Self and foreign peptides interact with intact and disassembled MHC class II antigen HLA-DR via tryptophan pockets

The acid release of endogenous peptides from immunoaffinity-pure human major histocompatibility complex (MHC) class II proteins HLA-DR1 is accompanied by an 18% decrease in intrinsic tryptophan fluorescence. The effect is totally reversible upon readdition of an autologous endogenous peptide fraction. High-performance size-exclusion chromatographic (HPSEC) binding and release studies with a nonfluorescent HLA-DR1-restricted influenza matrix peptide IM(18-29) prove the fact that Trp residues of the HLA protein change their fluorescence intensities. Since the far-UV circular dichroism spectra of HLA molecules before and after peptide release, DR1NAT] and DR1REL], show very small differences, we can rule out the breakdown of secondary structural elements under release conditions, although DRREL] consists of disassembled alpha- and beta-subunits, as evidenced by HPSEC. Quenching of DR1NAT] and DR1REL] using the neutral quencher acrylamide results in a 20% increase in total accessibility of the nine-residue Trp population whereas quenching by iodide yields only a 5% increase. Both results taken together tell us that two Trp residues, preferentially ones located in apolar pockets, become accessible upon the release of peptides. The significantly smaller fluorescence enhancement upon binding IM(18-29) of DR3REL], exclusively lacking Trp-9(beta 1), and the missing tendency to reassemble under the influence of IM(18-29) compared to DR1REL] suggest an important role for position 9(beta 1). The region around Trp-43(alpha 1) should be responsible for the binding of IM(18-29) to the alpha-subunits of DR1 and DR3, respectively, as verified by fluorometric HPSEC and SDS-PAGE. Obviously, our findings are in total agreement with the hypothetical MHC class II model, whereafter Trp-9(beta 1) and Trp-43(alpha 1) besides Trp-61(beta 1) are constituents of the binding groove of DR1. Extending the homology to MHC class I products, we postulate the existence of three hydrophobic pockets in the binding site of DR1 with the cited Trp residues being juxtaposed to contacting apolar peptide side chains in HLA-peptide complexes. According to the deduced two-residue-contact model the minimal consensus motif for DR1-restricted peptide antigens consists of two hydrophobic residues lying 14-16 A apart in the bound state of the peptide.