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Biophys J, October 2000, p. 2010-2023, Vol. 79, No. 4

Differential Effects of Surfactant Protein A on Regional Organization of Phospholipid Monolayers Containing Surfactant Protein B or C

Svetla G. Taneva* and Kevin M. W. Keough*dagger

 *Department of Biochemistry and  dagger Discipline of Pediatrics, Memorial University of Newfoundland, St. John's, Newfoundland A1B 3X9, Canada

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 Ca2+. Independently of the presence of Ca2+ in the subphase, SP-A at a bulk concentration of 0.68 µg/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 <=  pi  <=  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 ~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. Ca2+ in the subphase appeared to disperse this phase into tiny probe-excluding particles, likely comprising Ca2+-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 Ca2+-independent manner that led to the formation of a separate potentially protein-rich phase in the films.

Biophys J, October 2000, p. 2010-2023, Vol. 79, No. 4
© 2000 by the Biophysical Society   0006-3495/00/10/2010/14  $2.00


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