Complete amino acid sequence of human plasma Zn-alpha 2-glycoprotein and its homology to histocompatibility antigens.

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Abstract
In the present study the complete amino acid sequence of human plasma Zn-.alpha.2-glycoprotein was determined. This protein whose biological function is unknown consits of a single polypeptide chain of 276 amino acid residues including 8 tryptophan residues and has a pyroglutamyl residue at the amino terminus. The location of the two disulfide bonds in the polypeptide chain was also established. The three glycans, whose structure was elucidated with the aid of 500 MHz 1H NMR spectroscopy, were sialylated N-biantennas. The molecular weight calculated from the polypeptide and carbohyrate structure is 38,478, which is close to the reported value of .apprxeq. 41,000 based on physicochemical measurements. The predicted secondary structure appeared to be comprised of 23% .alpha.-helix, 27% .beta.-sheet, and 22% .beta.-turns. The three N-glycans were found to be located in .beta.-turn regions. An unexpected finding was made by computer analysis of the sequence data; this revealed that Zn-.alpha.2-glycoprotein is closely related to antigens of the major histocompatibility complex in amino acid sequence and in domain structure. There was an unusually high degree of sequence homology with the .alpha. chains of class I histocompatibility antigens. Moreover, this plasma protein was shown to be a member of the immunoglobulin gene superfamily. Zn-.alpha.2-glycoprotein appears to be a truncated secretory major histocompatibility complex-related molecule, and it may have a role in the expression of the immune response.