Omnus Deike J, Ljungdahl Per O
Department of Molecular Biosciences, The Wenner-Gren Institute, Stockholm University, S-106 91 Stockholm, Sweden.
Department of Molecular Biosciences, The Wenner-Gren Institute, Stockholm University, S-106 91 Stockholm, Sweden
Mol Biol Cell. 2014 Nov 15;25(23):3823-33. doi: 10.1091/mbc.E14-06-1140. Epub 2014 Sep 24.
The Ssy1-Ptr3-Ssy5 (SPS)-sensing pathway enables yeast to respond to extracellular amino acids. Stp1, the effector transcription factor, is synthesized as a latent cytoplasmic precursor with an N-terminal regulatory domain that restricts its nuclear accumulation. The negative regulatory mechanisms impinging on the N-terminal domain are poorly understood. However, Stp1 latency depends on three inner nuclear membrane proteins, Asi1, Asi2, and Asi3. We report that the N-terminal domain of Stp1 contains a small motif, designated RI, that fully accounts for latency. RI is modular, mediates interactions with the plasma membrane, and can retain histone Htb2 in the cytoplasm. A novel class of STP1 mutations affecting RI were isolated that are less efficiently retained in the cytoplasm but remain under tight negative control by the Asi proteins. Intriguingly, these mutant proteins exhibit enhanced stability in strains lacking ASI1. Our results indicate that RI mediates latency by two distinct activities: it functions as a cytoplasmic retention determinant and an Asi-dependent degron. These findings provide novel insights into the SPS-sensing pathway and demonstrate for the first time that the inner nuclear membrane Asi proteins function in a degradation pathway in the nucleus.
Ssy1-Ptr3-Ssy5（SPS）传感途径使酵母能够对细胞外氨基酸做出反应。效应转录因子Stp1作为一种潜在的细胞质前体被合成，其N端调控结构域限制了它在细胞核中的积累。影响N端结构域的负调控机制尚不清楚。然而，Stp1的潜伏性依赖于三种内核膜蛋白Asi1、Asi2和Asi3。我们报告称，Stp1的N端结构域包含一个小基序，命名为RI，它完全解释了潜伏性。RI是模块化的，介导与质膜的相互作用，并能将组蛋白Htb2保留在细胞质中。我们分离出了一类影响RI的新型STP1突变体，它们在细胞质中的保留效率较低，但仍受到Asi蛋白的严格负调控。有趣的是，这些突变蛋白在缺乏ASI1的菌株中表现出更高的稳定性。我们的结果表明，RI通过两种不同的活性介导潜伏性：它作为细胞质保留决定因素和Asi依赖的降解结构域发挥作用。这些发现为SPS传感途径提供了新的见解，并首次证明内核膜Asi蛋白在细胞核的降解途径中发挥作用。
