ESCRT-III protein Snf7 mediates high-level expression of the SUC2 gene via the Rim101 pathway

The yeast (Saccharomyces cerevisiae) Snf7 family consists of six highly charged, coiled-coil-forming proteins involved in multivesicular body (MVB) formation. Although all proteins perform a common function at late endosomes, individual mutants also show distinct phenotypes. This suggests that Snf7 homologues have additional functions separate from their role in MVB formation. In this report, we explored the molecular basis for the sucrose-nonfermenting phenotype of snf7 mutants. Our Northern blotting experiments provide evidence that Snf7 is involved in the regulation of SUC2 transcription. The Snf7-dependent regulation of SUC2 transcription does not appear to involve the transcription factor Mig1, since Mig1 phosphorylation after glucose derepression was not affected in a Deltasnf7 mutant. Instead, we show that Snf7 influences SUC2 expression by regulating the level of the transcription factor Nrg1. Snf7 exerts its effects on Nrg1 levels through activation of the transcription factor Rim101, which is part of the yeast alkaline response pathway ("Rim101 pathway"). This is supported by the findings that deletion of RIM101 or overexpression of NRG1 from the ADH1 promoter leads to the same SUC2 expression level as deletion of SNF7. In addition, deletion of other components of the Rim101 pathway, like RIM13 and RIM20, led to the same growth phenotype on raffinose media as deletion of SNF7. Furthermore, Snf7 turned out to be dispensable for SUC2 expression in an NRG1 deletion background. Thus, the effects of Snf7 on SUC2 expression can be completely accounted for by its effect on Nrg1 levels.