SP-A permeabilizes lipopolysaccharide membranes by forming protein aggregates that extract lipids from the membrane
Olga Cañadas 1, Ingnacio García-Verdugo 1, Kevin M.W. Keough 2 and Cristina Casals 1*
1 UNIVERSIDAD COMPLUTENSE DE MADRID
2 MEMORIAL UNIVERSITY OF NEWFOUNDLAND
* To whom correspondence should be addressed. E-mail: ccasalsc{at}bio.ucm.es.
Submitted on May 13, 2008
Revised on June 6, 2008
Accepted on 13 June 2008
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Abstract |
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Surfactant protein A (SP-A) is known to cause bacterial permeabilization. The aim of this work was to gain insight into the mechanism by which SP-A induces permeabilization of rough lipopolysaccharide (Re-LPS) membranes. In the presence of calcium, large interconnected aggregates of fluorescently labeled TR-SP-A were observed on the surface of Re-LPS films by epifluorescence microscopy. Using Re-LPS monolayer relaxation experiments at constant surface pressure, we demonstrated that SP-A induced Re-LPS molecular loss by promoting the formation of three-dimensional lipid-protein aggregates in Re-LPS membranes. This resulted in decreased van der Waals interactions between Re-LPS acyl chains, as determined by differential scanning calorimetry, which rendered the membrane leaky. We also showed that the coexistence of gel and fluid lipid phases within the Re-LPS membrane conferred susceptibility to SP-A-mediated permeabilization. Taken together, our results seem to indicate that the calcium-dependent permeabilization of Re-LPS membranes by SP-A is related to the extraction of LPS molecules from the membrane due to the formation of calcium-mediated protein aggregates that contain LPS.
Key Words:
DPH fluorescence, differential scanning calorimetry, epifluorescence microscopy, lung surfactant, monolayer relaxation kinetics
