Mitochondria contain their own genome that is expressed by nuclear-encoded factors imported into the organelle. This review provides a summary of the current state of knowledge regarding the mechanism of protein translation in human mitochondria and the factors involved in this process.
- human mitochondrion
- mitochondrial elongation factor
- mitochondrial initiation factor
- translation factor
Mitochondria play a vital role in a wide variety of cellular processes, including ATP production, signal transduction and apoptosis. Human mitochondria possess their own genome that is central to their multiple functions, therefore a detailed investigation of the mechanism of mitochondrial gene expression is required to understand mitochondrial dysfunctions associated with numerous human diseases. mtDNA (mitochondrial DNA) from mammals encodes two ribosomal RNAs, 22 tRNAs and 13 proteins. All protein products of the mitochondrial genome are part of the multi-enzymatic complexes involved in oxidative phosphorylation. The synthesis of these proteins is carried out on specialized translational apparatus located within the organelle.
General features of mitochondrial translational apparatus
A number of interesting features distinguish the mammalian mitochondrial translational apparatus from other protein-synthesizing systems. Mammalian mitoribosomes (mitochondrial ribosomes) differ significantly from their bacterial and cytosolic counterparts. They have a relatively low RNA content and, consequently, a low sedimentation coefficient (approx. 55 S) . The low RNA content is compensated for by a large number of mitoribosomal proteins. Proteomic studies have led to the identification of many of these proteins [2,3], about half of which are homologues of bacterial proteins, while the rest are unique to mitochondria. The polypeptides encoded by the mammalian mitochondrial genome are synthesized from nine monocistronic and two dicistronic transcripts, each with overlapping reading frames . Some of the transcripts lack a complete stop codon, and, at these locations, the post-transcriptional addition of poly(A) tails produces a functional termination codon . One of the most interesting features of mitochondrial genetic systems is deviation from the standard genetic code. Thus, for example, TGA, which is a stop codon elsewhere, is read as tryptophan in human mitochondria, whereas AGA and AGG, conventionally encoding arginine, are used as the stop codons in this system.
Four steps of protein synthesis in mitochondria
Many aspects of mitochondrial gene expression cannot be studied directly, as these organelles are not accessible to genetic manipulation. Moreover, there is no in vitro translational system derived from mitochondria, therefore our understanding of the mechanisms of mitochondrial translation initiation, elongation, termination and ribosome recycling is far from complete.
The initiation of mitochondrial translation differs significantly from that found in bacteria and the eukaryotic cytoplasm, as the mitochondrial mRNAs are not capped and essentially have no upstream leader sequences to facilitate ribosome binding . How mitoribosomes are directed to the initiation codon is one of the intriguing questions which still needs to be addressed.
Only two mammalian mitochondrial initiation factors (IF-2mt and IF-3mt) have been identified to date [7,8]. Detailed in vitro characterization of bovine IF-2mt has revealed functional similarity of this protein to the bacterial orthologue, and both proteins have been shown to be active on Escherichia coli ribosomes (reviewed in ). Three mitochondrial elongation factors, EF-Gmt, EF-Tsmt and EF-Tumt, with a strong resemblance to bacterial proteins, have also been identified [10,11]. Consequently, elongation of the mitochondrial polypeptides is assumed to process in a similar fashion as in bacteria; however, the details of the interactions of these factors with ribosomes remain to be comprehensively analysed.
The mechanism of translation termination in mammalian mitochondria is the least studied phase of mitochondrial translation. A putative mtRF1 (mitochondrial release factor 1) was identified by bioinformatics analysis ; however, no functional characterization has been published. Recently, our group identified and characterized a new protein, termed mtRF1a, which has the ability both in vitro and in vivo to act as a termination release factor . This factor specifically decodes the UAA and UAG termination triplets, which are the major stop codons in human mitochondria, as they are found at the ends of 11 of the 13 mitochondrial open reading frames. No release activity has been ascribed to mtRF1. This does not imply, however, that mtRF1a is the exclusive mitochondrial release factor, as mtRF1 may act to decode AGA and AGG codons. Thus the mechanism of translation termination should be investigated further.
No information is currently available on the final step of protein synthesis, which is ribosome recycling. After release of the nascent peptide, the post-terminational complex consisting of ribosome, mRNA and tRNA has to be disassembled and the components recycled for another round of translation. In prokaryotes, this process is catalysed by the co-ordinated actions of RRF (ribosome recycling factor) and EF-G (elongation factor G) (reviewed in ). Bacterial RRF is a universally conserved protein and is essential for cell viability . BLAST searches of the human EST (expressed sequence tag) database using Escherichia coli RRF as a query sequence revealed a potential candidate for a human mtRRF (mitochondrial RRF) . Analysis of the deduced amino acid sequence of the putative mtRRF suggests the presence of a pre-sequence that would localize it to mitochondria. This protein shares 25–30% identity with bacterial RRFs . To date, there have been no studies proving that the putative mtRRF is indeed a mitochondrial protein or demonstrating its involvement in the synthesis of mitochondrial proteins. Further characterization of the candidate protein should provide important insights into the molecular mechanisms directing ribosome recycling in mitochondria.
In summary, several factors involved in mitochondrial translation have been identified to date, particularly those involved in initiation and elongation. Most certainly, there must also be other, as yet unidentified, factors that play crucial roles in protein synthesis in mitochondria.
Central Nervous System: A Focus Topic at Life Sciences 2007, held at SECC Glasgow, U.K., 9–12 July 2007. Edited by C. Dart (Liverpool, U.K.), M. Houslay (Glasgow, U.K.), M. Ludwig (Edinburgh, U.K.), R. Porter (Trinity College Dublin, Ireland) and J. Potts (Misouri-Columbia, U.S.A.).
Abbreviations: IF-2mt, mitochondrial initiation factor 2; mtDNA, mitochondrial DNA; mtRF1a, mitochondrial release factor 1a; mtRRF, mitochondrial ribosome recycling factor; mitoribosome, mitochondrial ribosome
- © The Authors Journal compilation © 2007 Biochemical Society