EFFECT OF ACYLATION ON STRUCTURE AND FUNCTION OF SURFACTANT PROTEIN-CAT THE AIR-LIQUID INTERFACE

Pulmonary surfactant protein C (SP-C) is a small hydrophobic peptide t hat is palmitoylated on 2 adjacent cysteine residues. SP-C enhances th e adsorption of phospholipids into a monolayer. The function of the ac ylation is not clear yet. The experiments described in this article we re carried out in order to investigate the function of SP-C acylation in (protein-catalyzed) lipid monolayer formation, and in bilayer inter actions. Palmitoylated and nonpalmitoylated human recombinant SP-C wer e used. In addition, a nonacylated SP-C with a Cys --> Ser mutation wa s included in these studies. In Wilhelmy plate experiments using negat ively charged, protein-containing phospholipid monolayers and negative ly charged vesicles, CaCl2 was required to obtain a maximal insertion rate of lipids into the monolayer. If the negatively charged phospholi pids in the monolayer were replaced by neutral phospholipids, CaCl2 wa s only required to show a maximal SP-C-catalyzed insertion rate (if th e molecule is palmitoylated, but not if nonpalmitoylated proteins were added). In pressure area measurements, the palmitoylated protein show ed a different change in pressure as a function of the surface area, a s compared with the nonpalmitoylated proteins. Circular dichroism expe riments showed that all three proteins had a high content of alpha-hel ix. All three proteins showed a preferential orientation at the air-wa ter interface, but the palmitoylated protein has an orientation which is more parallel to the monolayer than that of the nonpalmitoylated pr oteins. It is concluded that acylation of SP-C alters structural and p hysical properties of this protein.