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

Abstract

The SRO7/SOP1 encoded tumor suppressor homologue of Saccharomyces cerevisiae is required for maintenance of ion homeostasis in cells exposed to NaCl stress. Here we show that the NaCl sensitivity of the sro7⌬ mutant is due to defective sorting of Ena1p, the main sodium pump in yeast. On exposure of sro7⌬ mutants to NaCl stress, Ena1p fails to be targeted to the cell surface, but is instead routed to the vacuole for degradation via the multivesicular 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 multicopy suppressors identified RSN1, encoding a transmembrane protein of unknown function. Overexpression of RSN1 restores NaCl tolerance of sro7⌬ mutants by retargeting Ena1p to the plasma membrane. We propose a model in which blocked exocytic sorting in sro7⌬ mutants, gives rise to quality control-mediated routing of Ena1p to the vacuole.

Transport to the plasma membrane is regulated differently early and late in the cell cycle in Saccharomyces cerevisiae

“…The effect of loss of the Rab proteins on plasma membrane traffic of the markers was assessed after growth of the cells to log phase at 30°C. the plasma membrane localization of the sodium transporter Ena1p under high-salt stress (Larsson et al, 1998;Wadskog et al, 2006). Our findings, however, show a differential requirement under normal growth conditions.…”

The pathway by which the yeast protein kinase Snf1p controls acquisition of sodium tolerance is different from that mediating glucose regulation

“…The centromeric plasmid pUG35 (Niedenthal et al, 1996) and pUG35-ENA1 were kindly provided by Lennart Adler (Wadskog et al, 2006) (a gift from Martha Cyert and Victoria Heath).…”

Function and Regulation of the Saccharomyces cerevisiae ENA Sodium ATPase System

“…Localization at the cell membrane has been demonstrated for S. cerevisiae Pmr2a/Ena1 and Pmr2b/Ena2 (94) and for S. occidentalis (7), although it has been reported that overexpression of ENA1 also results in the presence of the protein in intracellular membrane structures (11). Interestingly, SRO7/ SOP1, a homolog of the Drosophila Lgl tumor suppressor gene, encoding a protein involved in exocytic docking and fusion of Golgi apparatus-derived vesicles with the plasma membrane, was recently identified as being required for appropriate delivery of Ena1 to the plasma membrane (93). It is remarkable that the role of Sro7 could be relatively specific, since sensitivity to NaCl is the only evident phenotype of sro7 cells (45) and other membrane-directed proteins do not present altered membrane targeting (93).…”

“…Interestingly, SRO7/ SOP1, a homolog of the Drosophila Lgl tumor suppressor gene, encoding a protein involved in exocytic docking and fusion of Golgi apparatus-derived vesicles with the plasma membrane, was recently identified as being required for appropriate delivery of Ena1 to the plasma membrane (93). It is remarkable that the role of Sro7 could be relatively specific, since sensitivity to NaCl is the only evident phenotype of sro7 cells (45) and other membrane-directed proteins do not present altered membrane targeting (93).…”