Messenger RNAs are exported from the nucleus as
large ribonucleoprotein complexes (mRNPs). To date, proteins
implicated in this process include TAP/Mex67p and RAE1/Gle2p
and are distinct from the nuclear transport receptors of
the β-related, Ran-binding protein family. Mex67p is
essential for mRNA export in yeast. Its vertebrate homolog
TAP has been implicated in the export of cellular mRNAs
and of simian type D viral RNAs bearing the constitutive
transport element (CTE). Here we show that TAP is predominantly
localized in the nucleoplasm and at both the nucleoplasmic
and cytoplasmic faces of the nuclear pore complex (NPC).
TAP interacts with multiple components of the NPC including
the nucleoporins CAN, Nup98, Nup153, p62, and with three
major NPC subcomplexes. The nucleoporin-binding domain
of TAP comprises residues 508–619. In HeLa cells,
this domain is necessary and sufficient to target GFP-TAP
fusions to the nuclear rim. Moreover, the isolated domain
strongly competes multiple export pathways in vivo, probably
by blocking binding sites on the NPC that are shared with
other transport receptors. Microinjection experiments implicate
this domain in the export of specific CTE-containing RNAs.
Finally, we show that TAP interacts with transportin and
with two proteins implicated in the export of cellular
mRNAs: RAE1/hGle2 and E1B-AP5. The interaction of TAP with
nucleoporins, its direct binding to the CTE RNA, and its
association with two mRNP binding proteins suggest that
TAP is an RNA export mediator that may bridge the interaction
between specific RNP export substrates and the NPC.