Characterization of a novel isoform of alpha-nascent polypeptide-associated complex as IgE-defined autoantigen

The nascent polypeptide-associated complex is required for intracellular translocation of newly synthesized polypeptides in eukaryotic cells. It may also act as a transcriptional coactivator in humans and various eukaryotic organisms and binds to nucleic acids. Recently, we provided evidence that a component of nascent polypeptide-associated complex, alpha-nascent polypeptide-associated complex, represents an IgE-reactive autoantigen for atopic dermatitis patients. By oligonucleotide screening we isolated a complete cDNA coding for a so far unknown alpha-nascent polypeptide-associated complex isoform from a human epithelial cDNA library. Southern blot hybridization experiments provided further evidence that alpha-nascent polypeptide-associated complex is encoded by a gene family. Recombinant alpha-nascent polypeptide-associated complex was expressed in Escherichia coli as a soluble, His-tagged protein, and purified via nickel affinity chromatography. By circular dichroism analysis it is demonstrated that purified recombinant alpha-nascent polypeptide-associated complex represents a folded protein of mixed alpha-helical and beta-sheet conformation with unusual high thermal stability and remarkable refolding capacity. Complete recombinant alpha-nascent polypeptide-associated complex (215 amino acids) and its 86 amino acid C-terminal fragment specifically bound IgE autoantibodies. Recombinant alpha-nascent polypeptide-associated complex also inhibited IgE binding to natural alpha-nascent polypeptide-associated complex, demonstrating the presence of common IgE epitopes between the recombinant and natural protein. Furthermore, recombinant alpha-nascent polypeptide-associated complex induced specific lymphoproliferative responses in peripheral blood mononuclear cells of a sensitized atopic dermatitis patient. As has been proposed for environmental allergens it is possible that T cell responses to IgE-defined autoantigens may contribute to the chronic skin manifestations in atopic dermatitis.