RTG-dependent mitochondria to nucleus signaling is negatively regulated by the seven WD-repeat protein Lst8p

In cells with reduced mitochondrial function, RTG1, 2 and 3 are required for expression of genes involved in glutamate synthesis. Glutamate negatively regulates RTG-dependent gene expression upstream of Rtg2p, which, in turn, acts upstream of the bHLH/Zip transcription factors, Rtg1p and Rtg3p. Here we report that some mutations [lst8-(2–5)] in LST8, an essential gene encoding a seven WD40-repeat protein required for targeting of amino acid permeases (AAPs) to the plasma membrane, bypass the requirement for Rtg2p and abolish glutamate repression of RTG-dependent gene expression. The lst8-1 mutation, however, which reduces plasma membrane expression of AAP, cannot bypass the Rtg2p requirement, but still suppresses glutamate repression of RTG target gene expression. We show that Lst8p negatively regulates RTG gene function, acting at two sites, one upstream of Rtg2p, affecting glutamate repression of RTG-dependent gene expression through Ssy1p, an AAP-like sensor of external amino acids, and the other between Rtg2p and Rtg1p–Rtg3p. These data, together with genome-wide transcription profiling, reveal pathways regulated by glutamate, and provide insight into the regulation of cellular responses to mitochondrial dysfunction.

在线粒体功能减退的细胞中，RTG1、RTG2与RTG3基因是谷氨酸合成相关基因表达所必需的。谷氨酸在Rtg2p蛋白上游负向调控依赖于RTG通路的基因表达；而Rtg2p又作用于bHLH/Zip类转录因子Rtg1p和Rtg3p的上游。本研究发现，LST8基因——一种编码含7个WD40重复结构域蛋白的必需基因，该蛋白负责将氨基酸通透酶（amino acid permeases, AAPs）靶向运输至质膜——中的部分突变[lst8-(2–5)]可绕过Rtg2p的必需性，并消除谷氨酸对RTG通路依赖型基因表达的抑制作用。但能够降低氨基酸通透酶质膜表达量的lst8-1突变，则无法绕过Rtg2p的必需性，却仍可抑制谷氨酸对RTG通路靶基因表达的抑制作用。本研究证实，Lst8p蛋白可通过两个位点负向调控RTG通路的基因功能：其一位于Rtg2p上游，通过识别胞外氨基酸的AAP类传感器Ssy1p影响谷氨酸对RTG通路依赖型基因表达的抑制作用；其二则位于Rtg2p与Rtg1p-Rtg3p复合物之间。本研究数据结合全基因组转录组分析结果，揭示了谷氨酸所调控的信号通路，并为理解细胞对线粒体功能障碍的应答调控机制提供了新的见解。