The role of human and Drosophila NXF proteins in nuclear mRNA ...

Introduction 12
Lipowsky et al., 1999). As intron removal has been suggested to occur normally before 5'
end processing, this inability to discriminate between intron-containing and intron-free
tRNAs should not result in export of immature molecules under natural conditions (Lund
and Dahlberg, 1998). It has also been proposed that aminoacylation facilitates nuclear
export or is even a prerequisite for it (Grosshans et al., 2000; Lund and Dahlberg, 1998;
Sarkar et al., 1999), although exportin-t can also bind to deacylated tRNAs (Arts et al.,
1998b; Lipowsky et al., 1999). This controversy can be explained by the existence of an
alternative tRNA export pathway which is aminoacylation-dependent (see below).
The functional homologue of exportin-t in S. cerevisiae is Los1p, which is also
implicated in tRNA export. However, Los1p is not essential for viability (Hurt et al.,
1987), and certain tRNA species are not affected when the LOS1 gene is disrupted
(Grosshans et al., 2000). This suggests that at least some tRNAs can reach the cytoplasm
independently of Los1p, either by passive diffusion (as suggested by Gorlich and Kutay,
1999) or by an alternative active export pathway (as suggested by Grosshans et al., 2000;
Bohnsack et al., 2002).
Recently, exportin5, which can form complexes with aminoacylated tRNAs in the
presence of RanGTP, has been proposed to mediate tRNA export in parallel with exportin-t
(Bohnsack et al., 2002). Exportin5 has also been shown to mediate nuclear export of the
translation elongation factor eEF1A, which binds exportin5 via aminoacylated tRNAs. As
exportin-t and exportin5 seem to recognize different sets of tRNAs, they may fulfill
complementary functions.
1.2.2 Nuclear export of rRNAs
Three of the four ribosomal RNAs (rRNAs) are produced as a single RNA
polymerase I transcript which is later extensively processed and modified to yield the 18S,
5.8S and 28S rRNAs. The 5S rRNA is transcribed by RNA polymerase III and recruited
separately to the assembling ribosome. About 80 ribosomal proteins associate with the
primary RNA polymerase I transcript in the nucleolus to generate a pre-ribosomal particle
which undergoes a series of maturation steps including the splitting into a large and a small
subunit. The particles are then exported to the cytoplasm, most likely as separate
precursors of the small (40S, containing the 18S rRNA) and large (60S, containing 28S,
5.8S and 5S rRNA) subunits (reviewed by: Aitchison and Rout, 2000; Lei and Silver,
2002b). The nuclear export signals are generally believed to be provided by the proteins in
the ribosomal particles rather than by the RNAs themselves.
In S. cerevisiae, both the export of the large and the small subunit have been shown
to be dependent on the RanGTPase system, suggesting that members of the importin β-like
family are involved (Hurt et al., 1999; Moy and Silver, 1999, 2002). Particularly, Crm1p
seems to play a key role, as the drug leptomycin B (LMB) which inhibits Crm1p interferes