Disturbed intramitochondrial phosphatidic acid transport impairs cellular stress signaling

Lipid transfer proteins of the Ups1/PRELID1 family facilitate the transport of phospholipids across the intermembrane space of mitochondria in a lipid-specific manner. Heterodimeric complexes of yeast Ups1/Mdm35 or human PRELID1/TRIAP1 shuttle phosphatidic acid (PA) synthesized in the endoplasmic reticulum (ER) to the inner membrane, where it is converted to cardiolipin (CL), the signature phospholipid of mitochondria. Loss of Ups1/PRELID1 proteins impairs the accumulation of CL and broadly affects mitochondrial structure and function. Unexpectedly and unlike yeast cells lacking the cardiolipin synthase Crd1, Ups1 deficient yeast cells exhibit glycolytic growth defects, pointing to functions of Ups1-mediated PA transfer beyond CL synthesis. Here, we show that the disturbed intramitochondrial transport of PA in ups1D cells leads to altered phospholipid composition of the ER membrane, independent of disturbances in CL synthesis. The impaired flux of PA into mitochondria is associated with the increased synthesis of phosphatidylcholine (PC) and a reduced phosphatidylethanolamine (PE)/PC ratio in the ER of ups1D cells which suppresses the unfolded protein response (UPR). Moreover, we observed inhibition of TORC1 signaling in these cells. Activation of either UPR by ER protein stress or of TORC1 signaling by disruption of its negative regulator, the SEACIT complex, increased cytosolic protein synthesis and restored glycolytic growth of ups1D cells. These results demonstrate that PA influx into mitochondria is required to preserve ER membrane homeostasis and that its disturbance is associated with impaired glycolytic growth and cellular stress signaling.Ups1/PRELID1家族脂质转运蛋白能够以脂质特异性方式，介导磷脂跨越线粒体膜间间隙进行转运。酵母Ups1/Mdm35或人源PRELID1/TRIAP1形成的异二聚体复合物，可将内质网（ER）合成的磷脂酸（PA）穿梭转运至线粒体内膜，在此处磷脂酸被转化为心磷脂（CL）——线粒体的标志性磷脂。Ups1/PRELID1蛋白的缺失会损害心磷脂的积累，并广泛影响线粒体的结构与功能。出乎意料的是，与缺失心磷脂合酶Crd1的酵母细胞不同，敲除Ups1的酵母细胞会出现糖酵解生长缺陷，这表明Ups1介导的磷脂酸转运所承担的功能不止于心磷脂合成。本研究证实，ups1Δ细胞中受损的线粒体内磷脂酸转运会导致内质网膜的磷脂组成发生改变，且该改变并不依赖于心磷脂合成异常。磷脂酸向线粒体的内流受损，与ups1Δ细胞内质网中磷脂酰胆碱（PC）合成增加、磷脂酰乙醇胺（PE）/PC比值降低相关，而这一变化会抑制未折叠蛋白反应（UPR）。此外，本研究还观察到这些细胞中的TORC1信号通路受到抑制。通过内质网蛋白应激激活UPR，或通过破坏其负调控因子SEACIT复合物激活TORC1信号通路，均可提升胞质蛋白合成水平，并恢复ups1Δ细胞的糖酵解生长能力。本研究结果表明，磷脂酸向线粒体的内流对于维持内质网膜稳态不可或缺，而磷脂酸内流受损与糖酵解生长缺陷及细胞应激信号通路异常密切相关。