NEUTRALIZATION OF THE POSITIVE CHARGES OF SURFACTANT PROTEIN-C - EFFECTS ON STRUCTURE AND FUNCTION

Pulmonary surfactant protein C (SP-C) is a small, extremely hydrophobi c peptide with a highly conservative primary structure. The protein is characterized by two adjacent palmitoylated cysteine residues, two po sitively charged residues (one arginine residue and one lysine residue ) in the N-terminal region, and a long hydrophobic stretch. SP-C enhan ces the adsorption of phospho lipids into an air-water interface. To d etermine the importance of the positively charged residues, we carried out experiments with natural porcine SP-C and modified porcine SP-C ( SP-C-m) in which the positive charges had been blocked by phenylglyoxa l. Circular dichroism experiments showed that SP-C-m had an increased content of alpha-helix. Natural SP-C, but not SP-C-m, catalyzed insert ion of phospholipids into a monolayer at the air-water interface. This reduced insertion was due to a strong reduction of binding of phospho lipid vesicles to the monolayer. The insertion catalyzed by the natura l porcine SP-C was decreased by an increased pH of the subphase. In co ntrast to natural SP-C, SP-C-m induced lipid mixing between phospholip id vesicles. The extent of lipid mixing was a function of the SP-C con tent. We conclude that the positively charged residues of SP-C are imp ortant for the binding of phospholipid vesicles to the monolayer, a pr ocess that precedes the insertion of phospholipids into the monolayer.