The amino acid at position 97 is involved in folding and surface expression of HLA-B27

Abstract

HLA-B27 confers susceptibility to ankylosing spondylitis (AS) but the mechanism linking this association remains unknown. Other properties unrelated to its natural role of antigen presenting function may be important in disease pathogenesis. We determined here the impact of N97D substitution on the folding and expression of HLA-B*2704 transfected in the 721.221 cell line. The mutation at position 97 abolishes the surface expression of non-conformational (HC10) and conformational (ME1) forms. The expression of ME1 forms was found to be absent in B*2704 N97D by immunoprecipitation and flow cytometry of fixed and permeabilized cell experiments with the conformation-sensitive ME1 antibody. However, immunoblotting cell lysates with HC10 revealed the presence of unfolded heavy chain (HC) and HC-dimer forms. The impact of the N97D mutation in the exit from the endoplasmic reticulum (ER) was analysed by western blot after endoglycosidase-H treatment, and it was found that B*2704 N97D was retained and accumulated as unfolded molecules. We tested for mutant association with transporter associated with antigen processing (TAP), calnexin (CNX), calreticulin (CLR) and beta2 microglobulin (beta2m). The wild-type B*2704 and N97D mutants were associated with TAP, CNX and CLR, although HC10 forms of mutant N97D interact more weakly with TAP. Only folded molecules of HLA-B*2704 were associated with beta2m. Surprisingly, the peptide-binding assay demonstrated the ability of unfolded N97D molecules to bind high-affinity peptides. It has been suggested that AS may arise because of aberrant folding of HLA-B27 molecules within the ER. Future work must therefore aim to clarify the functional connection between the unfolded protein response pathway in response to the accumulation of HLA-B27 in the ER. This mutant could be useful as a model for the misfolding of HLA-B27.