Role of an RNA-binding protein in import of tRNA into Leishmania mitochondria

Nuclear-encoded cytoplasmic tRNAs are imported into the mitochondria of kinetoplastid protozoa by an unknown mechanism, In a Leishmania in organello system, ATP-dependent import of a cloned, unspliced tRNA(Tyr)(GUA) transcript was demonstrated by protection from ribonuclease, whereas import of a tRNA(Gln) (CUG) transcript was much less efficient, Specific binding of tRNA(Tyr) to two mitochondrial surface proteins of 15 and 22 kilodaltons was observed. Tubulin antisense-binding protein (TAB), the 15-kilodaton species, was purified to apparent homogeneity by RNA affinity chromatography. TAB forms stable complexes with the D stem-loop region of tRNA(Tyr). Immunocytochemical and cell fractionation experiments, combined with limited proteolysis, suggested the association of TAB with the outer mitochondrial membrane, Importantly, anti-TAB antibody specifically inhibited binding as well as import of tRNA(Tyr) and of a synthetic structural homolog, These results support the role of TAB as a membrane-bound receptor or carrier for RNA import into Leishmania mitochondria.