Aryl hydrocarbon receptor (AhR)/AhR nuclear translocator (ARNT) activity is unaltered by phosphorylation of a periodicity/ARNT/single-minded (PAS)-region serine residue
Sl. Levine et Gh. Perdew, Aryl hydrocarbon receptor (AhR)/AhR nuclear translocator (ARNT) activity is unaltered by phosphorylation of a periodicity/ARNT/single-minded (PAS)-region serine residue, MOLEC PHARM, 59(3), 2001, pp. 557-566
The aryl hydrocarbon nuclear translocator (ARNT) protein belongs to the fam
ily of basic helix-loop-helix (HLH)-periodicity/ARNT/single-minded [Per/ARN
T/Sim (PAS)] transcription factors and regulates a range of cellular proces
ses by either homodimerizing or heterodimerizing with other basic HLH-PAS p
roteins. To date, it has been shown that both the HLH and PAS domains are r
equired for aryl hydrocarbon receptor (AhR) ARNT heterodimerization and tha
t phosphorylation of ARNT is also required for this heterodimerization. Pre
sently, regulation of ARNT with respect to phosphorylation is poorly unders
tood. In an earlier study, murine ARNT was shown to be a phosphoprotein, to
display charge heterogeneity, and to have a shift in its predominant isofo
rms after heterodimerization with the AhR. It was hypothesized that this sh
ift may represent a change in ARNT phosphorylation status. Metabolic [P-32]
orthophosphate labeling of human ARNT-transfected COS-1 cells, in conjunct
ion with phosphoamino acid analysis, Edman degradation, and phosphopeptide
mapping, demonstrated that ARNT is predominantly phosphorylated on serine r
esidues and that serine 348 (S348) in the PAS domain is phosphorylated. Ala
nine and glutamic acid substitutions were used to demonstrate that loss of
phosphorylation at this site did not influence AhR-mediated xenobiotic resp
onse elements-driven or ARNT-mediated class B E-box-driven signaling. Addit
ionally, the phosphorylation pattern of ARNT was unaltered after AhR hetero
dimerization. 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.