5 S rRNA and tRNA import into human mitochondria. Comparison of in vitro requirements.

In vivo, human mitochondria import 5 S rRNA and do not import tRNAs from the cytoplasm. We demonstrated previously that isolated human mitochondria are able to internalize a yeast tRNA(Lys) in the presence of yeast soluble factors. Here, we describe an assay for specific uptake of 5 S rRNA by isolated human mitochondria and compare its requirements with the artificial tRNA import. The efficiency of 5 S rRNA uptake by isolated mitochondria was comparable with that found in vivo. The import was shown to depend on ATP and the transmembrane electrochemical potential and was directed by soluble proteins. Blocking the pre-protein import channel inhibited internalization of both 5 S rRNA and tRNA, which suggests this apparatus be involved in RNA uptake by the mitochondria. We show that human mitochondria can also selectively internalize several in vitro synthesized versions of yeast tRNA(Lys) as well as a transcript of the human mitochondrial tRNA(Lys). Either yeast or human soluble proteins can direct this import, suggesting that human cells possess all factors needed for such an artificial translocation. On the other hand, the efficiency of import directed by yeast or human protein factors varies significantly, depending on the tRNA version. Similarly to the yeast system, tRNA(Lys) import into human mitochondria depended on aminoacylation and on the precursor of the mitochondrial lysyl-tRNA synthetase. 5 S rRNA import was also dependent upon soluble protein(s), which were distinct from the factors providing tRNA internalization.