The evolutionarily conserved mRNA export receptor Mex67/NXF1 associates with mRNAs through its adaptor, Yra1/REF, allowing mRNA ribonucleoprotein (mRNP) exit through nuclear pores. However, alternate adaptors should exist, since Yra1 is dispensable for mRNA export in Drosophila and Caenorhabditis elegans. Here we report that Mex67 interacts directly with Nab2, an essential shuttling mRNA-binding protein required for export. We further show that Yra1 enhances the interaction between Nab2 and Mex67, and becomes dispensable in cells overexpressing Nab2 or Mex67. These observations appoint Nab2 as a potential adaptor for Mex67, and define Yra1/REF as a cofactor stabilizing the adaptor-receptor interaction. Importantly, Yra1 ubiquitination by the E3 ligase Tom1 promotes its dissociation from mRNP before export. Finally, loss of perinuclear Mlp proteins suppresses the growth defects of Tom1 and Yra1 ubiquitination mutants, suggesting that Tom1-mediated dissociation of Yra1 from Nab2-bound mRNAs is part of a surveillance mechanism at the pore, ensuring export of mature mRNPs only.[Keywords: Mex67/NXF1; Nab2; Yra1/REF; Tom1; mRNA export; ubiquitination] Supplemental material is available at http://www.genesdev.org.

The adenovirus VA1 RNA (VA1), a 160-nucleotide (nt)-long RNA transcribed by RNA polymerase III, is efficiently exported from the nucleus to the cytoplasm of infected cells, where it antagonizes the interferon-induced antiviral defense system. We recently reported that nuclear export of VA1 is mediated by a cis-acting RNA export motif, called minihelix, that comprises a double-stranded stem (>14 nt) with a base-paired 5 end and a 3-8-nt protruding 3 end. RNA export mediated by the minihelix motif is Ran-dependent, which indicates the involvement of a karyopherin-related factor (exportin) that remained to be determined. Here we show using microinjection in Xenopus laevis oocytes that VA1 is transported to the cytoplasm by exportin-5, a nuclear transport factor for double-stranded RNA binding proteins. Gel retardation assays revealed that exportin-5 directly interacts with VA1 RNA in a RanGTP-dependent manner. More generally, in vivo and in vitro competition experiments using various VA1-derived, but also artificial and cellular, RNAs lead to the conclusion that exportin-5 preferentially recognizes and transports minihelix motif-containing RNAs.Nucleo-cytoplasmic transport of most RNAs and proteins is dependent on soluble receptors called karyopherins that can dock at and translocate through the nuclear pore complex. Interaction between cargo and karyopherin ␤ is governed by the GTPase Ran. The asymmetric distribution of the Ran regulatory proteins provides a steep gradient of RanGDP (cytoplasmic)/RanGTP (nuclear) across the nuclear envelope that ensures the directionality of nuclear transport (1, 2). Nuclear import receptors unload their cargo upon binding to RanGTP in the nucleus, whereas RanGTP is used to assemble export complexes which are in turn destabilized by dissociation of RanGTP in the cytoplasm (3, 4).Our understanding of the nuclear export of RNAs has been greatly facilitated by the study of viral RNAs. For this reason, we focused our attention on the adenovirus VA1 RNA (VA1), a 160-nt 1 -long RNA transcribed by RNA polymerase III that massively accumulates in the cytoplasm of infected cells. It serves to antagonize the interferon-induced cellular antiviral defense system. Indeed, VA1 binds and inhibits the doublestranded RNA-dependent protein kinase R (PKR), which otherwise phosphorylates eIF2␣ and leads to the inhibition of protein synthesis (5, 6). Adenovirus VA1 RNA contains a new cis-acting RNA export motif that comprises a double-stranded stem (Ͼ14 nt) with a base-paired 5Ј end and a 3-8-nt protruding 3Ј end and that can tolerate some mismatches and bends (7). This export signal, called minihelix, is present not only in VA1 but in a large family of small viral and cellular RNAs transcribed by polymerase III. RNA export mediated by the minihelix motif is Ran-dependent, which indicates the involvement of a karyopherin-related factor (exportin). This exportin is distinct from Crm1 and exportin-t (7, 8). Therefore, we sought to identify cellular factors that bind to and mediate the export of miniheli...

The mRNA nuclear export receptor Mex67͞Mtr2 is recruited to mRNAs through RNA-binding adaptors, including components of the THO͞ TREX complex that couple transcription to mRNA export. Here we show that the ubiquitin-associated (UBA) domain of Mex67 is not only required for proper nuclear export of mRNA but also contributes to recruitment of Mex67 to transcribing genes. Our results reveal that the UBA domain of Mex67 directly interacts with polyubiquitin chains and with Hpr1, a component of the THO͞TREX complex, which is regulated by ubiquitylation in a transcription-dependent manner. This interaction transiently protects Hpr1 from ubiquitin͞protea-some-mediated degradation and thereby coordinates recruitment of the mRNA export machinery with transcription and early messenger ribonucleoproteins assembly.nuclear export ͉ THO complex