MUTATIONAL ANALYSIS OF ARG(197) OF RAT SURFACTANT PROTEIN-A - HIS(197) CREATES SPECIFIC LIPID UPTAKE DEFECTS

In previous studies, tandem mutagenesis of Glu(195) and Arg(197) of su rfactant protein A (SP-A) has implicated both residues as critical par ticipants in the interaction of the molecule with alveolar type II cel ls and phospholipids. We substituted Ala, Lys, His, Asp, and Asn mutat ions for Arg to evaluate the role of a basic amino acid at position 19 7 in SP-A action. Unexpectedly, Ala(197) retained complete activity in the SP-A functions of carbohydrate binding type HL cell binding inhib ition of surfactant secretion, lipid binding, lipid aggregation, and l ipid uptake by type II cells. The results unambiguously demonstrate th at Arg(197) is not mechanistically essential for SP-A function, The Ly s(197) mutation displayed all functions of the wild type protein but e xhibited a 2-fold increase in lipid uptake activity. The His(197) muta tion displayed all SP-B functions studied except for lipid uptake. The results obtained with the His(197) mutation clearly demonstrate that lipid aggregation alone by SP-A is insufficient to promote lipid uptak e by type II cells, These findings indicate that specific interactions between type II cells send SP-A are involved in the phospholipid upta ke processes.