A novel, non-canonical mechanism for the nuclear entry of large mitochondrial enzymes

为了实现效率，电池的内部被很好地隔开。线粒体是细胞中的细胞器，许多特殊的蛋白质在这里工作，以产生能量，为细胞的所有功能提供燃料。细胞核是细胞内基因所在的另一个细胞器。由于基因的关键重要性，细胞核的内部被围绕在细胞核周围的特殊膜很好地保护着。进入和退出细胞核只通过被称为核孔(NP)的特殊通道，这种通道只允许低于大小限制的蛋白质进入。此外，来自细胞另一部分的蛋白质必须在其两端之一有一个特定的片段，称为“核定位序列”(NLS)，特定的交通伴侣可以将其结合并引导它通过NP进入细胞核。 我们最近发现，一种被认为只存在于线粒体中的非常重要的酶，出人意料地也在细胞核中被发现。这种酶(称为丙酮酸脱氢酶复合体，PDC)在细胞核内发挥功能，产生基因调控的关键代谢物。但PDC是生物学中已知的最大的酶(其大小远远超过NPs的大小限制)，并且没有NLS。因此，PDC如何进入细胞核仍然是一个谜，它的存在表明，可能存在一种未知的、新的机制，使大蛋白进入细胞核。 本研究项目旨在发现这一新颖的进入机制。我们将从我们的初步实验中寻找早期证据，即线粒体移动并与细胞核接触。线粒体和核膜中都存在一种名为Mitofusin2(Mfn2)的蛋白质，它被用来将这两个细胞器连接在一起。这使得PDC能够进入外部核膜，并与内部核膜的主要蛋白质之一，即层粘连蛋白相互作用，使其能够进入核。 我们相信这是体内所有细胞的一个特征，因此我们将使用先进的分子和成像技术，包括超分辨率共聚焦显微镜，研究许多不同类型的细胞。我们的工作有望显著增强线粒体与细胞核以及不同细胞器之间的通讯途径这一相对较新且尚未完全了解的概念。它的潜在含义是多方面的，从胚胎发育过程中细胞的功能，到新陈代谢应激期间细胞的功能，再到衰老的机制。这项研究计划为研究生的培训提供了极好的机会，并有望导致跨越生物学许多不同方面的重要出版物的发表。

To achieve efficiency, the interior of a cell is well-compartmentalized. Mitochondria are organelles in the cell where a number of specialized proteins work toward the production of energy to fuel all cellular functions. The nucleus is another organelle within the cell where the genes are located. Because of the critical importance of the genes, the interior of the nucleus is well protected with specialized membranes that surround the nucleus. Entry and exit from the nucleus occur only through specialized channels called nuclear pores (NP), that only allow proteins below a size limit to entry. In addition, a protein originating from another part of the cell, has to have a specific segment in one of its two ends, called “nuclear localization sequence” (NLS), to which specific traffic chaperones can bind it and guide it through the NP into the nucleus. We recently found that a very important enzyme only thought to exist within the mitochondria, was unexpectedly found in the nucleus as well. This enzyme (called pyruvate dehydrogenase complex, PDC) was functional in the nucleus, producing a critical metabolite for gene regulation. But PDC is the largest enzyme known in biology (its size far exceeds the size limit of NPs) and has no NLS. Thus it remains a mystery how PDC can enter the nucleus and its presence there suggests that an unknown, novel mechanism may exist for the entrance of large proteins in the nucleus. This research program aims to discover this novel entry mechanism. We will pursue early evidence from our preliminary experiments, that mitochondria travel and make contact points with the nucleus. A protein called Mitofusin2 (MFN2) that is found in both the mitochondria and the nucleus membranes, is used to tether the 2 organelles together. This allows the PDC to enter the external nuclear membrane and interact with one of the main proteins of the internal nuclear membrane, i.e. lamin, enabling its nuclear entry. We believe that this is a feature of all cells in the body and thus we will study many different cell types using advance molecular and imaging techniques, including super-resolution confocal microscopy. Our work is expected to significantly enhance the relative recent and incompletely understood concept of communication of mitochondria with the nucleus and, in general, the communication pathways between different cellular organelles. Its potential implications are many and range from the function of cells during embryo development, to the function of cells during metabolic stress, to the mechanism of aging. This research program provides excellent opportunities of the training of graduate students and is expected to result in important publications that cross many different aspects of biology.