Aryl hydrocarbon receptor (AHR)/AhR nuclear translocator (ARNT) activity is unaltered by phosphorylation of a periodicity/ARNT/single-minded (PAS)-region serine residue

The aryl hydrocarbon nuclear translocator (ARNT) protein belongs to the family of basic helix-loop-helix (HLH)-periodicity/ ARNT/single-minded [Per/ARNT/Sim (PAS)] transcription factors and regulates a range of cellular processes by either homodimerizing or heterodimerizing with other basic HLH-PAS proteins. To date, it has been shown that both the HLH and PAS domains are required for aryl hydrocarbon receptor (AhR) ARNT heterodimerization and that phosphorylation of ARNT is also required for this heterodimerization. Presently, regulation of ARNT with respect to phosphorylation is poorly understood. In an earlier study, murine ARNT was shown to be a phosphoprotein, to display charge heterogeneity, and to have a shift in its predominant isoforms after heterodimerization with the AhR. It was hypothesized that this shift may represent a change in ARNT phosphorylation status. Metabolic [³²P]orthophosphate labeling of human ARNT-transfected COS-1 cells, in conjunction with phosphoamino acid analysis, Edman degradation, and phosphopeptide mapping, demonstrated that ARNT is predominantly phosphorylated on serine residues and that serine 348 (S348) in the PAS domain is phosphorylated. Alanine and glutamic acid substitutions were used to demonstrate that loss of phosphorylation at this site did not influence AhR-mediated xenobiotic response elements-driven or ARNT-mediated class B E-box-driven signaling. Additionally, the phosphorylation pattern of ARNT was unaltered after AhR heterodimerization. Although phosphorylation of S348 did not modulate AhR-ARNT or ARNT-ARNT signaling, phosphorylation of this PAS-region serine residue may be important in other ARNT-mediated gene expression systems.