Mitochondrial genomes are retained by selective constraints on protein targeting

http://www.pnas.org/content/early/2015/07/17/1421372112

Björkholm P, Harish A, Hagström E, Ernst AM, Andersson SG

Mitochondria originated through the fusion of two early bacterial forms of life: an endosymbiont giving rise to mitochondria, and a host to modern-day eukaryotic cells. Each of these original organisms contained their own genomes; however, over evolutionary time, there has been genetic transfer from mitchondrial DNA to nuclear DNA.

Several competing hypotheses exist to explain why any particular gene may be retained by mitochondria. One hypothesis is that a core set of genes must be retained by these organelles to retain local control over respiration (a similar argument exists for chloroplasts in photosynthesis, this is the CoRR hypothesis).

In this study, the authors provide evidence for the hydrophobicity of the gene products to determine gene retention. They predict that mitochondrially-encoded proteins have a larger insertion free energy than nuclear-encoded proteins. They show experimentally that when these proteins are expressed in the nucleus, they tend to be recognised by the signal recognition particle (SRP), and targeted to the endoplasmic reticulum, rather than mitochondria. This is due to the SRP's ability to bind to a hydrophobic domain. This may be problematic for gene therapies attempting to alleviate mitochondrial genetic diseases, by expression of such genes in the nucleus.

The authors discuss that hydrophobic proteins are unlikely to fold properly in the cytoplasm, and their import into double-membraned organelles like mitochondria, would be difficult (and potentially toxic if unfolded proteins were to accumulate). The authors emphasise that the hydrophobicity hypothesis is not mutually exclusive to the CoRR hypothesis, and many selective pressures are likely to operate on organelle genomes.