Heavy chain of HLA-A and HLA-B antigens is conformationally labile: a possible role for beta 2-microglobulin.

The three-dimensional organization of HLA antigens has been investigated by spectroscopic and immunochemical techniques. Measurement of the circular dichroism shows that in papain-solubilized HLA the heavy chain as well as the previously studied light chain (beta 2-microglobulin) consists predominantly of beta-pleated sheet structures. When heavy chain is separated from the light chain under denaturing conditions and is allowed to renature, about 50% of the beta structure is lost, concomitantly with most of the alloantigenic activity. Analysis of the two acid-cleaved fragments of HLA-B7 heavy chain shows that beta structure is preferentially lost from the COOH-terminal region of the heavy chain. Exposure to denaturants per se does not inevitably result in irreversible loss of antigenic activity. However, recovery of antigenic properties does seem to depend on reassociation of the two chains. The results reported here provide further evidence for (i) the similarity of HLA antigens and immunoglobulins at the three-dimensional level and (ii) two distinct and physiologically important conformations of the HLA heavy chain, depending upon whether it is associated with the light chain.