Biochemical characterization and crystalization of human Zn-alpha2-glycoprotein, a soluble class I major histocompatibility complex homolog.

Zn-alpha2-glycoprotein (ZAG) is a 41-kDa soluble protein that is present in most bodily fluids. In addition, ZAG accumulates in fluids from breast cysts and in 40% of breast carcinomas, which suggests that ZAG plays a role in the development of breast diseases. However, the function of ZAG under physiological and cancerous conditions remains unknown. Because ZAG shares 30-40% sequence identity with the heavy chains of class I major histocompatibility complex (MHC) proteins, we compared the biochemical properties of ZAG with those of classical class I MHC molecules. We purified human ZAG from breast cyst fluid and serum and produced a panel of anti-ZAG monoclonal antibodies. Binding assays and acid elution experiments revealed that, in contrast to class I MHC proteins, ZAG does not bind peptides or the class I light chain, beta2-microglobulin (beta2m). Nevertheless, CD studies indicated that ZAG is thermally stable in the absence of bound peptide or associated beta2m, as opposed to class I MHC molecules, which require the presence of both beta2m and peptides for stability. These data indicate 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 structures of ZAG and class I MHC molecules, we produced ZAG crystals that diffract beyond 2.7 A and have initiated an x-ray structure determination.