Surfactant protein SP-B induces ordering at the surface of model membrane bilayers.
The effects of bovine pulmonary surfactant-associated protein B (SP-B) on molecular packing of model membrane lipids (7:1 DPPC/DPPG) were studied by fluorescence anisotropy. The bilayer surface was markedly ordered by SP-B below the gel to fluid phase transition temperature (Tc) while it was only slightly ordered above this temperature as indicated by surface-sensitive probes 6-NBD-PC and 6-NBD-PG. The effects of SP-B on fluorescence anisotropy were concentration dependent, reaching maximal activity at 1-2% protein to phospholipid by weight. Anisotropy measurements of interior-selective fluorescent probes (cis-parinaric acid and DPH) imply that addition of SP-B into the phospholipid shifted the Tc of the model membrane but did not alter lipid order at the membrane interior. Since fluorescence anisotropy studies with trans-parinaric acid, an interior-sensitive probe with high affinity for gel-phase lipids, did not detect any changes in lipid packing or Tc, it is likely that SP-B resides primarily in fluid-phase domains. Fluorescence lifetime measurements indicated that two conformers of the NBD-PC probe exist in this DPPC/DPPG model membrane system. Fluorescence intensity measurements generated with NBD-PC and NBD-PG, in conjunction with information from lifetime measurements, support the concept that SP-B increases the distribution of the short-lifetime conformer in the gel phase. In addition, the anisotropy and intensity profiles of NBD-PG in the model membrane indicate that bovine SP-B interacts selectively with phosphatidylglycerol.
University of Cincinnati, College of Medicine, Pediatrics/Neonatology Division, OH 45267-0541.
J E
JE
Baatz
B
B
Elledge
J A
JA
Whitsett
D015060:1,2-Dipalmitoylphosphatidylcholine; D000595:Amino Acid Sequence; D000818:Animals; D002417:Cattle; D005454:Fluorescence Polarization; D005456:Fluorescent Dyes; D008051:Lipid Bilayers; D008563:Membrane Lipids; D008565:Membrane Proteins / Q000494:pharmacology*; D008969:Molecular Sequence Data; D010715:Phosphatidylglycerols; D011510:Proteolipids / Q000494:pharmacology*; D011663:Pulmonary Surfactants / Q000494:pharmacology*; D013499:Surface Properties
D005456:Fluorescent Dyes; D008051:Lipid Bilayers; D008563:Membrane Lipids; D008565:Membrane Proteins; D010715:Phosphatidylglycerols; D011510:Proteolipids; D011663:Pulmonary Surfactants; D015060:1,2-Dipalmitoylphosphatidylcholine; C030345:1,2-dipalmitoylphosphatidylglycerol