Sequence of the COOH-terminal hydrophilic region of histocompatibility antigens HLA-A2 and HLA-B7

Detergent-solubilized HLA antigens were isolated from a human lymphoblastoid cell using an anti-beta2-microglobulin immunoaffinity column. The HLA-A and HLA-B locus products were separated by thin layer isoelectric focusing. Cleavage of the p44 chain of HLA-A2 and -B7 antigens with cyanogen bromide led to the isolation of a 31-amino-acid fragment from each. The fragments were sequenced and shown to be from the COOH-terminal end of the intact chains using carboxypeptidase Y. The fragment from the HLA-B7 chain, 55% of whose amino acids were polar, contained the 2 cysteine residues not found in the papain-derived molecule. The tentative sequence of the fragment from the HLA-A2 chain was similar to that of the HLA-B7 fragment but appeared not to contain any cysteine residues. The hydrophilic COOH-terminal region of HLA antigens, which directly follows the hydrophobic, membrane-binding segment, began with a cluster of basic amino acids. This arrangement of amino acids resembles that found at the COOH terminus of the red blood cell membrane protein, glycophorin.