The mitochondrion is a machine within the cell that acts as the “power plant” of the cell. Mitochondria take oxygen and chemically combine it with energy-rich nutrients from our food, to make carbon dioxide and water (which we exhale) and ATP, the “energy currency” of the cell.
The mitochondrion is therefore a really essential part of the cell. Lots of other parts of the cell are essential too, though, so why have a whole SENS strand devoted to it? The answer is that, unlike any other part of the cell, mitochondria have their own DNA (mtDNA), separate from the nucleus. Being at the site of cellular respiration, the mtDNA is vulnerable to its reactive by-products. Worse yet, the mitochondria’s capacity for repairing DNA damage is much more limited than that of the nucleus. Thus we need a different system to combat the inevitable accumulation of such mutations.
Only 13 of the mitochondrion’s component proteins are still encoded by its own DNA (and thus mroe vulnerable to mutation) while the rest are in the nucleus. Rather than fixing mitochondrial mutations, we can make them harmless to us. By putting “backup copies” of these few remaining genes into the nucleus, we can prevent the harm caused by any mutations that may occur of the original versions.
To date, three of the thirteen OXPHOS genes still encoded in the mitochondria have been allotopically expressed (AE) in human cells with mutated versions of the same gene, and thereby rescued a respiratory defect: ATPase6, ND4, and ND1.
Now we have the first report of a new gene, COX2, being allotopically expressed in yeast, by mutating the gene to overcome the hydrophobicity of the mitochondrial membrane. The next step, of course, is to do it in mammalian cells — preferably, of our own species. And the same broad strategy likely applies to many of the other remaining 13; indeed, during his work sponsored by SENS Foundation, Mark Hamalainen developed software that models hydrophobicity of proteins, and it predicts that a relatively small number of relatively minor amino acid changes would lower the hydrophobicity of several of the 13 nuclear-encoded ETS structural components sufficiently to make them importable when the native gene likely is not.
It is conceivable that there are genes for which mitochondrial mRNA localization it won’t be helpful, which is fine so long as there’s an alternative solution available, as they demonstrate, in this case, that there is. And if both methods actually work, we will have our choice: we can compare the 2 methods’ resulting relative import rates, and also their level of rescue of respiration (which is likely to be higher by relocalized translation of the WT protein than by a protein with an altered AA sequence).