Translation initiation of leaderless and polycistronic transcripts in mammalian mitochondria

The synthesis of mitochondrial OXPHOS complexes is central to cellular metabolism, yet many molecular details of mitochondrial translation remain elusive. It has been commonly held view that translation initiation in human mitochondria proceeded in a manner similar to bacterial systems, with the mitoribosomal small subunit bound to the initiation factors, mtIF2 and mtIF3, along with initiator tRNA and an mRNA. However, unlike in bacteria, most human mitochondrial mRNAs lack 5' leader sequences that can mediate small subunit binding, raising the question of how leaderless mRNAs are recognized by mitoribosomes. By using novel in vitro mitochondrial translation initiation assays, alongside biochemical and genetic characterization of cellular knockouts of mitochondrial translation factors, we describe unique features of translation initiation in human mitochondria. We show that in vitro, leaderless mRNA transcripts can be loaded directly onto assembled 55S mitoribosomes, but not onto the mitoribosomal small subunit (28S), in a manner that requires initiator fMet-tRNAMet binding. In addition, we demonstrate that in human cells and in vitro, mtIF3 activity is not required for translation of leaderless mitochondrial transcripts but is essential for translation of ATP6 in the case of the bicistronic ATP8/ATP6 transcript. Furthermore, we show that mtIF2 is indispensable for mitochondrial protein synthesis. Our results demonstrate an important evolutionary divergence of the mitochondrial translation system and further our fundamental understanding of a process central to eukaryotic metabolism.

Cytosolic and mitochondrial translation elongation are coordinated through the molecular chaperone TRAP1 for the synthesis and import of mitochondrial proteins

A complex interplay between mRNA translation and cellular respiration has been recently unveiled, but its regulation in humans is poorly characterized in either health or disease. Cancer cells radically reshape both biosynthetic and bioenergetic pathways to sustain their aberrant growth rates. In this regard, we have shown that the molecular chaperone TRAP1 not only regulates the activity of respiratory complexes, behaving alternatively as an oncogene or a tumor suppressor, but also plays a concomitant moonlighting function in mRNA translation regulation. Herein we identify the molecular mechanisms involved, demonstrating that TRAP1: i) binds both mitochondrial and cytosolic ribosomes as well as translation elongation factors, ii) slows down translation elongation rate, and iii) favors localized translation in the proximity of mitochondria. We also provide evidence that TRAP1 is coexpressed in human tissues with the mitochondrial translational machinery, which is responsible for the synthesis of respiratory complex proteins. Altogether, our results show an unprecedented level of complexity in the regulation of cancer cell metabolism, strongly suggesting the existence of a tight feedback loop between protein synthesis and energy metabolism, based on the demonstration that a single molecular chaperone plays a role in both mitochondrial and cytosolic translation, as well as in mitochondrial respiration.

Assembly of the Mitochondrial Translation Initiation Complex

Mitochondria maintain their own translational machinery that is responsible for the synthesis of essential components of the oxidative phosphorylation system. The mammalian mitochondrial translation system differs significantly from its cytosolic and bacterial counterparts. Here, we describe detailed protocols for efficient in vitro reconstitution of the mammalian mitochondrial translation initiation complex, which can be further used for mechanistic analyses of different aspects of mitochondrial translation.

Dynamics of intracellular stress-induced tRNA trafficking

Stress is known to induce retrograde tRNA translocation from the cytoplasm to the nucleus but translocation kinetics and tRNA-spatial distribution have not been characterized previously. We microinject fluorescently-labeled tRNA into living cells and use confocal microscopy to image tRNA spatial distribution in single cells at various levels of starvation and to determine translocation rate constants. Retrograde tRNA translocation occurs reversibly, within minutes after nutrition depletion of the extracellular medium. Such nutritional starvation leads to down-regulation of tRNA nuclear import and nearly complete curtailment of its nuclear export. Nuclear tRNA accumulation is suppressed in cells treated with the translation inhibitor puromycin, but is enhanced in cells treated with the microtubule inhibitor nocodazole. tRNA in the cytoplasm exhibits distinct spatial distribution inconsistent with diffusion, implying that such distribution is actively maintained. We propose that tRNA biological complexes and/or cytoplasmic electric fields are the likely regulators of cytoplasmic tRNA spatial distribution.

New crystal forms ofEscherichia coliandSaccharomyces cerevisiaesoluble inorganic pyrophosphatases

We have obtained new crystal forms of Escherichia coli and Saccharomyces cerevisiae soluble inorganic pyrophosphatase with and without substrate, competitive inhibitor and divalent cation. They diffract to higher resolution than any forms previously reported. The best E. coli crystals are in space group R32 with cell dimensions of 111.4 x 111.4 x 76.6 A and diffract to 2.0 A. The best S. cerevisiae crystals were grown from a mixture of PEG 1000 and 4000 in the presence of metal ions. They are in space group P2(1)2(1)2(1), have cell dimensions of 54.2 x 68.5 x 161.7 A and diffract to 1.8 A.