Substitution of aspartic acid at beta 57 with alanine alters MHC class II peptide binding activity but not protein stability: HLA-DQ (alpha 1*0201,beta 1*0302) and (alpha 1*0201,beta 1*0303)

In class II major histocompatibility complex (MHC) proteins, residue beta 5 7 is usually aspartic acid. Alleles carrying serine, valine, or alanine ar this position are strongly correlated with the development of insulin-depen dent diabetes mellitus (IDDM). Asp beta 57 participates in a conserved salt bridge that bridges the alpha and beta subunits in the peptide-binding sit e. It has been proposed that the correlation between IDDM and MHC alleles l acking Asp beta 57 may be due to an instability of the protein caused by lo ss of this salt bridge. Using a pair of HLA-DQ proteins (alpha 1*0201, beta 1*0302) and (alpha 1*0201, beta 1*0303) differing only in having aspartic acid or alanine at position beta 57, we show that the polymorphism does not have a significant effect on protein stability for either the empty or pep tide-loaded forms. However, the circular dichroism spectra indicate that em pty and peptide-loaded Ala beta 57 proteins display slightly different seco ndary structures relative to their Asp beta 57 counterparts. A set: of thre e peptides shows different binding affinities for DQ(alpha 1*0201, beta 1*0 302) relative to DQ(alpha 1*0201, beta 1*0303). We propose that substitutio n of Asp beta 57 residue causes a local rearrangement within the DQ peptide -binding site that alters the peptide-binding specificity. This rearrangeme nt may help to explain the previously observed differences in SDS stability between Asp and non-Asp beta 57 DQ proteins. (C) American Society for Hist ocompatibility and Immunogenetics, 1999. Published by Elsevier Science Inc.