Secondary structure of the murine histocompatibility alloantigen, H-2Kb: relationship between heavy chain, .beta.2-microglobulin, and antigenic reactivity

Abstract
The far-UV circular dichroism (CD) spectra of the extracellular protion (papain-cleaved fragment) of the histocompatibility antigen H-2Kb and its noncovalently associated components, H chain and .beta.2-microglobulin (.beta.2m), indicate that the antigen is highly structured, containing about 30% .alpha.-helix, 41% .beta.-sheet, and 29% random coil. Separation of .beta.2m from the H chain produced a decrease in H chain .alpha.-helix and .beta.-sheet structure which correlated with a loss of alloantigenic reactivity. Reconstitution of the H chain.sbd..beta.2m complex resulted in an increase in secondary structure which was greater than the sum of the free chains and the recovery of considerable alloantigenic reactivity. This suggests that some of the secondary structure and much of the alloantigenic reactivity may depend on conformation associated with the binding of .beta.2m to H chain. A prediction of H chain secondary structure based on Chou-Fasman analysis of the primary amino acid sequence agreed with results from CD measurements and suggested that the segments of .alpha.-helix and .beta.-sheet structure are distributed throughout the molecule.