MOLECULAR COMPARISONS OF THE BETA-2-MICROGLOBULIN-BINDING SITE IN CLASS-I MAJOR-HISTOCOMPATIBILITY-COMPLEX ALPHA-CHAINS AND PROTEINS OF RELATED SEQUENCES

Beta-2-Microglobulin (beta-2m) binds non-covalently to the alpha-1, alpha-2 and alpha-3 domains of the alpha-chain of Class I major-histocompatibility-complex (MHC) molecules. On the basis of the crystal structures of human leucocyte antigens HLA-A2.1 and HLA-Aw68.1, we have used molecular-graphics analyses to define 44 contact points between 19 alpha-chain residues and 18 beta-2m residues. In 88 other alpha-chain sequences from the HLA-A, HLA-B, HLA-C, HLA-D, HLA-E, HLA-F and HLA-G locus products in man and the H-2, Qa and Tla loci in mouse, 37 contact sites were conserved to 90% or more, and in beta-2m sequences from seven other species 40% of contact sites were totally conserved. Four distinct regions form the contact points between the alpha-chain and beta-2m, one on each of the alpha-1 and alpha-2 domains and two on the alpha-3 domain. We have further studied the alpha-chain sequences of three non-MHC molecules, human CD1 and rat Fc receptor (FcRn), known to bind to beta-2m, and a third molecule, the putative product of the H301 (UL18) gene of human cytomegalovirus (CMV). CMV has been shown to bind beta-2m, and it has been postulated that the H301-gene product, which has sequence similarity to Class I HLA, is the protein responsible. These sequences exhibited much lower residue conservation with the MHC-linked group, although the alpha-3 domain was the most highly conserved, and gaps and insertions were required for optimal alignments with the 90 alpha-chain sequences. Of the 44 beta-2m-alpha-chain contacts defined for Class I HLA, 24-alpha-chain contact sites were conserved in CD1, 25 in FcRn and 17 in the H301-gene product. For CD1 and FcRn, the majority of the conserved beta-2m contacts were found in the alpha-2 domain and the major contact region in the alpha-3 domain. Together with the use of secondary-structure predictions, it was concluded that the binding of beta-2m in CD1 and FcRn was MHC-like at the alpha-3 domain, and probably also at the alpha-2 domain for FcRn, but non-MHC-like for the alpha-1 domain of both molecules and the alpha-3 domain of CD1. In the H301-gene product sequence, only the beta-2m contacts with the main region of the alpha-3 domain were noticeably conserved. Structural evidence for beta-2m binding in H301 in this alpha-3 contact region was, however, much weakened by a sequence insertion, the presence of proline residues, and marked deviations in the secondary-structure predictions. If the H301-gene product is responsible for the binding of beta-2m to cytomegalovirus, this occurs in a non-MHC-like manner.