SPECIFIC BINDING OF ANTIGENIC PEPTIDES TO SEPARATE ALPHA-CHAINS AND BETA-CHAINS OF CLASS-II MOLECULES OF THE MAJOR HISTOCOMPATIBILITY COMPLEX

Class II molecules of the major histocompatibility complex bind antigenic peptides and present them to T-helper cells. Class II molecules are heterodimers consisting of one α and one β chain. Here we report that each isolated α and β chain binds antigenic peptides and that this binding is specific. The specificity of peptide binding was investigated by employing the murine major histocompatibility complex haplotypes I-A(d) and I-E(k) and fluorescence-labeled peptides of chicken ovalbumin and pigeon cytochrome c, respectively, which are known to be specific for these haplotypes. The major histocompatibility complex molecules were incubated with these peptides and subjected to SDS/PAGE under nondenaturing conditions. The gels were then scanned for the fluorescent peptides and, after silver staining, for proteins. We found that the fluorescence-labeled peptide fragment of ovalbumin bound preferentially to the isolated α and β chains of I-A(d), whereas the fluorescence-labeled peptide fragment of pigeon cytochrome c bound preferentially to the isolated α and β chains of I-E(k). The α and β chains of each haplotype bound their specific peptides about equally well, suggesting comparable affinities. Our results indicate that in vivo the kinetic pathway for the formation of antigenic peptide complexes with the α/β heterodimers may involve the initial formation of complexes of the α and/or β chains with the specific antigenic peptides.