Interaction of the C-Terminal Peptide of Pulmonary Surfactant Protein B (SP-B) with a Bicellar Lipid Mixture Containing Anionic Lipid

Abstract

The hydrophobic lung surfactant SP-B is essential for respiration. SP-B promotes spreading and adsorption of surfactant at the alveolar air-water interface and may facilitate connections between the surface layer and underlying lamellar reservoirs of surfactant material. SP-B63–78 is a cationic and amphipathic helical peptide containing the C-terminal helix of SP-B. 2H NMR has been used to examine the effect of SP-B63–78 on the phase behavior and dynamics of bicellar lipid dispersions containing the longer chain phospholipids DMPC-d 54 and DMPG and the shorter chain lipid DHPC mixed with a 3∶1∶1 molar ratio. Below the gel-to-liquid crystal phase transition temperature of the longer chain components, bicellar mixtures form small, rapidly reorienting disk-like particles with shorter chain lipid components predominantly found around the highly curved particle edges. With increasing temperature, the particles coalesce into larger magnetically-oriented structures and then into more extended lamellar phases. The susceptibility of bicellar particles to coalescence and large scale reorganization makes them an interesting platform in which to study peptide-induced interactions between lipid assemblies. SP-B63–78 is found to lower the temperature at which the orientable phase transforms to the more extended lamellar phase. The peptide also changes the spectrum of motions contributing to quadrupole echo decay in the lamellar phase. The way in which the peptide alters interactions between bilayered micelle structures may provide some insight into some aspects of the role of full-length SP-B in maintaining a functional surfactant layer in lungs.

Effects of Amphipathic Polypeptides on Membrane Organization Inferred from Studies Using Bicellar Lipid Mixtures

“…In particular, the spectrum at the transition is a superposition of spectral components from the two coexisting phases. In an earlier study using DMPC- d 54 /DMPG/DHPC (3:1:1), it was reported that SP-B 63–78 lowers the temperature for the transition from the magnetically orientable phase to the extended lamellar phase in lipid dispersions with this composition . The spectra of DMPC- d 54 /DMPG/DHPC (3:1:1) with 10% (w/w) SP-B 63–78 shown in Figure b are consistent with this observation.…”

“…In an earlier study using DMPC-d 54 /DMPG/DHPC (3:1:1), it was reported that SP-B 63−78 lowers the temperature for the transition from the magnetically orientable phase to the extended lamellar phase in lipid dispersions with this composition. 39 The spectra of DMPC-d 54 /DMPG/DHPC (3:1:1) with 10% (w/w) SP-B 63−78 shown in Figure 3b are consistent with this observation. The peptide-induced lowering of the magnetically orientable to extended lamellar phase transition has been interpreted as indicating that, for bicellar mixtures with an anionic long-chain lipid component, SP-B 63−78 promotes coalescence to the more extended lamellar phase at a lower temperature than in the absence of peptide.…”

“…The transition from the magnetically orientable intermediate-temperature phase to the higher-temperature extended lamellar phase is largely driven by increased mixing of the short-chain and long-chain lipid components as temperature is raised. The observation that SP-B 63−78 lowers the temperature for this transition in the DMPC/DMPG/ DHPC mixture is consistent with previous results39 and suggests that this peptide promotes lipid mixing. This possibility is supported by the observation that the DMPC-d 54 and DMPGd 54 spectra in panels (a) and (b) of Figure6are very similar, suggesting that the two long-chain lipid components occupy nearly identical environments.…”

“…The sensitivity of bicellar lipid mixture properties to peptide-induced perturbation has previously been used to study the effects of a neuropeptide, Met-enkephalin, and an antibiotic, gramicidin A, on model membrane systems. 37,38 In one earlier study, 39 deuterium nuclear magnetic resonance ( 2 H NMR) was used to assess the effect of the SP-B fragment, SP-B 63−78 , on the temperature dependence of particle aggregation in a bicellar system comprising a 3:1:1 mixture of chain-perdeuterated 1,2dimyristoyl-sn-glycero-3-phophocholine (DMPC-d 54 ), 1,2-dimyristoyl-sn-glycero-3-phopho-(1′-rac-glycerol) (DMPG), and 1,2-dihexanoyl-sn-glycero-3-phophocholine (DHPC).…”

“…Such lipid mixtures, known as bicellar systems, are useful for studying membrane perturbation because they undergo easily detected transitions between differently organized states in response to factors that affect chain orientational order and/or mixing of their long-chain and short-chain lipid components. The sensitivity of bicellar lipid mixture properties to peptide-induced perturbation has previously been used to study the effects of a neuropeptide, Met-enkephalin, and an antibiotic, gramicidin A, on model membrane systems. , In one earlier study, deuterium nuclear magnetic resonance ( 2 H NMR) was used to assess the effect of the SP-B fragment, SP-B 63–78 , on the temperature dependence of particle aggregation in a bicellar system comprising a 3:1:1 mixture of chain-perdeuterated 1,2-dimyristoyl- sn -glycero-3-phophocholine (DMPC- d 54 ), 1,2-dimyristoyl- sn -glycero-3-phopho-(1′-rac-glycerol) (DMPG), and 1,2-dihexanoyl- sn -glycero-3-phophocholine (DHPC).…”