Endoplasmic reticulum-assisted mitochondrial precursor biogenesis and quality control

在过去的十年中，内质网(ER)在精选核编码生物发生中的作用 线粒体前体与突变、错位或非生产性进口蛋白的降解 从线粒体外膜(OM)开始出现。我们对这部出人意料的小说的兴趣 内质网相关的线粒体生物学是偶然的。在我们正在进行的表征脂质的努力中 磷脂酰丝氨酸脱羧酶1(PSD1在酵母中，PISD在酵母中的底物转运要求 人类)，是一种进化上保守的完整的线粒体内膜蛋白，它能产生 磷脂酰乙醇胺(PE)，这成为我们的优先事项，独立确定一小部分野生 正如最近声称的那样，类型(WT)PSD1是糖基化的，因此靶向于内膜系统。我们的 所产生的结果支持了不可避免的结论，即在酵母中，绝大多数，如果不是全部，功能 PSD1基因定位于线粒体。然而，我们确实发现了Psd1和ER之间的密切关系： 与WT蛋白不同，Psd1的非功能形式双重定位于内质网，它们位于内质网 糖基化，泛素化，并迅速降解。鉴于ER的角色是高效的 去除无功能的Psd1，然后我们问了两个问题-1)Psd1的生物发生是否得到了最近的 发现Djp1介导的内质网表面检索途径(ER-SURF)？和2)是否 内质网相关的无功能Psd1的积累依赖于线粒体膜外膜Msp1 已知的常驻AAA-ATPase可以去除错靶蛋白和非生产性进口的线粒体 外膜的前体呢？由此得到的答案为我们提出的目标和 确定PSD1为理想的模型底物以确定ER-SURF和蛋白质降解的新机制 这无疑将适用于其他线粒体蛋白质。目标1的目标是定义早期的、预先的 Psd1生物发生的线粒体步骤，由发生在内质网的特定相互作用介导。其他 除了Hsp40辅伴侣Djp1的关键作用外，人们对ER-SURF几乎一无所知。结果来自 AIM 1将通过这一新途径提供关于线粒体靶向的机械性见解，以及它是否 可以在特定情况下被强迫以支持非线粒体生物学。在过去的七年里，它 人们越来越意识到线粒体前体在这个细胞器外的积累 激活一系列细胞应激反应。在此背景下，我们发现非功能突变体Psd1 在被降解之前与ER膜临时结合尤其令人兴奋。目标2的目标是 目的：探讨非功能性PSD1的分子机制。Aim 2的结果将发生变化 我们对细胞如何处理无法到达正确目的地的线粒体前体的理解，以及 该领域出现了新的焦点。

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.