Isolation and partial characterization of a receptor to surfactant protein a expressed by rat type II pneumocytes

Surfactant protein A (SP-A), the most abundant protein component in pulmonary surfactant, has been shown to enhance surfactant phospholipid uptake by the type II alveolar epithelial cell. Recent evidence has shown that this process may be receptor-mediated. We undertook this study to isolate the putative receptor from type II cell membranes. We isolated two specific SP-A binding proteins from type II cells with apparent molecular weights (M_r) of 86 and > 200 kD under nonreducing conditions. Under reducing conditions, the higher-M_r protein was not present, but three proteins with apparent M_r of 65, 55, and 50 kD were visible, in addition to the 86-kD protein, indicating that the higher-M_r protein was composed of the smaller peptides which form disulfide bonds. The 86-kD protein is a glycoprotein with ∼ 30% of its mass as carbohydrate. The 50-kD protein is also a glycoprotein (∼ 30% of its mass as carbohydrate), and SP-A binds to the protein core. Polyclonal and monoclonal antibodies to these peptides saturably bind to the surface of type II cells but not other lung cells, as shown by immunohistochemistry. SP-A competitively inhibits binding of one monoclonal antibody to type II cells, and the monoclonal antibody was able to block the effect of SP-A on phospholipid uptake by type II cells, indicating that this complex is a receptor to SP-A which is expressed on type II cells. This novel receptor is fundamental to the biology of surfactant metabolism in the lung.