IMPORT OF RNA INTO LEISHMANIA MITOCHONDRIA OCCURS THROUGH DIRECT INTERACTION WITH MEMBRANE-BOUND RECEPTORS

Cytoplasmic tRNAs are imported into the kinetoplast mitochondrion of L eishmania, but the mechanism of import is unknown, particularly whethe r RNA is transferred as a ribonucleoprotein complex through the protei n import pathway or by a distinct receptor-mediated mechanism. Using i solated mitochondria, it was shown that a small, importable RNA, which is structurally homologous to tRNA, binds rapidly, specifically, and with high affinity to the mitochondrial surface in the absence of solu ble protein factors to form an import intermediate, Two classes of bin ding site of apparent K-d 0.3 and 10 an, respectively, were distinguis hed. tRNA from Leishmania, but not yeast, competitively inhibited the binding. Northwestern blot analysis revealed the presence of a 15-kDa RNA binding protein on the mitochondrial surface, whereas receptor bin ding was resistant to heparin and KCl, internalization was sensitive t o both reagents, These results are consistent with the presence of a d irect mechanism of receptor-mediated RNA import on Leishmania mitochon dria.