Differential effects of surfactant protein A on regional organization of phospholipid monolayers containing surfactant protein B or C.
Epifluorescence microscopy combined with a surface balance was used to study monolayers of dipalmitoylphosphatidylcholine (DPPC)/egg phosphatidylglycerol (PG) (8:2, mol/mol) plus 17 wt % SP-B or SP-C spread on subphases containing SP-A in the presence or absence of 5 mM Ca(2+). Independently of the presence of Ca(2+) in the subphase, SP-A at a bulk concentration of 0.68 microg/ml adsorbed into the spread monolayers and caused an increase in the molecular areas in the films. Films of DPPC/PG formed on SP-A solutions showed a pressure-dependent coexistence of liquid-condensed (LC) and liquid-expanded (LE) phases. Apart from these surface phases, a probe-excluding phase, likely enriched in SP-A, was seen in the films between 7 mN/m < or = pi < or = 20 mN/m. In monolayers of SP-B/(DPPC/PG) spread on SP-A, regardless of the presence of calcium ions, large clusters of a probe-excluding phase, different from probe-excluding lipid LC phase, appeared and segregated from the LE phase at near-zero surface pressures and coexisted with the conventional LE and LC phases up to approximately 35 mN/m. Varying the levels of either SP-A or SP-B in films of SP-B/SP-A/(DPPC/PG) revealed that the formation of the probe-excluding clusters distinctive for the quaternary films was influenced by the two proteins. Concanavalin A in the subphase could not replace SP-A in its ability to modulate the textures of films of SP-B/(DPPC/PG). In films of SP-C/SP-A/(DPPC/PG), in the absence of calcium, regions consisting of a probe-excluding phase, likely enriched in SP-A, were detected at surface pressures between 2 mN/m and 20 mN/m in addition to the lipid LE and LC phases. Ca(2+) in the subphase appeared to disperse this phase into tiny probe-excluding particles, likely comprising Ca(2+)-aggregated SP-A. Despite their strikingly different morphologies, the films of DPPC/PG that contained combinations of SP-B/SP-A or SP-C/SP-A displayed similar distributions of LC and LE phases with LC regions occupying a maximum of 20% of the total monolayer area. Combining SP-A and SP-B reorganized the morphology of monolayers composed of DPPC and PG in a Ca(2+)-independent manner that led to the formation of a separate potentially protein-rich phase in the films.
Taneva
S G
SG
Department of Biochemistry, Memorial University of Newfoundland, St. John's, Newfoundland A1B 3X9, Canada.
Keough
K M
KM
D015060:1,2-Dipalmitoylphosphatidylcholine / Q000737:chemistry; D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D002118:Calcium / Q000737:chemistry; D003208:Concanavalin A / Q000494:pharmacology; D066298:In Vitro Techniques; D008567:Membranes, Artificial; D008856:Microscopy, Fluorescence; D010715:Phosphatidylglycerols / Q000737:chemistry; D010743:Phospholipids / Q000737:chemistry*; D011510:Proteolipids / Q000737:chemistry* / Q000494:pharmacology; D037662:Pulmonary Surfactant-Associated Protein A; D037661:Pulmonary Surfactant-Associated Proteins; D011663:Pulmonary Surfactants / Q000737:chemistry* / Q000494:pharmacology; D013499:Surface Properties; D013552:Swine
D008567:Membranes, Artificial; D010715:Phosphatidylglycerols; D010743:Phospholipids; D011510:Proteolipids; D037662:Pulmonary Surfactant-Associated Protein A; D037661:Pulmonary Surfactant-Associated Proteins; D011663:Pulmonary Surfactants; D003208:Concanavalin A; D015060:1,2-Dipalmitoylphosphatidylcholine; D002118:Calcium