Characterization of the LxxLL motif in the aryl hydrocarbon receptor: Effects on subcellular localization and transcriptional activity

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that acts in concert with the AhR nuclear translocator (ARNT). Subcellular localization and transcriptional activation of target genes are mainly regulated by ligands such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We have previously reported that AhR migrates in cells as a nucleocytoplasmic shuttling protein mediated by its nuclear localization and export signals. A short sequence motif LxxLL (L is leucine and x is any amino acid) found in transcriptional co-activators has been reported to mediate the binding to liganded nuclear receptors. The role of the two LxxLL motifs, AhR[50-54] and [224-228], has now been analyzed by determining the localization of AhR and its transcriptional activity with Leu to Ala mutations in full-length AhR. Immunocytostaining revealed that mutation of the motif at AhR[50-54] promotes the efficiency of nuclear localization in the absence of ligand without altering HSP90 and ARA9 binding or nuclear export activity. Furthermore, this mutation decreases the transcriptional activity of the AhR/ARNT system, which is likely due to the suppression of AhR/ARNT/XRE complex formation. Another LxxLL motif at AhR[224-227] affects neither the subcellular localization nor transcriptional activity. These results indicate that the LxxLL motif at AhR[50-54] is important for the regulation of AhR activity.