The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

184
The Mitochondrial Free Radical Theory of Aging
is compelling evidence that some mitochondrial protein import is similar: in yeast, some
ribosomes engaged in translation are found bound to mitochondria. 24,25 This is actually no
great surprise, since the signal that targets most proteins to mitochondria is at their
N-terminus, which is synthesised first. Thus, this signal becomes “visible” to the targeting
machinery before the protein synthesis is complete. In other words, contranslational import
may not be obligatory; it may just happen by chance some of the time.
This interpretation is supported by a number of other points. Firstly, it seems that only
a minority of import is cotranslational: no transcripts have been found exclusively (or even
predominantly) associated with mitochondria-bound ribosomes rather than free ones. 25
Secondly, the acceleration of import that is sometimes achieved by duplicating a protein’s
leader sequence 18,11 (see Section 15.9) appears hard to explain on the basis of “more
strenuous” import, since (based on our current understanding) the import machinery would
not be expected to bind both sequences simultaneously; but it is easy to explain on the basis
of more rapid targeting to mitochondria leading to more cotranslational (hence successful)
import, since the targeting machinery will see a bigger signal more quickly on average.
These considerations might suggest that it would be difficult to exploit cotranslational
import for the present purpose. Since no protein is known which is predominantly imported
cotranslationally, we must presume that there is nothing about the signal sequence (other
than its size) which promotes such import, and we know (see Section 15.9) that bigger is
better but not good enough. If contranslational import is a matter of chance, therefore, we
would need to increase that chance. Pessimism may be premature, however. One approach
that might possibly achieve this would be to exploit the nuclear genome’s codon bias.* Codon
bias is thought to be self-sustaining, by virtue of rare codons being represented by small
numbers of tRNA genes or by tRNAs with low efficiency. Thus, the idea is to give the
transgenes deliberately terrible codon bias—to construct them with a large number of codons
which are rare in human nuclear DNA, on the basis that they will typically be more slowly
recognised and translated than normal, giving more time for import to begin (and, once
begun, to keep up with translation). In bacteria, codon choice can alter translation rate by as
much as sixfold, 26a so this approach has potential. Promotion of cotranslational import is a
possible reason why overexpressing a protein involved in nucleocytoplasmic transport
improves import of moderately hydrophic proteins. 26b
The consideration of cotranslational import suggests another possible obstacle to
mitochondrial gene therapy, however: it is quite conceivable that most mt-coded proteins
are cotranslationally exported into the inner membrane. If they are, then the problem of
folding (discussed in Section 15.7) becomes altogether more likely: it may very well be much
easier for a protein to go straight into the membrane as it comes off a ribosome than as it
comes through the Tim machinery, since the ribosome can face the membrane whereas the
Tim machinery is facing the wrong way.
* Codon bias is a numerical property of a collection of sequences of protein-coding genes—typically, of the
set of all sequenced genes of a given species. Amino acids are encoded by triplets of nucleotides, so there are
64 possible triplets (codons), but there are only 20 amino acids. Thus, most amino acids are encoded by
more than one codon—sometimes as many as six. One can therefore compare two synonymous codons
(ones that translate to the same amino acid) with regard to how often they each appear in the collection of
sequences. One will appear more often than the other: sometimes, it turns out, much more often. The
difference between these pairs of numbers is the set's codon bias.