Endoplasmic reticulum-assisted mitochondrial precursor biogenesis and quality control

在过去的十年中，内质网（ER）在选择核编码的生物发生中的作用 线粒体前体和突变、错误定位或非生产性输入蛋白质的降解 线粒体外膜（OM）的细胞膜已经开始出现。我们对这部意料之外的小说的兴趣 ER相关的线粒体生物学是偶然的。在我们持续的研究中， 磷脂酰丝氨酸（PS）脱羧酶1（酵母中的Psd 1， 人类），一种进化上保守的线粒体内膜蛋白， 磷脂酰乙醇胺（PE），它成为我们的优先事项，以独立地确定，如果一小部分野生型 （WT）型Psd 1被糖基化，因此靶向内膜系统，如最近所声称的。我们 产生的结果支持不可避免的结论，即在酵母中，绝大多数，如果不是全部，功能性的， psd 1是神经系统定位的。然而，我们确实发现了Psd 1和ER之间的亲密关系： 与WT蛋白不同，Psd 1的非功能形式双重定位于ER， 糖基化、泛素化和快速降解。考虑到急诊室作为有效的 去除无功能的Psd 1，我们然后问了两个问题-1）Psd 1生物起源是否由最近的 发现Djp 1介导的内质网表面修复途径（ER-SURF）？（2）是否 ER相关非功能性Psd 1的积累依赖于线粒体外膜Msp 1 已知常驻AAA-ATPase可去除错误靶向的蛋白质和非生产性输入的线粒体 外膜的前体由此产生的答案为我们提出的目标奠定了前提， 鉴定Psd 1作为理想模型底物以确定ER-SURF和蛋白质降解的新机制 这无疑也适用于其他线粒体蛋白。目标1的目标是确定早期、预 Psd 1生物合成的线粒体步骤，由在ER发生的特异性相互作用介导。其他 除了Hsp 40辅伴侣蛋白Djp 1的关键作用外，对ER-SURF几乎一无所知。结果 目的1将提供有关线粒体靶向通过这种新的途径，以及它是否 可以在特定的环境下被强制支持非线粒体生物学。在过去的七年里， 越来越多的人认识到，线粒体前体在细胞器外的积累， 激活一系列细胞应激反应在这种情况下，我们发现，非功能性突变Psd 1 在降解之前与ER膜暂时结合是特别令人兴奋的。目标2的目标是 以确定非功能性Psd 1分辨率的分子机制。目标2的结果将转变 我们对细胞如何科普未能到达正确目的地的线粒体前体的理解， 在该领域的新焦点。

Over the past decade, roles for the endoplasmic reticulum (ER) in the biogenesis of select nuclear-encoded mitochondrial precursors and the degradation of mutant, mis-localized, or non-productively imported proteins from the mitochondrial outer membrane (OM) have begun to emerge. Our interest in this unanticipated, novel ER-associated mitochondrial biology was serendipitous. In our ongoing efforts to characterize the lipid substrate trafficking requirements for phosphatidylserine (PS) decarboxylase 1 (Psd1 in yeast, PISD in humans), an evolutionarily conserved, integral inner mitochondrial membrane protein that produces phosphatidylethanolamine (PE), it became a priority for us to independently ascertain if a small fraction of wild type (WT) Psd1 is glycosylated and thus targeted to the endomembrane system, as recently claimed. Our generated results support the unavoidable conclusion that in yeast, the vast majority, if not all, of functional Psd1 is mitochondrially localized. However, we did uncover an intimate relationship between Psd1 and the ER: unlike the WT protein, non-functional forms of Psd1 are dually localized to the ER, where they are glycosylated, ubiquitinated, and rapidly degraded. Given the role of the ER as a staging ground for the efficient removal of non-functional Psd1, we then asked two questions ― 1) Is Psd1 biogenesis supported by a recently discovered Djp1-mediated Endoplasmic Reticulum Surface Retrieval pathway (ER-SURF)? and 2) Does the accumulation of ER-associated non-functional Psd1 depend on Msp1, an outer mitochondrial membrane resident AAA-ATPase known to remove mis-targeted proteins and non-productively imported mitochondrial precursors from the outer membrane? The resulting answers establish the premise for our proposed aims and identify Psd1 as an ideal model substrate to define novel mechanisms of ER-SURF and protein degradation that will undoubtedly apply to other mitochondrial proteins. The goal of Aim 1 is to define the early, pre- mitochondrial steps of Psd1 biogenesis that are mediated by specific interactions that occur at the ER. Other than a critical role for the Hsp40 cochaperone, Djp1, virtually nothing is known about ER-SURF. Results from Aim 1 will provide mechanistic insight about mitochondrial targeting through this novel pathway and whether it can be coerced in specific contexts to support non-mitochondrial biology. Over the past seven years, it has become increasingly appreciated that the accumulation of mitochondrial precursors outside of this organelle activates a range of cellular stress responses. In this context, our discovery that non-functional mutant Psd1 temporarily associates with ER membranes prior to being degraded is particularly exciting. The goal of Aim 2 is to determine the molecular mechanisms of non-functional Psd1 resolution. Results from Aim 2 will transform our understanding of how cells cope with mitochondrial precursors that fail to reach their correct destination, an emerging focus in the field.