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.
Sánchez
L M
LM
Division of Biology 156-29, California Institute of Technology, Pasadena, CA 91125, USA.
López-Otín
C
C
Bjorkman
P J
PJ
D001798:Blood Proteins / Q000737:chemistry* / Q000378:metabolism; D003460:Crystallization; D006023:Glycoproteins / Q000737:chemistry* / Q000378:metabolism; D006183:H-2 Antigens / Q000737:chemistry; D015395:Histocompatibility Antigens Class I / Q000737:chemistry* / Q000378:metabolism; D006358:Hot Temperature; D006801:Humans; D011485:Protein Binding; D011489:Protein Denaturation; D029589:Seminal Plasma Proteins; D017386:Sequence Homology, Amino Acid; D012995:Solubility; D014961:X-Ray Diffraction; D000090203:Zn-Alpha-2-Glycoprotein; D001613:beta 2-Microglobulin / Q000378:metabolism
D001798:Blood Proteins; D006023:Glycoproteins; D006183:H-2 Antigens; C024755:H-2K(K) antigen; D015395:Histocompatibility Antigens Class I; D029589:Seminal Plasma Proteins; D000090203:Zn-Alpha-2-Glycoprotein; D001613:beta 2-Microglobulin