The role of exportin-t in selective nuclear export of mature tRNAs

Exportin-t (Xpo-t) is a vertebrate nuclear export receptor for tRNAs that b inds tRNA cooperatively with GTP-loaded Ran, Xpo-t antibodies are shown to efficiently block tRNA export from Xenopus oocyte nuclei suggesting that it is responsible for at least the majority of tRNA export in these cells, We examine the mechanism by which Xpo-t-RanGTP specifically exports mature tR NAs rather than other forms of nuclear RNA, including tRNA precursors. Chem ical and enzymatic footprinting together with phosphate modification interf erence reveals an extensive interaction between the backbone of the T Psi C and acceptor arms of tRNA(Phe) and Xpo-t-RanGTP. Analysis of mutant or pre cursor tRNA forms demonstrates that, aside from these recognition elements, accurate 5' and 3' end-processing of tRNA affects Xpo-t-RanGTP interaction and nuclear export, while aminoacylation is not essential. Intron-containi ng, end-processed, pre-tRNAs can be bound by Xpo-t-RanGTP and are rapidly e xported from the nucleus if Xpo-t is present in excess, These results sugge st that at least two mechanisms are involved in discrimination of pre-tRNAs and mature tRNAs prior to nuclear export.