Cholesterol modulates the exposure and orientation of pulmonary surfactant protein SP-C in model surfactant membranes.
Cholesterol is the major neutral lipid in lung surfactant, accounting for up to 8-10% of surfactant mass, while surfactant protein SP-C ( approximately 4.2 kDa) accounts for no more than 1-1.5% of total surfactant weight but plays critical roles in formation and stabilization of pulmonary surfactant films. It has been reported that surfactant protein SP-C interacts with cholesterol in lipid/protein interfacial films and this interaction could have a potential role on modulating surfactant function. In the present study, we have analyzed the effect of cholesterol on the structure, orientation and dynamic properties of SP-C embedded in physiologically relevant model membranes. The presence of cholesterol does not induce substantial changes in the secondary structure of SP-C, as analyzed by Attenuated Reflection Fourier Transformed Infrared spectroscopy (ATR-FTIR). However, the presence of cholesterol modifies the orientation of the transmembrane helix and the dynamic properties of the protein, as demonstrated by hydrogen/deuterium exchange kinetics. The effect of cholesterol on SP-C reconstituted in zwitterionic, entirely fluid, membranes made of POPC (palmitoyloleoylphospatidylcholine) or in anionic membranes with coexistence of ordered and disordered phases, such as those made of dipalmitoylphosphatidylcholine (DPPC):POPC:Palmitoyloleoylphosphatidylglycerol (POPG) (50:25:15) is dual. Cholesterol decreases the exposure of the protein to the aqueous environment and the tilt of its transmembrane helical segment up to a ratio Cholesterol:SP-C of 4.8 and 2.4 (mol/mol) in the two lipid systems tested, respectively, and it increases the exposure and tilt at higher cholesterol proportions. The results presented here suggest the existence of an interaction between SP-C and cholesterol-enriched phases, with consequences on the behavior of the protein, which could be of relevance for cholesterol-dependent structure-function relationships in pulmonary surfactant membranes and films.
Gómez-Gil
L
L
Laboratory for the Structure and Function of Biological Membranes, Center for Structural Biology and Bioinformatics, Université Libre de Bruxelles, Campus Plaine CP 206/2, Blvd. du Triomphe, B-1050 Brussels, Belgium.
Pérez-Gil
J
J
Goormaghtigh
E
E
D015060:1,2-Dipalmitoylphosphatidylcholine / Q000737:chemistry; D000577:Amides / Q000737:chemistry; D041961:Deuterium Exchange Measurement; D008051:Lipid Bilayers / Q000737:chemistry; D010713:Phosphatidylcholines / Q000737:chemistry; D010715:Phosphatidylglycerols / Q000737:chemistry; D025341:Principal Component Analysis; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D037721:Pulmonary Surfactant-Associated Protein C / Q000737:chemistry*; D011663:Pulmonary Surfactants / Q000737:chemistry*; D017550:Spectroscopy, Fourier Transform Infrared
D000577:Amides; D008051:Lipid Bilayers; D010713:Phosphatidylcholines; D010715:Phosphatidylglycerols; D037721:Pulmonary Surfactant-Associated Protein C; D011663:Pulmonary Surfactants; D015060:1,2-Dipalmitoylphosphatidylcholine; C060037:1-palmitoyl-2-oleoylglycero-3-phosphoglycerol; C028694:1-palmitoyl-2-oleoylphosphatidylcholine