The yeast galactose switch operated by the Gal4p-Gal80p-Gal3p regulatory module is a textbook model of transcription regulation in eukaryotes. The Gal80 protein inhibits Gal4p-mediated transcription activation by binding to the transcription activation domain. In Saccharomyces cerevisiae, inhibition is relieved by formation of an alternative Gal80-Gal3 complex. In yeasts lacking a Gal3p ortholog, such as Kluyveromyces lactis, the Gal1 protein (KlGal1p) combines regulatory and enzymatic activity. The data presented here reveal a yet unknown role of the KlGal80 N terminus in the mechanism of Gal4p activation. The N terminus contains an NLS, which is responsible for nuclear accumulation of KlGal80p and KlGal1p and for KlGal80p-mediated galactokinase inhibition. Herein, we present a model where the N terminus of KlGal80p reaches the catalytic center of KlGal1p causing enzyme inhibition in the nucleus and stabilization of the KlGal1-KlGal80p complex. We corroborate this model by genetic analyses and structural modelling and provide a rationale for the divergent evolution of the mechanism activating Gal4p.

中文翻译：

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Gal80 N末端在Gal4p转录激活中起双重作用。

Gal4p-Gal80p-Gal3p调节模块操作的酵母半乳糖开关是真核生物转录调节的教科书模型。Gal80蛋白通过与转录激活域结合来抑制Gal4p介导的转录激活。在酿酒酵母中，抑制作用通过形成另一种Gal80-Gal3复合物而得以缓解。在缺乏Gal3p直系同源物的酵母中，例如乳酸克鲁维酵母，Gal1蛋白（KlGal1p）兼具调节和酶促活性。此处提供的数据揭示了K1Gal80 N末端在Gal4p激活机制中的未知作用。N末端包含一个NLS，它负责KlGal80p和KlGal1p的核积累以及KlGal80p介导的半乳糖激酶的抑制作用。在这里，我们提出了一个模型，其中KlGal80p的N末端到达KlGal1p的催化中心，导致细胞核中的酶抑制和KlGal1-KlGal80p复合物的稳定化。我们通过遗传分析和结构建模证实了该模型，并为激活Gal4p的机制的不同进化提供了理论依据。