“Ping-Pong” Interactions between Mitochondrial tRNA Import Receptors within a Multiprotein Complex

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Abstract
The mitochondrial genomes of a wide variety of species contain an insufficient number of functional tRNA genes, and translation of mitochondrial mRNAs is sustained by import of nucleus-encoded tRNAs. In Leishmania, transfer of tRNAs across the inner membrane can be regulated by positive and negative interactions between them. To define the factors involved in such interactions, a large multisubunit complex (molecular mass, ∼640 kDa) from the inner mitochondrial membrane of the kinetoplastid protozoon Leishmania, consisting of ∼130-Å particles, was isolated. The complex, when incorporated into phospholipid vesicles, induced specific, ATP- and proton motive force-dependent transfer of Leishmania tRNATyr as well as of oligoribonucleotides containing the import signal YGGYAGAGC. Moreover, allosteric interactions between tRNATyr and tRNAIle were observed in the RNA import complex-reconstituted system, indicating the presence of primary and secondary tRNA binding sites within the complex. By a combination of antibody inhibition, photochemical cross-linking, and immunoprecipitation, it was shown that binding of tRNAIle to a 21-kDa component of the complex is dependent upon tRNATyr, while binding of tRNATyr to a 45-kDa component is inhibited by tRNAIle. This “ping-pong” mechanism may be an effective means to maintain a balanced tRNA pool for mitochondrial translation.