ArnT is a member of the GT-C family of glycosyltransferases, and it has a similar fold to a bacterial oligosaccharyltransferase (OST) from Campylobacter lari (PglB) and to an archaeal OST from Archaeoglobus fulgidus (AglB). This entry represents the C-terminal periplasmic domain of Arnt proteins [].
The Ah (dioxin) receptor nuclear translocator protein (Arnt) belongs to the basichelix-loop-helix (bHLH) family of transcription factors and is required forAh receptor function. The Ah receptor binds, and mediates the carcinogeniceffects of a variety of environmental pollutants, including 2,3,7,8-tetrachlorodibenzo-p-dioxin and polycyclic aromatic hydrocarbons. Arnt is a structural component of theXRE-binding form of the Ah receptor. These proteins are class VII members of the basic helix-loop-helix (bHLH) family. The bHLH domain may be responsible for interacting with both the XRE and the ligand-bindingsubunit [, ].The activated Ah receptor and Arnt protein bind DNA as a heterodimer. Bothproteins contain PAS homology regions, which in Drosophila PER and SIMproteins function as dimerisation domains [].
Aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that mediates the biological and toxic effects of dioxin-like compounds []. Upon ligand binding, the receptor translocates into the nucleus where it heterodimerises with ARNT (AHR nuclear translocator) []. The aryl hydrocarbon receptor repressor (AHRR) competes with AHR for the binding of ARNT, and thereby repressing AHR's transcription activity [].
Aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that mediates the biological and toxic effects of dioxin-like compounds []. Upon ligand binding, the receptor translocates into the nucleus where it heterodimerises with ARNT (AHR nuclear translocator) []. The aryl hydrocarbon receptor repressor (AHRR) competes with AHR for the binding of ARNT, and thereby repressing AHR's transcription activity [].
Aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that mediates the biological and toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), dioxin-like compounds (DLC), as well as some drugs and endogenous tryptophan metabolites []. Without ligands, AHR is part of the Hsp90/XAP2 (heat shock protein 90/hepatitis B virus X-associated protein) multi-chaperone protein complex. Upon ligand binding, the receptor dissociates from the chaperone complex and translocates into the nucleus where it heterodimerises with ARNT (AHR nuclear translocator). The respective heterodimeric complex then modulates the cell's transcriptional activity by binding to specific xenobiotic response elements (XREs) in the promoters of AHR target genes [, ].
In Drosophila, single-minded (sim) is a transcription factor that acts as the master regulator of neurogenesis. Two mammalian homologues of Sim which have been identified, Sim1 and Sim2, are novel heterodimerisation partners for ARNT in vitro, and may function both as positive and negative transcriptional regulators in vivo, during embryogenesis and in the adult organism []. SIM2 is thought to contribute to some specific Down syndrome phenotypes []. There is a high level of homology among mammalian and Drosophila sim proteins in their amino-terminal half where the conserved bHLH, PAS () and PAC motifs are present (). The PAC region occurs C-terminal to the PAS domains and are proposed to contribute to the PAS domain fold [, , ]. In contrast, the carboxy-terminal parts are only conserved in vertebrates []. The Sim1 C terminus contains a Ser-rich region, whereas the Sim2 C terminus both contain Ser/Thr-rich regions, Pro/Ser-rich regions, Pro/Ala-rich regions, and positively charged regions. Sim2s, a splice variant of Sim2, still contains the Ser/Thr- and Pro/Ser-rich regions shown to harbor repressive activities, but is missing the Pro/Ala-rich repressor region [, , ].
