Evolutionary Relationship Between HLA‐DR Antigen β‐Chains, HLA‐A, B, C Antigen Subunits and Immunoglobulin Chains

Abstract

CDNA for a β‐chain of HLA‐DR antigens was cloned and the partial nucleotide sequence was determined. The data suggest that the β‐chain consists of approximately 230 amino acids, of which about 200 are exposed on the cell surface. The β‐chain appears to be composed of two exposed disulphide‐containing domains. The arrangement of the disulphide loops suggests that the β‐chain is similar in structure to the HLA‐A, B, C antigen subunits and the immunoglobulin chains. For the β‐chain domain closest to the membrane this similarity was verified at the level of primary structure. The partial amino acid sequence of the NH₂‐terminal domain did not display any apparent homology to HLA‐A, B, C antigens and immunoglobulins. However, the similarity established here between the two types of major histocompatibility antigen subunits and the immunoglobulin chains suggests a common ancestral origin for at least some regions of these molecules.