Quality control of proteins and mitochondria in organisms is essential for maintaining basic cell viability. Protein homeostasis in cells is mainly maintained through the coordinated operation of the chaperone protein system and two proteolytic systems, namely the ubiquitin-proteasome system and the autophagy-lysosomal system. As a cell's energy and metabolic center, mitochondria have relatively independent quality control systems, including molecular-level oxygen free radical scavenging systems, molecular chaperone protein systems and protease systems, and organelle-level fusion/division mechanisms and mitochondrial autophagy mechanisms. Imbalance of protein homeostasis and mitochondrial dysfunction are important factors in the occurrence of aging and aging-related diseases. The occurrence of the two may be a cause and effect.
Figure 1. An interaction between mitochondrial outer membrane protein FUNDC1 and chaperone HSC70 promotes degradation or aggregation of unfolded cytosolic proteins on the mitochondria, thereby maintaining cellular proteostasis but reducing mitochondrial fitness. (Li Y,et al. 2019)
Mitochondria and their functions
Mitochondria are two-layer membrane-coated organelles that are found in most cells. They are structures that make energy in cells. They are the main place for cells to perform aerobic respiration. They are called "power houses". Its diameter is around 0.5 to 1.0 microns. Except for amoeba, Giardia lamblia, and several microsporidia, most eukaryotic cells have more or less mitochondria, but they each have mitochondria in terms of size, number, and appearance. They are all different. Mitochondria have their own genetic material and genetic system, but their genome size is limited and they are a semi-autonomous organelle. In addition to energizing cells, mitochondria also participate in processes such as cell differentiation, cell information transmission, and apoptosis, and possess the ability to regulate cell growth and cell cycle.
Figure 2. Unbalanced mitochondrial dynamics and turnover during aging. Mitochondrial homeostasis is maintained by a series of protective mechanisms. (https://doi.org/10.1155/2019/9825061)
Mitochondrial Protein Synthesis
Proteins that are made outside the mitochondrion in the cellular cytoplasm undergo the process of translation from messenger RNA to the individual amino acids that make up the protein chain. These proteins are then shuttled to the mitochondrion through special cellular signals that match the proteins to the outer membrane surface. From the surface, the proteins interact with a protein called translocase, which sorts the proteins toward their final destination inside the mitochondrion. Extra-mitochondrial proteins can end up being folded into the outer or inner membranes, trapped inside the intermembrane space, or sorted within the interior matrix of the mitochondrion. Inside the mitochondrion, only a small fraction of proteins are made. These matrix proteins might not be numerous, but they perform critical functions inside the organelle. Parts of the mitochondrial ribosomes, small complexes of RNA and protein that translate mitochondrial RNA into proteins, are made within the mitochondrion. Another important protein made within the mitochondrion is a portion of the protein complex that makes ATP, known as ATP synthase. Think of these mitochondrial proteins as the wheels and gears inside a machine. Without the right cogs set up in the right places, the machine won't run. And without the right proteins in the proper order, mitochondria can't make the energy molecule that powers the cell and, in turn, powers a living organism.
Figure 3. An electron microscope image of two mitochondria, with the characteristic folds and creases of this organelle (From https://study.com/academy/lesson/mitochondrial-protein-synthesis.html)
Mitochondrial Protein functions
Mitochondrial ADP/ATP carrier (AAC) is the most important transporter on the inner membrane of mitochondria. AAC is responsible for exchanging ATP in mitochondria with ADP in cytoplasm and controlling the production of ATP in cells. In addition, some studies have suggested that AAC may be involved in the mitochondrial uncoupling process. The AAC protein transports ADP into the mitochondria in the direction of the mitochondrial matrix. At this time, the structural state of the AAC protein is called m-state; the state of transporting ATP into the cytoplasm in the direction of cytoplasm is called c-state. AAC has dynamic conformational changes of six movable components during ATP/ADP transport.
Figure 4. Schematic diagram of ATP / ADP transporter structure (Ruprecht J J. et al. 2019)
The mitochondrial autophagy receptor protein FUNDC1 located on the outer mitochondrial membrane can interact with the cytoplasmic molecular chaperone protein HSC70. Damaged or misfolded proteins in the cytoplasm can be recruited to mitochondria through this interaction, then enter the mitochondrial matrix through the TOM-TIM complex and be degraded by the mitochondrial protease LONP1, which is localized to the matrix. When the proteasome activity is inhibited in the cell, the interaction between FUNDC1 and HSC70 is enhanced, and the unfolded proteins entering the mitochondria are correspondingly increased. If these proteins cannot be removed in time from the mitochondrial matrix, they will participate in the formation of a specific multilayer structure. The new pathway mediated by FUNDC1 and HSC70 can help cells maintain protein homeostasis by enabling the mitochondrial protease system or the mitochondrial autophagy mechanism, but excessive accumulation of unfolded proteins on mitochondria can damage mitochondrial integrity, activate AMPK and Causes cell aging. Therefore, this kind of mitochondrial-mediated compensatory degradation of proteins comes at the expense of the normal function of mitochondria itself and the health and vitality of cells.
Directed transport of mitochondrial proteins
Mitochondrial proteins have limited ability to synthesize, and a large number of mitochondrial proteins are synthesized in the cytoplasm and transported to the mitochondria. These proteins exist as unfolded precursors before transport, and the molecular chaperones bound to them keep the precursor protein in an unfolded state. Usually a signal sequence at the N-terminus of the precursor protein is called leader peptide, leader peptide, or transit peptide. After the completion of the transfer, it is cut off by the signal peptidase to become a mature protein. This phenomenon is called Post-translation.
The characteristics of the mitochondrial precursor protein signal sequence are: ①Mostly located at the N-terminus of the peptide chain, consisting of about 20 amino acids; ②No negatively charged amino acids, forming an amphoteric alpha helix, positively charged amino acid residues and no The charged hydrophobic amino acid residues are located on both sides of the helix, which is believed to be related to the recognition of translocation factors. ③There is no specific requirement for the protein being pulled. Non-mitochondrial proteins connected to such signal sequences will also be transported to Mitochondria. In addition, some signal sequences are located inside the protein and will not be excised after completing the transfer, and some signal sequences are located at the C-terminus of the precursor protein, such as mitochondrial DNA helicase Hmil.
Figure 5. Directed transport of mitochondrial proteins (From Baidu)