The mitochondria, also dubbed the powerhouse, is an important organelle in cells. It produces a huge chunk of the body’s currency of energy–ATP. That said, it should come as no surprise that all eukaryotes should have cells that contain lots of them. Apart from producing energy, the mitochondria perform several important biologic functions like regulating the cell cycle, signaling time for a cell to die, regulating the maturation of cells, and cell growth. The structure of the mitochondria is made up of compartments like the cristae, the inner membrane, intermembrane space, and the matrix. Scientists are particularly interested in the cristae part of mitochondria. The cristae are the particular components that produce energy. Essentially, these cristae are folds on the inner membrane. These folds ensure that there is enough space for chemical reactions [aerobic respiration]. Also, the cristae are laden with many proteins and chemical substances to facilitate the chemical. interactions in aerobic respiration.
Structure of cristae
For a long while, the exact structure was not known, although different theories were postulated here and there about its structure. Recently a team of scientists used high-power microscopy technology to visualize the structure of mitochondria. They published this study in the Royal Society’s Open Biology.
The scientist used advanced technologies like electron tomography and live-cell super-resolution light microscopy. These cutting-edge technologies gave scientists an unprecedented view of cristae dynamism. The detailed structure of the cristae was reconstructed from several tilt images, then used to produce 3D tomograms. Furthermore, analysis of the structure and biochemical properties of the inner membranes of mitochondria, thylakoids, and myelin revealed that the structure of cristae was constantly changing.
To better our understanding of the study, the researchers elaborated the evolutionary paths via which cristae have developed over the years. Four major pathways that the cristae have undergone: from the earliest protist to its most recent modifications in the most advanced eukaryotes. Whichever the pathway, it has a couple of things that are common and central to all of them, which include: ATP synthase at the edges of the cristae, dimerization, organization of ‘mitochondrial contact site, and cristae organizing system’ (MICOS) at junctions of cristae. Others include remodeling of the membrane by a dynamin-related GTPase (Mgm1 and OPA1 in yeast and mammals respectively), alteration in the makeup of the membrane.
The first pathway involves ATP-synthase spontaneous dimerization and careful organization in an order that is specific for a given organism. This arrangement represents the baseline geometry. Unlike the case in respiratory complexes, I-IV which were arranged in a flat inner boundary membrane, the ATP synthase complex V is arranged in the inside of the cristae membrane. Amongst other subunits of ATP synthase, the Atp20 and Atp21 subunits are the most needed.
Furthermore, an overload of ADP tends to cause the intra-cristal spaces to swell up. On the other hand, the dearth of ADP forces the intra-cristal spaces to contract. Studies in giant amoeba Chaos carolinensis and mice collaborate on this fact.
At the second pathway, the MICOS contact sites are assembled. Critical proteins like the MIC60-related gene have always been present in the mitochondria of endosymbiotic organisms. The early ancestors had mitochondria that differed from late descendants in intracytoplasmic membrane structures. Species with very simplified mitochondria without cristae also lacked MICOS-related genes, however when MIC60 homologs were introduced the yeast mutant corrected that structural defect.
The third pathway involves proteins like dynamin-related GTPases, they organize the fusion and fission of the two membranes of the mitochondria. During fission, the dynamin-related GTPases undergo polymerization into contractile bands that contract squeezing the mitochondria. Researchers say the final results depend on how well this protein interacts with MICOS complex, transport systems, and cristae junctions.
The last pathway involves the membrane phospholipids. The mitochondrial membrane contains specific substances that produce phospholipids that are the building block. However, most of the lipids for the mitochondria are produced in the endoplasmic reticulum and have to be ferried across the membrane into the mitochondria. Afterward, the lipids are distributed by special intermembrane-space-localized transport.
The breakthrough in the visualization of the cristae means that at long last we can understand the genesis of different diseases rooted in a defect in mitochondria like autism, some cases of heart failure, myopathies, etc, and hopefully treat them.
The challenge of explaining the architecture of cristae has been finally solved. Lipids are the building block of the cristae and these lipids are influenced by important systems, MICOS complex, and ATP synthase dimerization.