Isolation and partial characterization of pneumocin, a novel apical membrane surface glycoprotein marker of rat type II cells.

Rat alveolar type II pneumocytes, in situ, label with Maclura pomifera agglutinin (MPA), a plant lectin that recognizes alpha-galactosyl oligosaccharide residues of glycoproteins and glycolipids. To study the glycoproteins recognized by the lectin, MPA lectin affinity chromatography was used to isolate a novel glycoprotein, pneumocin, from type II and whole rat lung cell membranes. Pneumocin isolated from adult rat lungs was a non-disulfide-linked sialoglycoprotein with an Mr of 165 kD. Asparagine-linked oligosaccharides contributed 5 to 10% to the Mr. Two-dimensional chymotryptic peptide maps of pneumocin isolated from whole lung membranes and type II cells were similar. The glycoprotein partitioned in the detergent phase on Triton X-114 phase separation. Murine monoclonal antibodies developed against the purified glycoprotein localized on apical membranes of type II pneumocytes in situ. The antibodies did not label type I cells or lamellar bodies but labeled luminal surfaces of vesicular structures of type II cells. Isolated type II cells labeled with antibodies after 1 d in culture but showed significantly less staining of cells after 4 d of culture. These observations demonstrate that pneumocin is a cell surface sialoglycoprotein marker of type II cells. Western blot analysis of liver and kidney cell membranes suggest that related glycoproteins may also be present in those tissues. The isolation technique and monoclonal antibodies should permit further characterization and functional studies of the glycoprotein.