Disparate folding and stability of the ankylosing spondylitis-associated HLA-B*1403 and B*2705 proteins.

OBJECTIVE

To investigate the folding, assembly, maturation, and stability of HLA-B1402 and B1403, which differ by 1 amino acid change and are differentially associated with ankylosing spondylitis (AS), and to compare these features with those of B*2705.

METHODS

Stable transfectants expressing B1402, B1403, and B*2705 were used. Folding rates were estimated from the ratio of unfolded heavy chains to folded heavy chains that had been immunoprecipitated with specific antibodies in pulse-chase experiments. Heavy chain misfolding was measured as the half-life of endoglycosidase H (Endo H)-sensitive beta2-microglobulin-free heavy chains. Maturation/export rates were measured by acquisition of Endo H resistance. Association with calnexin or tapasin was analyzed by coprecipitation with chaperone-specific antibodies, and surface expression was estimated by flow cytometry. Thermostability of HLA-peptide complexes was assessed by immunoprecipitation after incubation at various temperatures. Heavy chain expression was quantified by Western blotting.

RESULTS

The folding rates of B1402 and B1403 were similar, and both were faster and more efficient than B2705, but some unfolded heavy chains from both B14 subtypes remained in the endoplasmic reticulum (ER) with a long half-life. The export rates of B1402 and B1403 were slow, and the heterodimers partially dissociated after exiting the ER, as revealed by significant amounts of Endo H-resistant and surface-expressed free heavy chains. Both interaction with tapasin and thermostability were higher for B2705 than for B1402 and higher for B1402 than for B1403, suggesting that the repertoires of the B1402-bound peptide and especially the B1403-bound peptide were less optimized than that of B2705.

CONCLUSION

Our results indicate that the folding, maturation, and stability of B1403 differ more from B2705 than from B*1402. Thus, these features cannot account for the fact that only the 2 former allotypes are associated with AS.