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-B*1402 and B*1403, 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 B*1402, B*1403, 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 B*1402 and B*1403 were similar, and both were faster and more efficient than B*2705, but some unfolded heavy chains from both B14 subtypes remained in the endoplasmic reticulum (ER) with a long half-life. The export rates of B*1402 and B*1403 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 B*2705 than for B*1402 and higher for B*1402 than for B*1403, suggesting that the repertoires of the B*1402-bound peptide and especially the B*1403-bound peptide were less optimized than that of B*2705.


CONCLUSIONS
Our results indicate that the folding, maturation, and stability of B*1403 differ more from B*2705 than from B*1402. Thus, these features cannot account for the fact that only the 2 former allotypes are associated with AS.
Merino
Elena
E
Centro de Biología Molecular Severo Ochoa (Consejo Superior de Investigaciones Científicas and Universidad Autónoma de Madrid), Universidad Autónoma, Madrid, Spain.
Galocha
Begoña
B
Vázquez
Miriam N
MN
López de Castro
José A
JA
D000483:Alleles; D000954:Antigens, Surface / Q000737:chemistry / Q000378:metabolism; D002460:Cell Line; D019281:Dimerization; D004721:Endoplasmic Reticulum / Q000378:metabolism; D015235:HLA-B Antigens / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D059909:HLA-B14 Antigen; D015796:HLA-B27 Antigen / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D006358:Hot Temperature; D006801:Humans; D047468:Immunoprecipitation; D008214:Lymphocytes / Q000166:cytology; D026901:Membrane Transport Proteins / Q000378:metabolism; D017510:Protein Folding; D021381:Protein Transport / Q000276:immunology; D013167:Spondylitis, Ankylosing / Q000276:immunology* / Q000378:metabolism*; D014162:Transfection
D000954:Antigens, Surface; D015235:HLA-B Antigens; C464936:HLA-B*27:05 antigen; D059909:HLA-B14 Antigen; D015796:HLA-B27 Antigen; C054109:HLA-B65 antigen; D026901:Membrane Transport Proteins; C107762:tapasin