The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

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The Mitochondrial Free Radical Theory of Aging
mitochondrial protein import machinery may have become saturated by “stuck” proteins
and so unable to import other, normally nuclear-coded proteins. This would certainly cause
the observed OXPHOS inhibition.
15.9. Import of Very Hydrophobic Proteins
This section discusses the only problem facing the protein import project which is,
honestly, going to have to be attacked by trial and error all the way, and which is therefore,
arguably, the hardest. On the other hand, initial stabs at it have met with quite significant
success, so it may turn out to yield quite quickly after all.
In the description (Sections 2.1 and 10.2) of how we came to have and keep
mitochondrial DNA, and in particular the genes for exactly these 13 proteins, I noted that
there is a particular characteristic, hydrophobicity, which is shared by all these proteins
(because it helps them to become embedded in the inner membrane, where they function)
and which seems to make them difficult to import. In fact there are measures of
hydrophobicity that discriminate almost all mt-coded proteins from those that are
imported. 11 Thus, if we take the signal presequence from some random imported protein
and attach it to a mitochondrially-encoded protein, we cannot by any means be sure that
import will occur—the protein may get stuck part-way through.
One can learn much about why this may be from examination of the signal presequences
of imported proteins. These proteins, of course, have varying degrees of hydrophobicity,
albeit less than the mt-coded ones. Interestingly, the more hydrophobic imported ones tend
to have a longer signal presequence. 17 On the basis of this observation, researchers have
examined the effect on importability of attaching longer-than-normal presequences. These
were made in the most simplistic way imaginable: just by attaching two copies of a standard
presequence end to end. And indeed, it was found that several proteins which could only be
imported very inefficiently were thereby imported more easily. 18,11
Unfortunately, the successfully imported proteins were not quite so hydrophobic as
most of the 13 of interest, and when ones with that greater degree of hydrophobicity were
tried, no import occurred even with the duplicated presequences. Thus, unlike any of the
difficulties discussed in previous sections, this obstacle to mitochondrial gene therapy by
protein import is still, unarguably real.
15.10. Hints from Other Organisms
It is all very well to say that the presequence design technique tried so far 18,11 is simplistic,
but that does not tell us what sophistication to try that might be more effective. I think we
may find it very difficult to guess a design strategy which will enable import of proteins that
are completely unimportable at the moment. We will need help. The source of such help is,
as usual, nature: organisms that already encode any of the relevant 13 on nuclear genes will
provide a far better example than extrapolation from the presequences of more easily
importable ones.
A further reason for optimism that other organisms can be of use is that the mt-coded
13 are all very highly conserved across taxa. It is of course likely that an example from another
animal will be more applicable to the human gene than one from a more distantly related
organism, but even very primitive eukaryotes have sufficiently similar homologues of these
genes that we should not ignore them.
At the time of writing, however, little effort has gone into this line of attack. Two
genes—subunits 2 and 3 of cytochrome c oxidase—have been found to be nuclear-coded in
certain plants (Vigna radiata 19 and Selaginella elegans, 20 respectively), but I know of no
attempt to import their human homologues using their presequences. A blue-green alga,