Department of Molecular Biosciences, The Wenner-Gren Institute, Stockholm University, Stockholm, Sweden.
Traffic. 2019 Oct;20(10):775-784. doi: 10.1111/tra.12681. Epub 2019 Aug 29.
Evidence from multiple laboratories has implicated Ssy1, a nontransporting amino acid permease, as the receptor component of the yeast plasma membrane (PM)-localized SPS (Ssy1-Ptr3-Ssy5)-sensor. Upon binding external amino acids, Ssy1 is thought to initiate signaling events leading to the induction of amino acid permease gene expression. In striking contrast, Kralt et al (2015) (Traffic 16:135-147) have questioned the role of Ssy1 in amino acid sensing and reported that Ssy1 is a component of the endoplasmic reticulum (ER), where it reportedly participates in the formation of ER-PM junctions. Here, we have re-examined the intracellular location of Ssy1 and tested the role of ER-PM junctions in SPS sensor signaling. We show that the C-terminal of Ssy1 carries a functional ER-export motif required for proper localization of Ssy1 to the PM. Furthermore, ER-PM junctions are dispensable for PM-localization and function of Ssy1; Ssy1 localizes to the PM in a Δtether strain lacking ER-PM junctions (ist2Δ scs2Δ scs22Δ tcb1Δ tcb2Δ tcb3Δ), and this strain retains the ability to initiate signals induced by extracellular amino acids. The data demonstrate that Ssy1 functions as the primary amino acid receptor and that it carries out this function at the PM.
来自多个实验室的证据表明，非转运氨基酸渗透酶 Ssy1 是酵母质膜（PM）定位 SPS（Ssy1-Ptr3-Ssy5）传感器的受体组成部分。当结合外部氨基酸时，Ssy1 被认为会启动信号事件，导致氨基酸渗透酶基因表达的诱导。相比之下，Kralt 等人（2015 年）（Traffic 16:135-147）对 Ssy1 在氨基酸感应中的作用提出了质疑，并报道 Ssy1 是内质网（ER）的组成部分，据称它参与 ER-PM 连接的形成。在这里，我们重新检查了 Ssy1 的细胞内位置，并测试了 ER-PM 连接在 SPS 传感器信号转导中的作用。我们表明，Ssy1 的 C 末端带有一个功能 ER 出口基序，该基序对于 Ssy1 正确定位到 PM 是必需的。此外，ER-PM 连接对于 Ssy1 的 PM 定位和功能是可有可无的；在缺乏 ER-PM 连接的 Δtether 菌株（ist2Δ scs2Δ scs22Δ tcb1Δ tcb2Δ tcb3Δ）中，Ssy1 定位于 PM，并且该菌株保留了启动细胞外氨基酸诱导信号的能力。数据表明，Ssy1 作为主要的氨基酸受体发挥作用，并且它在 PM 上执行此功能。
