The mitochondrial rough-ER (mt-rER), a new type of endoplasmic reticulum: from structure to function

细胞器间的接触位点存在于细胞的几乎所有区室之间，并且它们的研究已经变得非常热门，因为越来越多的生理和代谢过程通过细胞器之间的物理相互作用来协调。最具特征的细胞器间接触类型之一是发生在线粒体和smoothER之间的接触，其用于交换钙并调节呼吸，Ca 2+稳态和信号传导，磷脂生物合成等。然而，可以发现线粒体也与含核糖体的粗糙ER相关，但这种关联的功能尚不清楚，并且这种与线粒体相关的粗糙ER（mt-rER）是否是一种特殊类型的ER仍有待确定。 在这里，我们分享了大量令人信服的初步数据，提出了一个研究计划，其目的是表明mt-rER是一种结构独特的ER类型，通过翻译和区室化其途径的酶和运输它们的脂蛋白来控制胆固醇和脂肪酸的合成。 为了实现这一目标，我们将利用专门开发的方案，允许纯化小鼠肝脏mt-rER，并利用我们在冷冻电子显微镜，冷冻电子断层扫描，3D成像，转录组学和蛋白质组学方面的经验，以证明： 1：mt-rER未连接到小区的ER网络 2：mt-rER具有与ER不同的蛋白质组 3：smoothER和peption之间的接触位点（称为MAM或MERC）是mt-rER的一个子域 4：脂质和胆固醇途径的蛋白质直接在mt-rER翻译 5：脂质和胆固醇途径的蛋白质定位于mt-rER和线粒体之间的界面。 6：mt-rER含有一个活跃的胆固醇生物合成途径。 最终，这项研究将表明，mt-rER和peption之间的并置用于划分脂质和胆固醇途径的底物，酶和前体，从而解释细胞如何有效地合成，处理和运输大量这些分子。 这些发现将对许多人产生重大影响。对于细胞生物学家来说，因为他们将建立mt-rER作为一种新型的、结构和功能独特的ER类型;对于细胞生理学家来说，因为他们将赋予mt-rER通过合成脂质和胆固醇进入循环的关键作用脂蛋白;对于MAM的研究人员/MERC领域，因为他们将提供一个框架，研究机制，调节这一重要的亚细胞区室的生物发生和重塑。

Inter-organellar contact sites exist between nearly all the compartments of the cell, and their study has become very topical because more and more physiological and metabolic processes are coordinated by physical interactions between organelles. One of the best characterized type of inter-organellar contact is the one that occurs between mitochondria and the smoothER, which serves to exchange calcium and regulate respiration, Ca2+ homeostasis and signalling, phospholipid biosynthesis, and more. However, mitochondria can be found associated also to the ribosome-containing roughER, but the function of this association is unknown, and whether or not this mitochondria-associated roughER (mt-rER) is a specialized type of ER remains to be determined. Here, we share a wealth of compelling preliminary data to propose a research program that has the objective to show that the mt-rER is a structurally distinct type of ER that controls cholesterol and fatty acids synthesis by translating and compartmentalizing the enzymes of their pathway and the lipoproteins that traffic them. To this goal, we will capitalize on an ad-hoc developed protocol allowing the purification of mouse liver mt-rER and on our experience in cryo-electron microscopy, cryo-electron tomography, 3D imaging, transcriptomics and proteomics to show that: 1: the mt-rER is not connected to the ER network of the cell 2: the mt-rER has a proteome that is different from that of the ER 3: the contact site between the smoothER and the mitochondrion (known as MAM or MERC) is a subdomain of the mt-rER 4: proteins of the lipids and cholesterol pathway are translated directly at the mt-rER 5: proteins of the lipids and cholesterol pathway are localized at the interface between the mt-rER and the mitochondrion 6: the mt-rER contains an active cholesterol biosynthetic pathway. Ultimately, this research will show that the juxtaposition between the mt-rER and the mitochondrion serves to compartmentalize substrates, enzymes and precursors of the lipid and cholesterol pathway, thereby explaining how the cell can efficiently synthesize, handle and transport huge amounts of these molecules. These discoveries will have a major impact for many. For cell biologists, because they will establish the mt-rER as a novel, structurally and functionally distinct type of ER; for cell physiologists, because they will assigning to the mt-rER a key role in putting lipids and cholesterol in circulation through the synthesis of lipoproteins; and for researchers in the MAM/MERC field because they will provide a framework for studying the mechanisms regulating the biogenesis and remodelling of this important sub-cellular compartment.