NXT1 (p15) is a crucial cellular cofactor in TAP-dependent export of intron-containing RNA in mammalian cells

TAP, the human homologue of the yeast protein Mex67p, has been proposed to serve a role in mRNA export in mammalian cells. We have examined the abilit y of TAP to mediate export of Rev response element (RRE)-containing human i mmunodeficieney virus (HIV) RNA, a well-characterized export substrate in m ammalian cells. To do this, the TAP gene was fused in frame to either RevM1 0 or Rev Delta 78-79. These proteins are nonfunctional Rev mutant proteins that can bind to HIV RNA containing the RRE in vivo but are unable to media te the export of this RNA to the cytoplasm. However, the fusion of TAP to e ither of these mutant proteins gave rise to chimeric proteins that were abl e to complement Rev function. Significantly, cotransfection with a vector e xpressing NXT1 (p15), an NTF2-related cellular factor that binds to TAP, le d to dramatic enhancement of the ability of the chimeric proteins to mediat e RNA export. Mutant-protein analysis demonstrated that the domain necessar y for nuclear export mapped to the C-terminal region of TAP and required th e domain that interacts with NXT1, as well as the region that has been show n to interact with nucleoporins. RevM10-TAP function was leptomycin B insen sitive. In contrast, the function of this protein was inhibited by Delta CA N, a protein consisting of part of the FG repeat domain of CAN/Nup214. Thes e results show that TAP can complement Rev nuclear export signal function a nd redirect the export of intron-containing RNA to a CRM1-independent pathw ay. These experiments support the role of TAP as an RNA export factor in ma mmalian cells. In addition, they indicate that NXT1 serves as a crucial cel lular cofactor in this process.