The Saccharomyces cerevisiae integral membrane protein Ssy1p functions with Ssy5p and Ptr3p to sense extracellular amino acids. Signal transduction leads to processing and nuclear localization of Stp1p and Stp2p, transcriptional activators of many amino acid transporter genes. Ssy1p is structurally related to amino acid permeases, but unable to transport amino acids. We isolated SSY1 mutants that constitutively activate a target promoter. Dose-response analysis showed that the mutants are hyperresponsive, requiring less inducer to give strong signaling than does the wild type. Another mutant (Ssy1p(T639I)) turned out to be hyporesponsive, i.e., it signals only at high inducer concentration. In accordance with a transporter-like mechanism for Ssy1p function we suggest that the hyper- and hyporesponsive mutant forms differ from the wild-type sensor by being more and less inclined, respectively, to adopt an outward-facing, signaling conformation. Coordinate conformational dynamics of the sensor complex was supported by additive effects of combinations of constitutive SSY1, PTR3 and SSY5 alleles. Assuming structural similarity of Ssy1p to the distantly related bacterial leucine transporter LeuT(Aa), several activating substitutions were located near the substrate binding site while others were on the periphery of Ssy1p. We suggest analyses of transporter-like sensors as an approach to understand key features of transporters.

中文翻译：

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酿酒酵母Ssy1氨基酸传感器的高反应性和低反应性突变体形式。

酿酒酵母整合膜蛋白Ssy1p与Ssy5p和Ptr3p共同作用以感测细胞外氨基酸。信号转导导致Stp1p和Stp2p（许多氨基酸转运蛋白基因的转录激活因子）的加工和核定位。Ssy1p在结构上与氨基酸渗透酶有关，但无法转运氨基酸。我们分离出组成性激活目标启动子的SSY1突变体。剂量反应分析表明，这些突变体具有高反应性，与野生型相比，需要更少的诱导剂才能产生较强的信号。另一个突变体（Ssy1p（T639I））表现出反应低下，即仅在高诱导物浓度下发出信号。根据Ssy1p功能的转运蛋白样机制，我们建议高反应性和低反应性突变体形式与野生型传感器不同，它们分别或多或少地倾向于采用向外的信号传导构象。组成型SSY1，PTR3和SSY5等位基因组合的加和效应支持了传感器复合物的坐标构象动力学。假设Ssy1p与远距离相关的细菌亮氨酸转运蛋白LeuT（Aa）的结构相似，一些活化取代位于底物结合位点附近，而其他活化取代位于Ssy1p的外围。我们建议对转运蛋白样传感器进行分析，以了解转运蛋白的关键特征。组成型SSY1，PTR3和SSY5等位基因组合的加和效应支持了传感器复合物的坐标构象动力学。假设Ssy1p与远距离相关的细菌亮氨酸转运蛋白LeuT（Aa）的结构相似，一些活化取代位于底物结合位点附近，而其他活化取代位于Ssy1p的外围。我们建议对转运蛋白样传感器进行分析，以了解转运蛋白的关键特征。组成型SSY1，PTR3和SSY5等位基因组合的加和效应支持了传感器复合物的坐标构象动力学。假设Ssy1p与远距离相关的细菌亮氨酸转运蛋白LeuT（Aa）的结构相似，一些活化取代位于底物结合位点附近，而其他活化取代位于Ssy1p的外围。我们建议对转运蛋白样传感器进行分析，以了解转运蛋白的关键特征。