Most nucleocytoplasmic traffic through the nuclear pore complex is mediated by soluble receptors of the importin/exportin or karyopherin family. mRNA export is unique in that no receptor of this family has been implicated in trafficking of the bulk of mRNAs. Instead, many diverse proteins have been linked to mRNA export, but an all-encompassing model remains elusive. Understanding how these proteins interact with each other is central to the development of such a model. Here, we have focused on the interactions between three proteins implicated in mRNA export, Nup98, Rae1/ Gle2, and TAP. We have defined the binary complexes that form among these proteins. We find that Gle2 requires two sites within TAP for stable interaction. Strikingly, rather than a general affinity for all nucleoporin FG repeats, TAP has highest affinity for a specific region within the GLFG domain of Nup98, indicating that not all repeats are identical in function. We have established that the ternary complex can form through simultaneous binding of both Gle2 and TAP to adjacent sites on Nup98. In contrast, Nup98 competes with TAP for Gle2 binding; when bound to Nup98, Gle2 no longer interacts directly with TAP. From these interactions, we propose that Gle2 may act to deliver TAP to Nup98 and that this may represent the first in a series of interactions between an export complex and a nucleoporin.Traffic between the nucleus and cytoplasm proceeds exclusively through the nuclear pore complex, a multiprotein complex of ϳ125 MDa in vertebrate cells (1-3). This immense structure is assembled from surprisingly few constituent proteins or nucleoporins; ϳ30 proteins make up the NPC in both yeast and metazoans (4,5). This seemingly low number may reflect the highly symmetrical nature of the pore. The bulk of the pore structure has 8-fold symmetry around a central axis perpendicular to the plane of the envelope with most nucleoporins present in 8, 16, or 32 copies per pore. The exact mechanism by which the nuclear pore carries out translocation is still uncertain, although several models are under investigation.In addition to the pore complex itself, a number of soluble proteins are essential for nucleocytoplasmic transport. A large family of nuclear transport receptors termed importins/exportins or karyopherins recognize cargoes through specific import or export signal sequences (6 -8). Assembly and disassembly of the receptors and their cargo are highly regulated by the GTPase Ran. Strict compartmentalization of the Ran accessory factors, the cytoplasmic Ran GAP and the nuclear Ran GEF (RCC1), generates a Ran gradient across the nuclear envelope, which regulates the assembly and disassembly of receptorcargo complexes (9). Different members of the importin/ exportin receptor family specialize in the trafficking of specific cargoes. Import of the classical, basic nuclear localization sequence is mediated by the importin ␣/␤ heterodimer, and other protein classes, such as ribosomal proteins, have dedicated import receptors. The workhorse of nuclear pr...

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