*

*Department of Cell and Molecular Biology, Microbiology, Göteborg University, SE-405 30 Göteborg, Sweden; {ddagger}Department of Cell and Developmental Biology, University of North Carolina School of Medicine, Chapel Hill, NC 27599; {sect}Department of Biochemistry, University of Wisconsin-Madison, Madison, WI 53706; ||Department of Plant Biology and Forest Genetics, Swedish University of Agricultural Sciences, SE-750 07 Uppsala, Sweden; Department of Physics, Göteborg University, SE-412 96 Göteborg, Sweden; #Department of Medical Biochemistry and Microbiology, Uppsala University, SE-751 23 Uppsala, Sweden

Monitoring Editor: Akihiko Nakano

The SRO7/SOP1 encoded tumor suppressor homologue of S. cerevisiae is required for maintenance of ion homeostasis in cells exposed to NaCl stress. Here we show that the NaCl sensitivity of the sro7{Delta} mutant is due to defective sorting of Ena1p, the main sodium pump in yeast. On exposure of sro7{Delta} mutants to NaCl stress, Ena1p fails to be targeted to the cell surface, but is instead routed to the vacuole for degradation via the multi-vesicular endosome pathway. SRO7 deficient mutants accumulate post-Golgi vesicles at high salinity, in agreement with a previously described role for Sro7p in late exocytosis. However, Ena1p is not sorted into these post-Golgi vesicles, in contrast to what is observed for the vesicles that accumulate when exocytosis is blocked in sec6-4 mutants at high salinity. These observations imply that Sro7p has a previously unrecognized role for sorting of specific proteins into the exocytic pathway. Screening for multi-copy suppressors identified RSN1, encoding a transmembrane protein of unknown function. Overexpression of RSN1 restores NaCl tolerance of sro7{Delta} mutants by retargeting Ena1p to the plasma membrane. We propose a model in which blocked exocytic sorting in sro7{Delta} mutants, gives rise to quality control-mediated routing of Ena1p to the vacuole.

{dagger}Present address: Department of Chemical Engineering, Jönköping University, Box 1026, SE-551 11 Jönköping, Sweden.

Address correspondence to: Ingrid Wadskog (ingrid.wadskog{at}ing.hj.se) or Patrick Brennwald (pjbrennw{at}med.unc.edu)




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