A structural characterization of the Mo3 activation antigen expressed on the plasma membrane of human mononuclear phagocytes

Mo3 is an activation Ag expressed by human monocytic cells after stimulation in vitro by PMA, LPS, certain cytokines, and muramyl dipeptide. The structural characterization of Mo3 has been made possible by the development of a mAb (anti-Mo3f) that immunoprecipitates Mo3 from Nonidet P-40 lysates of radiolabeled PMA-stimulated U-937 cells and LPS-activated monocytes. On SDS-PAGE (nonreducing conditions) of anti-Mo3f immunoprecipitates, U-937 Mo3 is a single broad band of 39 to 66 kDa, whereas monocyte Mo3 is smaller with an apparent molecular mass of 32 to 56 kDa. Under reducing conditions, there is an increase in the m.w. of both species of Mo3 suggesting the existence of internal disulfide bonds. Mo3 is a glycoprotein with carbohydrate of the N-linked complex type as evidence by a reduction in m.w. by 40 to 50% after treatment with endoglycosidase F or N-glycanase; neuraminidase treatment produces a 3-kDa reduction in m.w. Deglycosylated Mo3 isolated from U-937 and monocytes have similar m.w. suggesting that the molecular heterogeneity of the native Mo3 may be due to differences in glycosylation. Mo3 is sensitive to phosphatidylinositol-specific phospholipase C with the release of native Mo3 from the surface of PMA-stimulated U-937 cells. These results indicate that Mo3 is a member of the glycosylphosphatidylinositol-linked family of surface glycoproteins.