Surfactant protein A amino acids Glu195 and Arg197 are essential for receptor binding, phospholipid aggregation, regulation of secretion, and the facilitated uptake of phospholipid by type II cells.

Pulmonary surfactant protein A (SP-A) is a mammalian lectin that regulates the uptake and secretion of surfactant by alveolar type II cells and is an important component of surfactant complexes. The domains of SP-A which mediate these functions have not been fully mapped. The binding of SP-A to its high affinity receptor on alveolar type II cells is thought to be dependent on a carbohydrate recognition domain (CRD), while the interaction with lipids has been attributed to the hydrophobic neck region of the molecule. To explore the role of the CRD in the interactions of SP-A with type II cells and lipids, we introduced mutations into the cDNA to encode for the substitutions Glu195-->Gln and Arg197-->Asp (SP-Ahyp,Gln195,Asp197) and expressed the mutant protein in insect (Sf9) cells using recombinant baculoviruses. Similar mutations introduced into mannose-binding protein A have been shown to switch the carbohydrate binding specificity from mannose > galactose to the converse. Wild type SP-A produced in Sf9 cells does not contain hydroxyproline (SP-Ahyp), but like rat SP-A it binds to carbohydrate affinity columns, lipids, and the SP-A receptor and is a potent inhibitor of the secretion of surfactant from type II cells (IC50 = 0.5-1.0 micrograms/ml). The SP-Ahyp,Gln195,Asp197 also bound to affinity matrices of galactose-Sepharose and mannose-Sepharose but the indicated mutations rendered the binding at least 100 times more susceptible than SP-Ahyp to competition by free galactose. The SP-Ahyp,Gln195,Asp197 did not compete with rat SP-A for occupancy of its high affinity receptor on type II cells and the mutant protein was 25-50-fold less potent as an inhibitor of the secretion of surfactant from type II cells (IC50 = 26.0 micrograms/ml). Unlike SP-Ahyp, the inhibition of secretion of surfactant by SP-Ahyp,Gln195,Asp197 was reversed by 0.25 M alpha-methylmannoside or galactose. In addition, the SP-Ahyp,Gln195,Asp197 bound avidly to phospholipid but did not aggregate vesicles or augment the uptake of phospholipid into type II cells. We conclude that the binding of SP-A to its receptor and the inhibition of surfactant secretion are critically dependent on the carbohydrate binding specificity of the CRD. Furthermore, phospholipid aggregation and augmentation of phospholipid uptake into type II cells are mediated by the COOH-terminal region of SP-A by a mechanism that is distinct from phospholipid binding.