Disposition of T200 Glycoprotein in the Plasma Membrane of a Murine Lymphoma Cell Line

T200 glycoprotein, a major cell surface component of murine hematopoietic cells, is a phosphorylated transmembrane glycoprotein. Two distinct regions of the molecule can be defined by radiolabeling with a variety of metabolic precursors or by lactoperoxidase-catalyzed iodination, in combination with protease treatments, immunoprecipitation techniques, and peptide "mapping" analysis. A relative protease-resistant domain, which is exposed on the cell surface and contains the antigenic site recognized by a monoclonal anti-T200 antibody known to react with the exterior cell surface, contains most if not all of the mannose-containing oligosaccharide units of the glycoprotein and all of the amino acid residues labeled by lactoperoxidase-catalyzed iodination of intact viable cells. This protease-resistant fragment migrates with an apparent molecular weight of approximately 100,000 in sodium dodecyl sulfate-polyacrylamide gels. The remaining portion of the molecule contains a region, extensively digested by trypsin, which is exposed on the cytoplasmic side of the plasma membrane and contains phosphoserine residues which can be labeled with 32PO4 in vivo. A 125I-labeled tryptic peptide derived from this region of the molecule was obtained if membrane preparations from cells disrupted by nitrogen cavitation were labeled by lactoperoxidase-catalyzed iodination.