NUCLEAR-LOCALIZATION AND EXPORT SIGNALS OF THE HUMAN ARYL-HYDROCARBONRECEPTOR

The aryl hydrocarbon receptor (Ahr) is a ligand activated transcriptio n factor that binds DNA in the form of a heterodimer with the Ahr nucl ear translocator (hypoxia-inducible factor 1 beta). We found in this s tudy that Ahr contains both nuclear localization and export sig nals i n the NH2-terminal region. A fusion protein composed of beta-galactosi dase and full-length Ahr translocates from the cytoplasm to the nucleu s in a ligand-dependent manner. However, a fusion protein lacking the PAS (Per-Ahr nuclear translocator-Sim homology) domain of the Ahr show ed strong nuclear localization activity irrespective of the presence o r absence of ligand. A minimum bipartite Ahr nuclear localization sign al (NLS) consisting of amino acid residues 13-39 was identified by mic roinjection of fused proteins with glutathione S-transferase-green flu orescent protein, A NLS having mutations in bipartite basic amino acid s lost nuclear translocation activity completely, which may explain th e reduced binding activity to the NLS receptor, PTAC58. A 21-amino aci d peptide (residues 55-75) containing the Ahr nuclear export signal is sufficient to direct nuclear export of a microinjected complex of glu tathione S-transferase-Ahr-green fluorescent protein. These findings s trongly suggest that Ahr act as a ligand- and signal-dependent nucleoc ytoplasmic shuttling protein.