| Abstract: | We previously reported the initial characterization of a polymorphic major cell surface glycoprotein of about 80,000 daltons from mouse embryo 3T3 cells. This glycoprotein has now been purified 1800-fold to apparent homogeneity by monoclonal antibody affinity chromatography. The purified molecule retained the total antigenic activity of the cell, as determined by antibody binding assays. The quantity of the glycoprotein, 0.06% of the total protein of the crude cell extract, confirmed its presence as a major constituent of the cell plasma membrane. The monoclonal antibody was also used to detect related antigens in cells and tissues of C57BL/6J mice. The antigen was present in high concentration in macrophages and subpopulations of bone marrow and blood polymorphonuclear cells. Much lower concentrations of antigen were detected in spleen cells, thymocytes, and extracts of solid tissues. The apparent Mr of the target antigen of myeloid cells was 92,000. This molecule was a major surface constituent of myeloid cells with 10(6) antibody binding sites per cell containing 10% of total 125I incorporated by the lactoperoxidase procedure. The macrophage glycoprotein labeled on the cell surface with 125I was highly sensitive to trypsin, yielding an antigenically active soluble glycopolypeptide of about 65,000 daltons, that contained all of the incorporated 125I. A similar 65,000-dalton glycopeptide was released from 3T3 cells by trypsin cleavage. These data indicate that a major cell surface constituent of mouse myeloid cells is a 92,000-dalton glycoprotein closely related to the 80,000-dalton glycoprotein of mouse embryo 3T3 cells. |
