Effect of Cholesterol and Palmitoylation on the Structure, Orientation and Lipid-Protein Interactions of Pulmonary Surfactant Protein SP-C

Abstract

Pulmonary surfactant is a specialized lipid-protein complex, which essential function is to stabilize the gas exchange surface of the respiratory epithelium against physical forces tending to its collapse. Hydrophobic surfactant proteins SP-B and SP-C are critical to promote very rapid adsorption of phospholipids into the interface and to stabilize the surface active film along the compression-expansion respiratory cycles. The composition of pulmonary surfactant varies slightly between species but in general it consists of 90% lipids by weight and 8-10% proteins, including the specific surfactant proteins SP-A, SP-B, SP-C and SP-D. The main feature of protein SP-C (35 amino acids, 3.7kDa) is its extreme hidrophobicity. Structurally it consists of a transmembrane α-helical stretch and a short N-terminal segment. The positions 5 and 6 of SP-C sequence are in most species occupied by two cysteines that are palmitoylated. It has been proposed that SP-C interacts with cholesterol-rich phases of surfactant membranes and films, contributing to modulate the interfacial behavior of surfactant in the alveoli but evidence remains uncertain.Our aim in this work was to elucidate how palmitoylation influences the interaction of SP-C with lipids in different surfactant-mimetic systems, in the presence or absence of cholesterol. To this end, we have compared the secondary structure, tilting and lipid-protein interactions of native palmitoylated SP-C purified from porcine lungs with the behavior of non-palmitoylated recombinant versions of SP-C produced in bacteria, by means of ATR-FTIR. This technique is sensitive to the orientation, structure and interactions of SP-C in membranes, which can be correlated with the contribution of SP-C to stabilize compressed cholesterol-containing surfactant films.