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.

译文

:酵母(Saccharomyces cerevisiae)Snf7家族由六种高度带电荷的卷曲螺旋形成蛋白组成,这些蛋白参与多囊泡体(MVB)的形成。尽管所有蛋白质都在晚期内体中发挥共同的功能,但单个突变体也显示出不同的表型。这表明Snf7同源物除了在MVB形成中的作用外,还具有其他功能。在本报告中,我们探讨了snf7突变体蔗糖非发酵表型的分子基础。我们的RNA印迹实验提供了Snf7参与SUC2转录调控的证据。 SUC2转录的Snf7依赖性调节似乎不涉及转录因子Mig1,因为在Deltasnf7突变体中葡萄糖抑制后的Mig1磷酸化不受影响。相反,我们显示Snf7通过调节转录因子Nrg1的水平影响SUC2表达。 Snf7通过激活转录因子Rim101来发挥其对Nrg1水平的影响,Rim101是酵母碱性反应途径(“ Rim101途径”)的一部分。这由以下发现支持:RIM101的缺失或ADH1启动子中NRG1的过表达导致与SNF7缺失相同的SUC2表达水平。此外,Rim101途径的其他成分(如RIM13和RIM20)的缺失导致棉子糖培养基上的生长表型与SNF7的缺失相同。此外,事实证明,Snf7对于NRG1缺失背景中的SUC2表达而言是可有可无的。因此,Snf7对SUC2表达的影响可以完全通过其对Nrg1水平的影响来解释。

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