BIOCHEMICAL-CHARACTERIZATION OF THE HUMAN DIABETES-ASSOCIATED HLA-DQ8ALLELIC PRODUCT - SIMILARITY TO THE MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-II I-A(G7) PROTEIN OF NONOBESE DIABETIC MICE

The human HLA-DQ8 (A10301/B1*0302) allelic product manifests a strong association with insulin-dependent diabetes mellitus (IDDM). Previous biochemical studies of the major histocompatibility complex (MHC) cla ss II I-A(g7) protein of IDDM-prone non-obese diabetic mice produced c ontroversial results. To better define the biochemical properties of I DDM-associated MHC class II molecules, we analyzed DQ8 proteins, in co mparison to other DQ allelic products, by partially denaturing sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). We now report that DQ8 proteins have a normal peptide occupancy and lifespan in cells. Similar to I-A(g7), DQ8 proteins formed only a minor fractio n of SDS-stable complexes with peptides. Although this phenotype was n ot unique to DQ8, some DQ allelic products such as IDDM-protective DQ6 proteins were SDS resistant. The DQ9 allelic product, differing from DQ8 only at position (P) beta 57, was SDS stable, suggesting that non- Asp residues at beta 57 might decrease the SDS stability of DQ protein s. We identified a single peptide which specifically induced an SDS-st able conformation in DQ8 as well as in I-A(g7) molecules. The residues at anchor P1 in this peptide were found to influence the SDS stabilit y of both molecules. Together with our previous observation of similar binding motifs of I-A(g7) and DQ8, these results demonstrate an overa ll biochemical similarity of mouse and human diabetes-associated MHC c lass II molecules. This similarity might contribute to a common immuno logical mechanism of IDDM in both species.