Effects of the lung surfactant protein B construct Mini-B on lipid bilayer order and topography
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The hydrophobic lung surfactant protein, SP-B, is essential for survival. Cycling of lung volume during respiration requires a surface-active lipid–protein layer at the alveolar air–water interface. SP-B may contribute to surfactant layer maintenance and renewal by facilitating contact and transfer between the surface layer and bilayer reservoirs of surfactant material. However, only small effects of SP-B on phospholipid orientational order in model systems have been reported. In this study, N-terminal (SP-B8–25) and C-terminal (SP-B63–78) helices of SP-B, either linked as Mini-B or unlinked but present in equal amounts, were incorporated into either model phospholipid mixtures or into bovine lipid extract surfactant in the form of vesicle dispersions or mechanically oriented bilayer samples. Deuterium and phosphorus nuclear magnetic resonance (NMR) were used to characterize effects of these peptides on phospholipid chain orientational order, headgroup orientation, and the response of lipid–peptide mixtures to mechanical orientation by mica plates. Only small effects on chain orientational order or headgroup orientation, in either vesicle or mechanically oriented samples, were seen. In mechanically constrained samples, however, Mini-B and its component helices did have specific effects on the propensity of lipid–peptide mixtures to form unoriented bilayer populations which do not exchange with the oriented fraction on the timescale of the NMR experiment. Modification of local bilayer orientation, even in the presence of mechanical constraint, may be relevant to the transfer of material from bilayer reservoirs to a flat surface-active layer, a process that likely requires contact facilitated by the formation of highly curved protrusions.
KeywordsLung surfactant Pulmonary surfactant Surfactant protein B (SP-B) Deuterium NMR Oriented bilayer Orientational order
This work was supported by the Canadian Institutes of Health Research and by the Natural Sciences and Engineering Research Council of Canada, and by the National Institutes of Health (HL-094641). We acknowledge the CREAIT network at Memorial for assisting with NMR facility maintenance and training, and in particular Dr. Celine Schneider. We would like to thank Lauren MacEachern for her help with the simulations, Donna Jackman for her help with sample preparation, as well as Frans Walther for the assistance he provided.
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