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Surfactant Protein A Forms Extensive Lattice-Like Structures on 1,2-Dipalmitoylphosphatidylcholine/Rough-Lipopolysaccharide- Mixed Monolayers

Ignacio García-Verdugo ^*, Olga Cañadas ^*, Svetla G. Taneva , Kevin M. W. Keough  and Cristina Casals ^*

^* Departamento de Bioquímica y Biología Molecular I and CIBER Enfermedades Respiratorias, Complutense University of Madrid, 28040-Madrid, Spain; and Department of Biochemistry and Discipline of Pediatrics, Memorial University of Newfoundland, St. John's, Newfoundland, Canada A1B 3X9

Correspondence: Address reprint requests to Cristina Casals, PhD, Dept. of Biochemistry and Molecular Biology I, Faculty of Biology, Complutense University of Madrid, 28040 Madrid, Spain. Tel.: 34-91-394-4261; Fax: 34-91-394-4672; E-mail: ccasalsc{at}bio.ucm.es; or Kevin M. W. Keough, PhD, Dept. of Biochemistry and Discipline of Pediatrics, Memorial University of Newfoundland, St. John's, Newfoundland, Canada A1B 3X9. Tel.: 780-423-5727; Fax: 780-429-3509; E-mail: kevin.keough{at}ahfmr.ab.ca.

Due to the inhalation of airborne particles containing bacterial lipopolysaccharide (LPS), these molecules might incorporate into the 1,2-dipalmitoylphosphatidylcholine (DPPC)-rich monolayer and interact with surfactant protein A (SP-A), the major surfactant protein component involved in host defense. In this study, epifluorescence microscopy combined with a surface balance was used to examine the interaction of SP-A with mixed monolayers of DPPC/rough LPS (Re-LPS). Binary monolayers of Re-LPS plus DPPC showed negative deviations from ideal behavior of the mean areas in the films consistent with partial miscibility and attractive interaction between the lipids. This interaction resulted in rearrangement and reduction of the size of DPPC-rich solid domains in DPPC/Re-LPS monolayers. The adsorption of SP-A to these monolayers caused expansion in the lipid molecular areas. SP-A interacted strongly with Re-LPS and promoted the formation of DPPC-rich solid domains. Fluorescently labeled Texas red-SP-A accumulated at the fluid-solid boundary regions and formed networks of interconnected filaments in the fluid phase of DPPC/Re-LPS monolayers in a Ca²⁺-independent manner. These lattice-like structures were also observed when TR-SP-A interacted with lipid A monolayers. These novel results deepen our understanding of the specific interaction of SP-A with the lipid A moiety of bacterial LPS.

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