Eukaryotic tRNAs are synthesized in the nucleus
and need to be exported to the cytoplasm where they function
in translation. tRNA export is mediated by exportin-t,
which binds tRNA directly and with high affinity. tRNAs
are initially synthesized as precursor molecules. Maturation
to functional tRNA takes place in the nucleus, precedes
export, and includes trimming of the 5′ and 3′
ends, posttranscriptional addition of the 3′ CCA
end, nucleoside modifications, and in some cases splicing.
Here we address the question of how tRNA maturation is
coordinated with export and thus how cytoplasmic accumulation
of inactive maturation intermediates is avoided. This could,
in principle, be achieved by nuclear retention of immature
tRNA or by selective export of the fully mature form. We
show that exportin-t has a strong preference for tRNA with
correctly processed 5′ and 3′ ends and nucleoside
modification. tRNA recognition by exportin-t can thus be
considered as a quality control mechanism for these maturation
steps prior to tRNA export. Surprisingly however, exportin-t
can efficiently bind unspliced tRNA and intron-containing
tRNA is exported when the rate of splicing is slow. During
characterization of the exportin-t/tRNA interaction we
found that exportin-t recognizes features in the tRNA that
are conserved between prokaryotic and eukaryotic tRNAs.
Our data suggest that correct tRNA shape, the 5′
and 3′ terminal ends, and the TΨC loop are critical
for exportin-t binding.