LETTER TO THE EDITOR Human mitochondrial tRNA is exported to the cytoplasm and associates with the Argonaute 2 protein ELISAVET MANIATAKI and ZISSIMOS MOURELATOS

Argonaute (Ago) proteins are the effector proteins of RNA interference (RNAi) and related silencing mechanisms that are mediated by small RNAs. Ago proteins bind directly to microRNAs (miRNAs) and to short interfering RNAs and are the core protein components of RNA induced silencing complexes (RISCs) and microRNPs (miRNPs). Here we report that an ~70-nt RNA associates specifically with immunopurified Ago2 expressed in human 293 cells. By directional cloning we identified this RNA as the mitochondrial tRNA (mt tRNA). Various exported (mt) tRNAs were detected in the cytosol of 293 cells, but Ago2 was found selectively bound to (mt) tRNA. The association in the cytosol of exported (mt) tRNA with Ago2 complements genetic and microscopic data that link mitochondria with RNAi-related components and events.