Effects of the lung surfactant protein B construct Mini-B on lipid bilayer order and topography

Abstract

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.

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

“…One possibility that has been considered is that the observed disruption of mechanical orientation induced by SP-B fragments might reflect interactions between the peptide and randomly oriented bilayers, at the hydration stage of sample preparation, that then interfered with flattening of the bilayers when orientational constraints were subsequently applied through the stacking of mica support plates [42]. Thus while the perturbation of mechanically oriented bilayers does provide some insight into interactions between SP-B fragment peptides and surfactant model bilayers, the constraints imposed by mechanical orientation may preclude observation of more dynamic perturbations of bilayer orientation or flatness by amphipathic peptides.…”

“…Studies of the extent to which different SP-B fragments and constructs interfere with mechanical bilayer organization suggest that such peptides can perturb bilayer surface geometry [41], [42]. In studies using bilayers deposited on mica plates, SP-B 8–25 and SP-B 63–78 were both found to promote the formation of bilayer populations with bilayer normal directions distributed over a range of angles with respect to the mechanically-oriented bilayer normal [41], [42].…”

“…In studies using bilayers deposited on mica plates, SP-B 8–25 and SP-B 63–78 were both found to promote the formation of bilayer populations with bilayer normal directions distributed over a range of angles with respect to the mechanically-oriented bilayer normal [41], [42]. It was suggested that this might reflect a peptide-induced coupling of randomly oriented bilayers, at the hydration stage of sample preparation, that would then interfere with the ability of the bilayers to spread and flatten under the influence of the orienting mica surfaces during the stacking phase of sample preparation [42]. If so, then the observed perturbation of bilayer orientation in mechanically-constrained systems could, in effect, reflect peptide-induced interbilayer interactions during sample preparation rather than the peptide induced perturbation of already flat surfaces.…”

Role of the N-Terminal Seven Residues of Surfactant Protein B (SP-B)

“…Pake quadrupole doublets split by half of the corresponding oriented sample doublet splittings. Doublets in the oriented component of the spectrum also broaden in a way consistent with increased mosaic spread [55]. The presence of SP-B (1–25,63–78) thus appears to disrupt the mechanical orientation in a small fraction of the bilayer material.…”

“…The presence of SP-B (1–25,63–78), a fragment which contains the N-terminal 7 residues as well as the first and last helices of SP-B, caused a substantial effect on the 2 H NMR splittings and hence acyl chain orientational order, particularly deep in the bilayer ( Table 1 ). The magnitude of the SP-B (1–25,63–78)-induced effect was particularly striking when compared to the effect of SP-B (8–25,63–78) [55], i.e. an identical peptide except for the absence of residues 1–7.…”