"Ping-pong" interactions between mitochondrial tRNA import receptors within a multiprotein complex

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 tRNA^Tyr as well as of oligoribonucleotides containing the import signal YGGYAGAGC. Moreover, allosteric interactions between tRNA^Tyr and tRNA^Ile 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 tRNA^Ile to a 21-kDa component of the complex is dependent upon tRNA^Tyr, while binding of tRNA^Tyr to a 45-kDa component is inhibited by tRNA^Ile. This "ping-pong" mechanism may be an effective means to maintain a balanced tRNA pool for mitochondrial translation.