BIOCHEMICAL-CHARACTERIZATION AND CRYSTALLIZATION OF HUMAN ZN-ALPHA(2)-GLYCOPROTEIN, A SOLUBLE CLASS-I MAJOR HISTOCOMPATIBILITY COMPLEX HOMOLOG

Zn-alpha(2)-glycoprotein (ZAG) is a 41-kDa soluble protein that is pre sent in most bodily fluids, In addition, ZAG accumulates in fluids fro m breast cysts and in 40% of breast carcinomas, which suggests that ZA G; plays a role in the development of breast diseases, However, the fu nction of ZAG under physiological and cancerous conditions remains unk nown. Because ZAG shares 30-40% sequence identity with the heavy chain s of class I major histocompatibility complex (MI-IC) proteins, we com pared the biochemical properties of ZAG with those of classical class I MHC molecules, We purified human ZAG from breast cyst fluid and seru m and produced a panel of anti-ZAG monoclonal antibodies, Binding assa ys and acid elution experiments revealed that, in contrast to class I MHC proteins, ZAG does not bind peptides or title class I light chain, beta(2)-microglobulin (beta(2)m). Nevertheless, CD studies indicated that ZAG is thermally stable in the absence of bound peptide or associ ated beta 2m, as opposed to class I MHC molecules, which require the p resence of both beta 2m and peptides for stability, These data indicat e that the function of ZAG has diverged from the peptide presentation and T-cell interaction functions of class I molecules. To gain insight into the function of ZAG and to compare the three-dimensional structu res of ZAG and class I MHC molecules, me produced ZAG crystals that di ffract beyond 2.7 Angstrom and have initiated an x-ray structure deter mination.