The nuclear export of the unspliced type D retrovirus mRNA depends on the cis-acting constitutive transport RNA element (CTE) that has been shown to interact with the human TAP (hTAP) protein promoting the export of the CTE-containing mRNAs. We report here that hTAP is a 619-amino-acid protein extending the previously identified protein by another 60 residues at the N terminus and that hTAP shares high homology with the predicted rat and mouse TAP proteins. We found that hTAP is a nuclear protein that accumulates in the nuclear rim and the nucleoplasm. We further demonstrated that hTAP is able to shuttle between the nucleus and the cytoplasm. Identification of the signals responsible for nuclear import (NLS) and export (NES) revealed that they are distinct but partially overlapping. NLS and NES of hTAP are active transferable signals that do not share similarities with known elements. The C-terminal portion contributes further to hTAP's nuclear retention and contains a signal(s) for nuclear rim association. Taken together, our data show that hTAP is a dynamic protein capable of bidirectional trafficking across the nuclear envelope. These data further support hTAP's role as an export factor of the CTE-containing mRNAs.Posttranscriptional regulation is an essential regulatory step of many retroviruses and is necessary for virus production. This key regulatory step mediates the export of the unspliced, fulllength viral RNA, which requires the interaction of viral and/or cellular factors. This controlled export of the viral RNA to the cytoplasm ensures the availability of the genomic RNA for packaging into the progeny virions and the production of the Gag/Pol polyproteins. Among the best-studied export systems are those used by the simian type D retroviruses (SRV/D) and the lentiviruses, such as human immunodeficiency virus type 1 (HIV-1) (for reviews see references 6, 14, and 28).SRV/D expression is controlled by the essential cis-acting constitutive transport element (CTE) (5,12,54,63). The SRV/D CTE (11, 55) and a related CTE-like element in a murine intracisternal A-particle retroelement (54) fold into an extended RNA stem-loop structure containing two conserved internal loops and an AAGA bulge. These loops and the bulge, the spacing of the loops within the RNA element, as well as the overall secondary structure of the element, have been shown to be essential features for CTE function (11,55). Recently, we showed that the human TAP protein (hTAP) binds specifically to these internal loops and promotes nucleocytoplasmic transport of the CTE-containing intron lariat from the Xenopus oocyte nucleus (22). TAP had previously been identified as a factor binding to Tip, a herpesvirus saimiri protein responsible for cell transformation (60). The role of hTAP interaction with Tip is still unclear.Whereas the SRV/D retroviruses have been proposed to utilize the cellular hTAP protein to export their unspliced mRNA (22), HIV-1 uses the viral Rev protein to promote the transport of the Rev responsive element (RRE)-containing...

Neuregulin 1 (NRG1) is essential for the development and function of multiple organ systems, and its dysregulation has been linked to diseases such as cancer and schizophrenia. Recently, altered expression of a novel isoform (type IV) in the brain has been associated with schizophrenia-related genetic variants, especially rs6994992 (SNP8NRG243177). Here we have isolated and characterized full-length NRG1 type IV cDNAs from the adult and fetal human brain and identified novel splice variants of NRG1. Full-length type IV spans 1.8 kb and encodes a putative protein of 590 amino acids with a predicted molecular mass of ϳ66 kDa. The transcript consists of 11 exons with an Ig-like domain, an epidermal growth factor-like (EGF) domain, a ␤-stalk, a transmembrane domain, and a cytoplasmic "a-tail," placing it in the ␤1a NRG1 subclass. NRG1 type IV was not detected in any tissues except brain and a putative type IV NRG1 protein of 66 kDa was similarly brain-specific. Type IV transcripts are more abundantly expressed in the fetal brain, where, in addition to the full-length structure, two novel type IV variants were identified. In vitro luciferase-reporter assays demonstrate that the 5 promoter region upstream of type IV is functional, with differential activity associated with genetic variation at rs6994992, and that promoter competition may impact on type IV expression. Our data suggest that type IV is a unique brain-specific NRG1 that is differentially expressed and processed during early development, is translated, and its expression regulated by a schizophrenia risk-associated functional promoter or single nucleotide polymorphism (SNP).

In this study, we examined the mechanism of translation of the human immunodeficiency virus type 1 tat mRNA in eucaryotic cells. This mRNA contains the tat open reading frame (ORF), followed by rev and nef ORFs, but only the first ORF, encoding tat, is efficiently translated. Introduction of premature stop codons in the tat ORF resulted in efficient translation of the downstream rev ORF. We show that the degree of inhibition of translation of rev is proportional to the length of the upstream tat ORF. An upstream ORF spanning 84 nucleotides was predicted to inhibit 50% of the ribosomes from initiating translation at downstream AUGs. Interestingly, the distance between the upstream ORF and the start codon of the second ORF also played a role in efficiency of downstream translation initiation. It remains to be investigated if these conclusions relate to translation of mRNAs other than human immunodeficiency virus type 1 mRNAs. The strong inhibition of rev translation exerted by the presence of the tat ORF may reflect the different roles of Tat and Rev in the viral life cycle. Tat acts early to induce high production of all viral mRNAs. Rev induces a switch from the early to the late phase of the viral life cycle, resulting in production of viral structural proteins and virions. Premature Rev production may result in entrance into the late phase in the presence of suboptimal levels of viral mRNAs coding for structural proteins, resulting in inefficient virus production.

, and Nup214, indicating that the rim association occurs through components of the nuclear pore complex. In summary, Ce-NXF-1 belongs together with hTAP and Mex67p to a family of proteins that participate in mRNA export and can provide a direct molecular link between mRNAs and components of the nuclear pore complex. Therefore, despite differences in mRNA metabolism between these species, they utilize a conserved mRNA transport mechanism.