A bifunctional tRNA import receptor from Leishmania mitochondria

In kinetoplastid protozoa, import of cytosolic tRNAs into mitochondria occurs through tRNAs interacting with membrane-bound proteins, the identities of which are unknown. The inner membrane RNA import complex of Leishmania tropica contains multiple proteins and is active for import in vitro. RIC1, the largest subunit of this complex, is structurally homologous to the conserved a subunit of F1 ATP synthase. The RIC1 gene complemented an atpA mutation in Escherichia coli. Antisense-mediated knockdown of RIC1/F1 alpha in Leishmania resulted in depletion of several mitochondrial tRNAs belonging to distinct subsets (types I and II) that interact cooperatively or antagonistically within the import complex. The knockdown-induced defect in import of type I tRNAs was rectified in a reconstituted system by purified RIC1/F1 alpha alone, but recovery of type II tRNA import additionally required a type I tRNA. RIC1/F1 alpha formed stable complexes with type I, but not type II, tRNAs through the cooperation of its nucleotide binding and C-terminal domains. Thus, RIC1/F1 alpha is a type I tRNA import receptor. As expected of a bifunctional protein, RIC1/F1 alpha is shared by both the import complex and by respiratory complex V. Alternative use of ancient respiratory proteins may have been an important step in the evolution of tRNA import.