Identifying target genes of the aryl hydrocarbon receptor nuclear translocator (Arnt) using DNA microarray analysis

The aryl hydrocarbon receptor nuclear translocator (Arnt) is a basic helix-loop-helix (bHLH) protein that also contains a Per-Arnt-Sim (PAS) domain. In addition to forming heterodimers with many other bHLH-PAS proteins, including the aryl hydrocarbon receptor (AhR) and hypoxia-inducible factors 1alpha, 2alpha and 3alpha, Arnt can also form homodimers when expressed from its cDNA in vitro or in vivo. However, target genes of the Arnt/Arnt homodimer remain to be identified. In this study, we have elucidated the profile of genes responsive to the reintroduction of Arnt expression in an Arnt-deficient mouse hepatoma cell line (c4), using DNA microarray analysis. The expression of 27 genes was upregulated by 1.5-fold or more in c4 cells infected with a retroviral vector expressing mouse Arnt, while no genes were found to be downregulated. Among the upregulated genes, BCL2/adenovirus E1B 19 kDa-interacting protein 1 (NIP3), serine (or cysteine) proteinase inhibitor, clade E, member 1 (PAI1), and N-myc downstream regulated-like (NDR1), were confirmed to be induced by Arnt using real-time PCR. We also found that the 5' promoter region of 15 out of 20 upregulated genes contain the type 2 E-box 5'-CACGTG-3' Arnt/Arnt binding sequence, consistent with the notion that they represent target genes for Arnt.     

The structure of the NXF2/NXT1 heterodimeric complex reveals the combined specificity and versatility of the NTF2-like fold

NXF1-like members of the NXF (nuclear export factor) family orchestrate bulk nuclear export of mRNA, while functionally distinct NXF variant proteins carry out separate substrate-specific and tissue-specific RNA regulation. Metazoan organisms possess at least one NXF1-like gene and one or more NXF variant genes. Heterodimerization of both proteins with the NXT (NTF2-related export) protein is central to NXF family function; however, given the multiplicity of NXF/NXT complexes, the specificity and mechanism of heterodimerization remain unclear. Here, we report the structural and functional analyses of the Caenorhabditis elegans NXF variant ceNXF2 bound to ceNXT1. Contacts crucial for NXF/NXT heterodimer stability and specificity, including a probable site for phosphoregulation, have been identified. The ceNXF2 NTF2 domain bears at least two nucleoporin (Nup) binding pockets necessary for the colocalization of ceNXF2/ceNXT1 at the nuclear envelope. Unexpectedly, one Nup binding pocket is formed at the heterodimer interface of the ceNXF2/ceNXT1 complex, demonstrating that NXT binding directly regulates NXF function.