Purification in large scale and characterization of the human leukocyte adhesion glycoprotein GP90 (CD18)

The leukocyte adhesion 90-kDa glycoprotein GP90 (antigen CD18) is non-covalently associated separately with cell-surface glycoproteins GP160 (antigens CD11a, TA-1, LFA-1), GP155 (antigens CD11b, OKM1, MO1) or GP130 (antigens CD 11c, Leu-M5). Large amounts of these protein complexes were purified to homogeneity from blood mononuclear leukocytes by immunoaffinity chromatography using a monoclonal antibody. GP90 was further isolated by preparative gel electrophoresis in the presence of sodium dodecyl sulfate. Rabbit antiserum towards the complex inhibited phorbol-ester-induced adhesion of leukocytes. The antiserum towards purified GP90 reacted more strongly with the denatured GP90 protein, but showed reactivity also with GP160 protein, indicating structural homologies between GP90 and GP160. The amino acid composition of GP90 was determined. Its N-terminus was found to be blocked. Treatment of GP90 with endo-beta-N-acetylglucosaminidase F, but not with endo-beta-N-acetylglucosaminidase H, reduced the apparent molecular mass to 75 kDa, indicating the presence of five or six N-linked complex-type oligosaccharides/molecule.