The ins and outs of tRNA transport.

Transfer RNAs (tRNAs) are involved in many cellular functions distributed throughout the cellular space. In addition to their role in protein translation, these molecules are engaged in tasks as diverse as the regulation of gene expression, amino-acid synthesis, protein degradation, cell-wall synthesis, porphyrin bio synthesis, priming of replication, RNA interference and the transport of macromolecules. The nucleus and the mitochondria of eukaryotic cells, as well as the chloroplasts in plants, contain their own genome that encodes a range of proteins and nucleic acids. Early evidence suggested that some of the RNA content of mitochondria could be trans cribed from non-mitochondrial DNA (Suyama, 1967). It is now well established that a variable number of tRNA species present in the mitochondria are indeed nucleus-encoded (Fig 1). The pheno menon of mitochondrial tRNA import has been reported in plants, marsupials, the yeast Saccharomyces cerevisiae, and the protozoa Leishmania, Trypanosoma and Tetrahymena. When certain tRNA species are not encoded by the mitochondrial genome, there is a predictable requirement for the import of nucleusencoded tRNAs into the mitochondria (Schneider & MarechalDrouard, 2000). This is the case, for example, for the mitochondria of Leishmania, which are completely devoid of tRNA-encoding genes. However, there are a few examples of tRNA import into mitochondria that already have a complete set of mitochondriaencoded tRNAs. Other tRNA transport systems have also been identified (Fig 1), which include their retrograde transport from the cytoplasm to the nucleus (Takano et al, 2005; Shaheen & Hopper, 2005), the export of mitochondria-encoded tRNA to the cytoplasm (Maniataki & Mourelatos, 2005), and the packaging of tRNAs into retroviruses (Waters & Mullin, 1977). The roles of some of these transport systems have yet to be defined. In yeast, the cytoplasmic, nucleus-encoded tRNA(Lys,CUU) is imported into the mitochondria, although the mitochondrial genome encodes the full set of tRNAs, including an authentic tRNALys (Tarassov & Entelis, 1992). Furthermore, the yeast mitochondrial lysyl-tRNA synthetase (mLysRS) cannot aminoacylate the nucleus-encoded tRNA(Lys,CUU). This is because the mitochondrial, prokaryotic-type enzyme requires an adenine at position 73 to aminoacylate its cognate tRNA, whereas the eukaryotic enzyme accepts any of the four nucleotides at this position. Therefore, the physiological significance of this import is unclear. When the import of a tRNA species is not a prerequisite for mitochondrial protein synthesis, one interesting possibility is that import of that tRNA is a means to transport a protein into the mitochondrial compartment in a co-import mechanism. This suggests that the co-imported protein is not able to use the protein-import system. Such a mechanism has been described for the nuclear export of human elongation factor EF1A, which is efficiently excluded from the nucleus by a co-transport process involving exportin 5 and tRNA as a carrier molecule (Bohnsack et al, 2002). This explanation might hold true in some circumstances for mitochondrial import, but is not a universal answer to the problem. Indeed, the nucleusencoded tRNA(Lys,CUU) referred to above is actually co-imported through the protein-import pathway. The essential requirement of tRNA import for protein synthesis to occur in the mitochondria of some organisms stimulated the initial research in this field (Marechal-Drouard et al, 1988; Hancock & Hajduk, 1990; Lye et al, 1993; Schneider et al, 1994). However, no unifying mechanism has emerged from the various in vitro and in vivo studies performed so far. In S. cerevisiae, cytosolic factors are involved in this process. Co-import of a tRNA(Lys,CUU) with the mitochondrial precursor of LysRS has been established (Tarassov et al, 1995), but recruitment of this tRNA to the mitochondrial compartment also requires the glycolytic enzyme, enolase (Entelis et al, 2006). In this system, it is likely that the protein-import channels are used to translocate tRNA(Lys,CUU). By contrast, import of tRNA in Leishmania mitochondria does not require cytosolic factors and the existence of a receptor-mediated import pathway has been uncovered (Mahapatra et al, 1994; Mahapatra & Adhya, 1996). A large multisubunit complex, isolated from the inner mitochondrial membrane of this organism, promotes tRNA import when incorporated into phospholipid vesicles (Bhattacharyya et al, 2003). Transport is ATP-dependent. Previous studies identified two components of this RNA import complex—RIC1 and RIC8A—as tRNA import receptors. RIC1 is homologous to the α-subunit of Nucleus Mitoondrion