title: Structural analysis of an HLA-B27 population variant, B27f. Multiple patterns of amino acid changes within a single polypeptide segment generate polymorphism in HLA-B27.
abstract: The structure of a new HLA-B27 variant, B27f, distinguishable from other HLA-B27 subtypes by isoelectric focusing and serologic criteria, has been established by comparative peptide mapping and radiochemical sequence analysis. HLA-B27f differs from the major B27.1 subtype in three clustered amino acid replacements: Asp74, Asp77, and Leu81 in B27.1 are changed to Tyr74, Asn77, and Ala81, respectively in B27f. This pattern of differences is analogous to that of HLA-B27.2 in that this subtype also differs from B27.1 in multiple clustered substitutions within the same segment. Thus, polymorphism within the HLA-B27 system is being achieved by introducing different sets of amino acid changes within a particular short segment of the alpha 1 domain. The most likely mechanism for the introduction of multiple changes within this segment is a nonreciprocal recombination event, such as gene conversion. The structural analogies and ethnic distribution of B27f and B27.2 as compared with those of B27.3, and B27.4 support a dynamic model of HLA-B27 evolution in which polymorphism has been created after the separation of the major ethnic groups. In this model, a Caucasian branch would be characterized by subtypes differing from B27.1 in a few changes within the alpha 1 domain, which were probably generated by single genetic steps. An Oriental branch would include those subtypes which differ from B27.1 by changes in both alpha 1 and alpha 2, involving multiple genetic steps for their generation.
mesh_terms: D000595:Amino Acid Sequence; D044466:Asian People; D002460:Cell Line; D005785:Gene Conversion; D005802:Genes, MHC Class II; D006680:HLA Antigens / Q000235:genetics* / Q000302:isolation & purification; D015796:HLA-B27 Antigen; D006801:Humans; D007525:Isoelectric Focusing; D008214:Lymphocytes / Q000276:immunology; D010449:Peptide Mapping; D010802:Phylogeny; D011110:Polymorphism, Genetic; D044465:White People
chemical_list: D006680:HLA Antigens; D015796:HLA-B27 Antigen