Interaction between perdeuterated dimyristoylphosphatidylcholine and low molecular weight pulmonary surfactant protein SP-C

Enhanced structure/function of mTSPO translocator in lipid:surfactant mixed micelles

TSPO is a ubiquitous transmembrane protein used as a pharmacological marker in neuroimaging. The only known atomic structure of mammalian TSPOs comes from the solution NMR of mouse TSPO (mTSPO) bound to the PK11195 ligand and in a DPC surfactant environment. No structure is available in a biomimetic environment and without PK11195 which strongly stiffens the protein. We measured the effect of different amphiphilic environments on ligand-free mTSPO to study its structure/function and find optimal solubilization conditions. By replacing the SDS surfactant, where the recombinant protein is purified, with mixed lipid:surfactant (DMPC:DPC) micelles at different ratios (0:1, 1:2, and 2:1, w:w), the α-helix content and interactions and the intrinsic tryptophan (Trp) fluorescence of mTSPO are gradually increased. Small-angle X-ray scattering (SAXS) shows a more extended mTSPO/belt complex with the addition of lipids: Dmax ∼95 Å in DPC alone versus ∼142 Å in DMPC:DPC (1:2). SEC-MALLS shows that the molecular composition of the mTSPO belt is ∼98 molecules for DPC alone and ∼58 DMPC and ∼175 DPC for DMPC:DPC (1:2). Additionally, DMPC:DPC micelles stabilize mTSPO compared to DPC alone, where the protein has a greater propensity to aggregate. These structural changes are consistent with the increased affinity of mTSPO for the PK11195 ligand in presence of lipids (Kd ∼70 μM in DPC alone versus ∼0.91 μM in DMPC:DPC, 1:2), as measured by microscale thermophoresis (MST). In conclusion, mixed lipid:surfactant micelles open new possibilities for the stabilization of membrane proteins and for their study in solution in a more biomimetic amphiphilic environment.

Rapid 2H NMR Transverse Relaxation of Perdeuterated Lipid Acyl Chains of Membrane with Bound Viral Fusion Peptide Supports Large-Amplitude Motions of These Chains That Can Catalyze Membrane Fusion

An early step in cellular infection by a membrane-enveloped virus like HIV or influenza is joining (fusion) of the viral and cell membranes. Fusion is catalyzed by a viral protein that typically includes an apolar "fusion peptide" (fp) segment that binds the target membrane prior to fusion. In this study, the effects of nonhomologous HIV and influenza fp's on lipid acyl chain motion are probed with ²H NMR transverse relaxation rates (R₂'s) of a perdeuterated DMPC membrane. Measurements were made between 35 and 0 °C, which brackets the membrane liquid-crystalline-to-gel phase transitions. Samples were made with either HIV "GPfp" at pH 7 or influenza "HAfp" at pH 5 or 7. GPfp induces vesicle fusion at pH 7, and HAfp induces more fusion at pH 5 vs 7. GPfp bound to DMPC adopts an intermolecular antiparallel β sheet structure, whereas HAfp is a monomer helical hairpin. The R₂'s of the no peptide and HAfp, pH 7, samples increase gradually as temperature is lowered. The R₂'s of GPfp and HAfp, pH 5, samples have very different temperature dependence, with a ∼10× increase in R₂^CD2 when temperature is reduced from 25 to 20 °C and smaller but still substantial R₂'s at 10 and 0 °C. The large R₂'s with GPfp and HAfp, pH 5, are consistent with large-amplitude motions of lipid acyl chains that can aid fusion catalysis by increasing the population of chains near the aqueous phase, which is the chain location for transition states between membrane fusion intermediates.

Physiological fluid interfaces: Functional microenvironments, drug delivery targets, and first line of defense

Fluid interfaces, i.e. the boundary layer of two liquids or a liquid and a gas, play a vital role in physiological processes as diverse as visual perception, oral health and taste, lipid metabolism, and pulmonary breathing. These fluid interfaces exhibit a complex composition, structure, and rheology tailored to their individual physiological functions. Advances in interfacial thin film techniques have facilitated the analysis of such complex interfaces under physiologically relevant conditions. This allowed new insights on the origin of their physiological functionality, how deviations may cause disease, and has revealed new therapy strategies. Furthermore, the interactions of physiological fluid interfaces with exogenous substances is crucial for understanding certain disorders and exploiting drug delivery routes to or across fluid interfaces. Here, we provide an overview on fluid interfaces with physiological relevance, namely tear films, interfacial aspects of saliva, lipid droplet digestion and storage in the cell, and the functioning of lung surfactant. We elucidate their structure-function relationship, discuss diseases associated with interfacial composition, and describe therapies and drug delivery approaches targeted at fluid interfaces. STATEMENT OF SIGNIFICANCE: Fluid interfaces are inherent to all living organisms and play a vital role in various physiological processes. Examples are the eye tear film, saliva, lipid digestion & storage in cells, and pulmonary breathing. These fluid interfaces exhibit complex interfacial compositions and structures to meet their specific physiological function. We provide an overview on physiological fluid interfaces with a focus on interfacial phenomena. We elucidate their structure-function relationship, discuss diseases associated with interfacial composition, and describe novel therapies and drug delivery approaches targeted at fluid interfaces. This sets the scene for ocular, oral, or pulmonary surface engineering and drug delivery approaches.

Divide & Conquer: Surfactant Protein SP-C and Cholesterol Modulate Phase Segregation in Lung Surfactant

Lung surfactant (LS) is an essential system supporting the respiratory function. Cholesterol can be deleterious for LS function, a condition that is reversed by the presence of the lipopeptide SP-C. In this work, the structure of LS-mimicking membranes has been analyzed under the combined effect of SP-C and cholesterol by deuterium NMR and phosphorus NMR and by electron spin resonance. Our results show that SP-C induces phase segregation at 37°C, resulting in an ordered phase with spectral features resembling an interdigitated state enriched in dipalmitoylphosphatidylcholine, a liquid-crystalline bilayer phase, and an extremely mobile phase consistent with small vesicles or micelles. In the presence of cholesterol, POPC and POPG motion seem to be more hindered by SP-C than dipalmitoylphosphatidylcholine. The use of deuterated cholesterol did not show signs of specific interactions that could be attributed to SP-C or to the other hydrophobic surfactant protein SP-B. Palmitoylation of SP-C had an indirect effect on the extent of protein-lipid perturbations by stabilizing SP-C structure, and seemed to be important to maximize differences among the lipids participating in each phase. These results shed some light on how SP-C-induced lipid perturbations can alter membrane structure to sustain LS functionality at the air-liquid interface.

Interaction of a charged polymer with zwitterionic lipid vesicles

The interaction between polyethylenimine (PEI) and phospholipid bilayers plays an important role in several biophysical applications such as DNA transfection of target cells. Despite considerable investigation into the nature of the interaction between PEI and phospholipid bilayers, the physical process remains poorly understood. In this paper, we study the impact of PEI on 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) vesicles as a function of salt concentration using several techniques including dynamic (DLS) and static (SLS) light scattering, differential scanning calorimetry (DSC), and nuclear magnetic resonance (NMR). At low salt concentration, vesicles aggregate, leading to the formation of stable clusters whose final size depends on the PEI concentration. At high salt concentration the system does not aggregate; DSC and NMR data reveal that the PEI penetrates into the bilayer, and SLS measurements are consistent with PEI crossing the bilayer. The transfectional ability of PEI is discussed in terms of these results.

Effects of palmitoylation on dynamics and phospholipid-bilayer-perturbing properties of the N-terminal segment of pulmonary surfactant protein SP-C as shown by 2H-NMR

It has been proposed that palmitoylation of the N-terminal segment of surfactant protein SP-C is important for maintaining association of pulmonary surfactant complexes with interfacial films compressed to high pressures at the end of expiration. In this study, we examined surfactant membrane models containing palmitoylated and nonpalmitoylated synthetic peptides, based on the N-terminal SP-C sequence, in dipalmitoylphosphatidylcholine (DPPC)/egg phosphatidylglycerol (7:3, w/w) by (2)H-NMR. Perturbations of lipid properties by the peptide versions were compared in samples containing chain- and headgroup-deuterated lipid (DPPC-d(62) and DPPC-d(4) respectively). Also, deuterated peptide palmitate chains were compared with those of DPPC in otherwise identical lipid-protein mixtures. Palmitoylated peptide increased average DPPC-d(62) chain orientational order slightly, particularly for temperatures spanning gel and liquid crystalline coexistence, implying penetration of palmitoylated peptide into ordered membrane. In contrast, the nonpalmitoylated peptide had a small disordering effect in this temperature range. Both peptide versions perturbed DPPC-d(4) headgroup orientation similarly, suggesting little effect of palmitoylation on the largely electrostatic peptide-headgroup interaction. Deuterated acyl chains attached to the SP-C N-terminal segment displayed a qualitatively different distribution of chain order, and lower average order, than DPPC-d(62) in the same membranes. This likely reflects local perturbation of lipid headgroup spacing by the peptide portion interacting with the bilayer near the peptide palmitate chains. This study suggests that SP-C-attached acyl chains could be important for coupling of lipid and protein motions in surfactant bilayers and monolayers, especially in the context of ordered phospholipid structures such as those potentially formed during exhalation, when stabilization of the respiratory surface by surfactant is the most crucial.

Interaction of toposome from sea-urchin yolk granules with dimyristoyl phosphatidylserine model membranes: a 2H-NMR study

The yolk granule is the most abundant membrane-bound organelle present in sea urchin eggs and embryos. The major protein component of this organelle, toposome, accounts for approximately 50% of the total yolk protein and has been shown to be localized to the embryonic cell surface. Extensive characterization in several laboratories has defined a role for toposome in mediating membrane-membrane interactions. The current study expands the analysis of toposome-membrane interaction by defining toposome-induced changes to the lipid bilayer. The effect of toposome on the biophysical properties of phosphatidyl serine (PS) multibilayers was investigated using deuterium nuclear magnetic resonance and perdeuterated dimyristoyl PS (DMPS-d(54)). Toposome was found to have little effect on DMPS-d(54) chain orientational order in both the gel and liquid-crystalline phases. Timescales for DMPS-d(54) reorientation were investigated using quadropole echo decay. Echo decay times were sensitive to toposome in the liquid-crystalline phase but not in the gel phase. Additional information about the perturbation of bilayer motions by toposome was obtained by analyzing its effect on the decay of Carr-Purcell-Meiboom-Gill echo trains. Collectively, these results suggest that toposome interacts peripherally with DMPS bilayers and that it increases the amplitude of lipid reorientation, possibly through local enhancement of bilayer curvature.

Evolution of pulmonary surfactants for the treatment of neonatal respiratory distress syndrome and paediatric lung diseases

This review documents the evolution of surfactant therapy, beginning with observations of surfactant deficiency in respiratory distress syndrome, the basis of exogenous surfactant treatment and the development of surfactant-containing novel peptides patterned after SP-B. We critically analyse the molecular interactions of surfactant proteins and phospholipids contributing to surfactant function.

CONCLUSION

Peptide-containing surfactant provides clinical efficacy in the treatment of respiratory distress syndrome and offers promise for treating other lung diseases in infancy.

Interaction of the N-terminal segment of pulmonary surfactant protein SP-C with interfacial phospholipid films

Pulmonary surfactant protein SP-C is a 35-residue polypeptide composed of a hydrophobic transmembrane alpha-helix and a polycationic, palmitoylated-cysteine containing N-terminal segment. This segment is likely the only structural motif the protein projects out of the bilayer in which SP-C is inserted and is therefore a candidate motif to participate in interactions with other bilayers or monolayers. In the present work, we have detected intrinsic ability of a peptide based on the sequence of the N-terminal segment of SP-C to interact and insert spontaneously into preformed zwitterionic or anionic phospholipid monolayers. The peptide expands the pi-A compression isotherms of interfacial phospholipid/peptide films, and perturbs the lipid packing of phospholipid films during compression-driven liquid-expanded to liquid-condensed lateral transitions, as observed by epifluorescence microscopy. These results demonstrate that the sequence of the SP-C N-terminal region has intrinsic ability to interact with, insert into, and perturb the structure of zwitterionic and anionic phospholipid films, even in the absence of the palmitic chains attached to this segment in the native protein. This effect has been related with the ability of SP-C to facilitate reinsertion of surface active lipid molecules into the lung interface during respiratory compression-expansion cycling.

Interaction of pulmonary surfactant protein SP-A with DPPC/egg-PG bilayers

In the mixture of lipids and proteins which comprise pulmonary surfactant, the dominant protein by mass is surfactant protein A (SP-A), a hydrophilic glycoprotein. SP-A forms octadecamers that interact with phospholipid bilayer surfaces in the presence of calcium. Deuterium NMR was used to characterize the perturbation by SP-A, in the presence of 5 mM Ca(2+), of dipalmitoyl phosphatidylcholine (DPPC) properties in DPPC/egg-PG (7:3) bilayers. Effects of SP-A were uniformly distributed over the observed DPPC population. SP-A reduced DPPC chain orientational order significantly in the gel phase but only slightly in the liquid-crystalline phase. Quadrupole echo decay times for DPPC chain deuterons were sensitive to SP-A in the liquid-crystalline mixture but not in the gel phase. SP-A reduced quadrupole splittings of DPPC choline beta-deuterons but had little effect on choline alpha-deuteron splittings. The observed effects of SP-A on DPPC/egg-PG bilayer properties differ from those of the hydrophobic surfactant proteins SP-B and SP-C. This is consistent with the expectation that SP-A interacts primarily at bilayer surfaces.

The N-terminal segment of pulmonary surfactant lipopeptide SP-C has intrinsic propensity to interact with and perturb phospholipid bilayers

In the present study, 13-residue peptides with sequences corresponding to the native N-terminal segment of pulmonary SP-C (surfactant protein C) have been synthesized and their interaction with phospholipid bilayers characterized. The peptides are soluble in aqueous media but associate spontaneously with bilayers composed of either zwitterionic (phosphatidylcholine) or anionic (phosphatidylglycerol) phospholipids. The peptides show higher affinity for anionic than for zwitterionic membranes. Interaction of the peptides with both zwitterionic and anionic membranes promotes phospholipid vesicle aggregation, and leakage of the aqueous content of the vesicles. The lipid-peptide interaction includes a significant hydrophobic component for both zwitterionic and anionic membranes, although the interaction with phosphatidylglycerol bilayers is also electrostatic in nature. The effects of the SP-C N-terminal peptides on the membrane structure are mediated by significant perturbations of the packing order and mobility of phospholipid acyl chain segments deep in the bilayer, as detected by differential scanning calorimetry and spin-label ESR. These results suggest that the N-terminal region of SP-C, even in the absence of acylation, possesses an intrinsic propensity to interact with and perturb phospholipid bilayers, thereby potentially facilitating SP-C promoting bilayer-monolayer transitions at the alveolar spaces.

Superficial disposition of the N-terminal region of the surfactant protein SP-C and the absence of specific SP-B-SP-C interactions in phospholipid bilayers

A dansylated form of porcine surfactant-associated protein C (Dns-SP-C), bearing a single dansyl group at its N-terminal end, has been used to characterize the lipid-protein and protein-protein interactions of SP-C reconstituted in phospholipid bilayers, using fluorescence spectroscopy. The fluorescence emission spectrum of Dns-SP-C in phospholipid bilayers is similar to the spectrum of dansyl-phosphatidylethanolamine, and indicates that the N-terminal end of the protein is located at the surface of the membranes and is exposed to the aqueous environment. In membranes containing phosphatidylglycerol (PG), the fluorescence of Dns-SP-C shows a 3-fold increase with respect to the fluorescence of phosphatidylcholine (PC), suggesting that electrostatic lipid-protein interactions induce important effects on the structure and disposition of the N-terminal segment of the protein in these membranes. This effect saturates above 20% PG molar content in the bilayers. The parameters for the interaction of Dns-SP-C with PC or PG have been estimated from the changes induced in the fluorescence emission spectrum of the protein. The protein had similar K(d) values for its interaction with the different phospholipids tested, of the order of a few micromolar. Cooling of Dns-SP-C-containing dipalmitoyl PC bilayers to temperatures below the phase transition of the phospholipid produced a progressive blue-shift of the fluorescence emission of the protein. This effect is interpreted as a consequence of the transfer of the N-terminal segment of the protein into less polar environments that originate during protein lateral segregation. This suggests that conformation and interactions of the N-terminal segment of SP-C could be important in regulating the lateral distribution of the protein in surfactant bilayers and monolayers. Potential SP-B-SP-C interactions have been explored by analysing fluorescence resonance energy transfer (RET) from the single tryptophan in porcine SP-B to dansyl in Dns-SP-C. RET has been detected in samples where native SP-B and Dns-SP-C were concurrently reconstituted in PC or PG bilayers. However, the analysis of the dependence of RET on the protein density excluded specific SP-B-Dns-SP-C associations.

High-yield purification of lung surfactant proteins sp-b and sp-c and the effects on surface activity

Several protocols for purification of milligram quantities of lung surfactant proteins (SP)-B and SP-C were studied for separation efficiency and surface activity of the isolated proteins recombined with synthetic phospholipids (SPL). SP-B and SP-C were obtained from calf lung surfactant extract by C8 chromatography with isocratic elution by either of three solvent systems: 7:1:0.4 MeOH/CHCl(3)/5% 0.1 M HCl (solvent A), 7:1 MeOH/CHCl(3)+ 0.1% TFA (solvent B), and 7:1:0.4 MeOH/CHCl(3)/H(2)O + 0.1% TFA (solvent C). Solvents A and C yielded pure apoprotein in a single pass, with estimated total protein recoveries of >85 and >90%, respectively. Solvent B was less effective in purifying SP-B and SP-C, had a lower recovery efficiency, and gave isolates with less surface activity. Mixtures of SPL plus SP-B eluted with solvents A and C adsorbed to equilibrium surface tensions of 21-22 mN/m and reached minimum surface tensions <1 mN/m during dynamic cycling. Mixtures of SPL with SP-C obtained with solvents A and C had equilibrium surface tensions of 26-27 mN/m and minimum dynamic values of 2-7 mN/m. The ability to obtain milligrams of virtually lipid-free SP-B and SP-C in a single column pass will facilitate research on their biological, structural, and biophysical properties.

Selective labeling of pulmonary surfactant protein SP-C in organic solution

Pulmonary surfactant protein SP-C has been isolated from porcine lungs and treated with dansyl isothiocyanate in chloroform:methanol 2:1 (v/v) solutions,under conditions optimized to introduce a single dansyl group covalently attached to the N-terminalamine group of the protein without loss of its native thioesther-linked palmitic chains. The resulting derivative Dans-SP-C conserves the secondary structure of native SP-C as well as the ability to promote interfacial adsorption of DPPC suspensions and to affect the thermotropic behavior of DPPC bilayers. This derivative can be used to characterize lipid-protein and protein-protein interactions of a native-like SP-C in lipid/protein complexes.

Formation of three-dimensional protein-lipid aggregates in monolayer films induced by surfactant protein B

This study focuses on the structural organization of surfactant protein B (SP-B) containing lipid monolayers. The artificial system is composed of the saturated phospholipids dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG) in a molar ratio of 4:1 with 0.2 mol% SP-B. The different "squeeze-out" structures of SP-B were visualized by scanning probe microscopy and compared with structures formed by SP-C. Particularly, the morphology and material properties of mixed monolayers containing 0.2 mol% SP-B in a wide pressure range of 10 to 54 mN/m were investigated revealing that filamentous domain boundaries occur at intermediate surface pressure (15-30 mN/m), while disc-like protrusions prevail at elevated pressure (50-54 mN/m). In contrast, SP-C containing lipid monolayers exhibit large flat protrusions composed of stacked bilayers in the plateau region (app. 52 mN/m) of the pressure-area isotherm. By using different scanning probe techniques (lateral force microscopy, force modulation, phase imaging) it was shown that SP-B is dissolved in the liquid expanded rather than in the liquid condensed phase of the monolayer. Although artificial, the investigation of this system contributes to further understanding of the function of lung surfactant in the alveolus.

Method of purification affects some interfacial properties of pulmonary surfactant proteins B and C and their mixtures with dipalmitoylphosphatidylcholine

Two methods were employed for preparation of lipid extracts from porcine lung surfactant. Pulmonary surfactant proteins SP-B and SP-C were isolated from the extracts using gel-exclusion chromatography on LH-60 with chloroform:methanol acidified with hydrochloric acid. Monolayers of pure SP-B or SP-C isolated from butanol lipid extracts spread at the air-water interface showed larger molecular areas than those determined in films of SP-B or SP-C isolated from chloroform surfactant extracts. Aqueous dispersions of dipalmitoylphosphatidylcholine (DPPC) supplemented with 2.5 and 5.0 wt% of SP-B or SP-C obtained from butanol extracts adsorbed faster to the air-water interface than their counterparts reconstituted with proteins isolated from chloroform extracts. Surface pressure-area characteristics of spread monolayers of DPPC plus SP-B or SP-C did not depend on the method of isolation of the proteins. The diagrams of the mean molecular areas vs. composition for the monolayers of DPPC plus SP-B or SP-C showed positive deviations from the additivity rule, independently of the procedure used for preparation of lipid extract surfactant. Matrix-assisted laser desorption/ionization spectrometry of the proteins isolated from different extraction solvents was consistent with some differences in the chemical compositions of SP-Bs. Butylation of SP-B during extraction of surfactant pellet with butanol may account for the differences observed in the molecular masses of SP-Bs isolated by the two different extraction protocols. The study suggests that the method of purification of SP-B and SP-C may modify their ability to enhance the adsorption rates of DPPC/protein mixtures, and this may be relevant to the formulation of protein-supplemented lipids for exogenous treatment of pulmonary surfactant insufficiency.

Synthetic mimics of surfactant proteins B and C: in vitro surface activity and effects on lung compliance in two animal models of surfactant deficiency

Synthetic surfactant peptides SP-B1-78 and SP-C1-31 in a standard phospholipid mixture have been employed to examine the correlation between in vitro surface activity and in vivo function of synthetic surfactant preparations in the isolated rat lung and premature rabbit models of respiratory distress syndrome. Monolayer techniques showed that SP-B peptides have a high propensity for association with a phospholipid structure. By dynamic respreading, synthetic SP-B and SP-C showed rapid spreading and attained low surface tensions. Used as replacement surfactants in two animal models, these synthetic surfactant preparations partially restored lung compliance in lavaged rats and premature rabbits better than a pure phospholipid preparation and to a degree comparable to clinical surfactant, measured by pressure/volume curves. Our data confirm that in vitro functional determinations of synthetic surfactant peptides are instrumental in the preparation of replacement surfactants, and that dispersions thus selected represent viable therapeutic alternatives to current treatments for respiratory distress syndrome.

Effect of calcium on phospholipid interaction with pulmonary surfactant protein C

Porcine pulmonary surfactant-associated protein SP-C was incorporated into bilayers of chain-perdeuterated dipalmitoylphosphatidylglycerol (DPPG-d62) and chain-perdeuterated dipalmitoyl-phosphatidylcholine (DPPC-d62) and into bilayers containing 70 mol% dipalmitoyl-phosphatidylcholine (DPPC) and 30 mol% DPPG-d62 or 70 mol% DPPC-d62 and 30 mol% dipalmitoylphosphatidylglycerol (DPPG). The effect of SP-C on the phase behavior, lipid chain order, and dynamics in these bilayers was examined by using deuterium nuclear magnetic resonance. SP-C was found to have a similar effect on the chain order and phase behavior of DPPC-d62 and DPPG-d62 in bilayers with a single lipid component. In gel phase DPPC/DPPG (7:3) bilayers with one or the other lipid component chain-perdeuterated, SP-C was found to affect first spectral moment more strongly for DPPG-d62 than for DPPC-d62. This may indicate that SP-C induced a nonrandom lateral distribution in the mixed lipid bilayer. SP-C was also found to influence motions responsible for deuteron transverse relaxation in both the gel and liquid crystalline phases. The presence of 5 mM Ca2+ in the aqueous phase substantially altered the effect of SP-C on transverse relaxation in the bilayer.

Different modes of interaction of pulmonary surfactant protein SP-B in phosphatidylcholine bilayers

Pulmonary surfactant-associated protein B (SP-B) has been incorporated into vesicles of dipalmitoyl phosphatidylcholine (DPPC) or egg yolk phosphatidylcholine (PC) by two different procedures to characterize the dependence of lipid-protein interactions on the method of reconstitution. In method A the protein was dissolved in a small volume of either methanol or 60% (v/v) acetonitrile and injected into an aqueous phase containing phospholipid vesicles. In method B the vesicles were prepared by injection of a mixture of phospholipid and SP-B dissolved in methanol or aqueous acetonitrile. Both methods of reconstitution led to the extensive interaction of SP-B with PC bilayers as demonstrated by co-migration during centrifugation, marked protection against proteolysis, change in the fluorescence emission intensity of SP-B, and protection of SP-B tryptophan fluorescence from quenching by acrylamide. SP-B promoted the rapid adsorption of DPPC on an air/liquid interface irrespective of the method of protein reconstitution. However, the interfacial adsorption activity of SP-B reconstituted by method B remained stable for hours, but that of SP-B prepared by method A decreased with time. Electron microscopy showed that the injection of SP-B into an aqueous phase containing PC or DPPC vesicles (method A) induced a rapid aggregation of vesicles. By contrast, a much longer time was required for detecting vesicle aggregation when the protein was reconstituted by co-injection of SP-B and phospholipids (method B). The presence of 5% (w/w) SP-B in DPPC bilayers prepared by method B broadened the differential scanning calorimetry thermogram and decreased the enthalpy of the transition. In contrast, the injection of SP-B into preformed DPPC vesicles (method A) did not influence the gel-to-liquid phase transition of DPPC bilayers. Taken together, these results indicate that the mode and extent of interaction of SP-B with surfactant phospholipids depends on the conditions of preparation of lipid/protein samples, and that care should be taken in the interpretation of findings from reconstituted systems on the role of these surfactant proteins in the alveolar space.

Combinations of fluorescently labeled pulmonary surfactant proteins SP-B and SP-C in phospholipid films

Hydrophobic pulmonary surfactant (PS) proteins B (SP-B) and C (SP-C) modulate the surface properties of PS lipids. Epifluorescence microscopy was performed on solvent-spread monolayers of fluorescently labeled porcine SP-B (R-SP-B, labeled with Texas Red) and SP-C (F-SP-C, labeled with fluorescein) in dipalmitoylphosphatidylcholine (DPPC) (at protein concentrations of 10 and 20 wt%, and 10 wt% of both) under conditions of cyclic compression and expansion. Matrix-assisted laser desorption/ionization (MALDI) spectroscopy of R-SP-B and F-SP-C indicated that the proteins were intact and labeled with the appropriate fluorescent probe. The monolayers were compressed and expanded for four cycles at an initial rate of 0.64 A2 x mol(-1) x s(-1) (333 mm2 x s x [-1]) up to a surface pressure pi approximately 65 mN/m, and pi-area per residue (pi-A) isotherms at 22 +/- 1 degrees C were obtained. The monolayers were microscopically observed for the fluorescence emission of the individual proteins present in the film lipid matrix, and their visual features were video recorded for image analysis. The pi-A isotherms of the DPPC/protein monolayers showed characteristic "squeeze out" effects at pi approximately 43 mN/m for R-SP-B and 55 mN/m for F-SP-C, as had previously been observed for monolayers of the native proteins in DPPC. Both proteins associated with the expanded (fluid) phase of DPPC monolayers remained in or associated with the monolayers at high pi (approximately 65 mN/m) and redispersed in the monolayer upon its reexpansion. At comparable pi and area/molecule of the lipid, the proteins reduced the amounts of condensed (gel-like) phase of DPPC monolayers, with F-SP-C having a greater effect on a weight basis than did R-SP-B. In any one of the lipid/protein monolayers the amounts of the DPPC in condensed phase were the same at equivalent pi during compression and expansion and from cycle to cycle. This indicated that only minor loss of components from these systems occurred between compression-expansion cycles. This study indicates that hydrophobic PS proteins associate with the fluid phase of DPPC in films, some proteins remain at high surface pressures in the films, and such lipid-protein films can still attain high pi during compression.

Molecular structures and interactions of pulmonary surfactant components

The dominating functional property of pulmonary surfactant is to reduce the surface tension at the alveolar air/liquid interface, and thereby prevent the lungs from collapsing at the end of expiration. In addition, the system exhibits host-defense properties. Insufficient amounts of pulmonary surfactant in premature infants causes respiratory distress syndrome, a serious threat which nowadays can be effectively treated by airway instillation of surfactant preparations. Surfactant is a mixture of many molecular species, mainly phospholipids and specific proteins, surfactant protein A (SP-A), SP-B, SP-C and SP-D. SP-A and SP-D are water-soluble and belong to the collectins, a family of large multimeric proteins which structurally exhibit collagenous/lectin hybrid properties and functionally are Ca2+-dependent carbohydrate binding proteins involved in innate host-defence functions. SP-A and SP-D also bind lipids and SP-A is involved in organization of alveolar surfactant phospholipids. SP-B belongs to another family of proteins, which includes also lipid-interacting polypeptides with antibacterial and lytic properties. SP-B is a 17.4-kDa homodimer and each subunit contains three intrachain disulphides and has been proposed to contain four amphipathic helices oriented pairwise in an antiparallel fashion. SP-A, SP-B and SP-D all have been detected also in the gastrointestinal tract. SP-C, in contrast, appears to be a unique protein with extreme structural and stability properties and to exist exclusively in the lungs. SP-C is a lipopeptide containing covalently linked palmitoyl chains and is folded into a 3.7-nm alpha-helix with a central 2.3-nm all-aliphatic part, making it perfectly suited to interact in a transmembranous way with a fluid bilayer composed of dipalmitoylglycerophosphocholine, the main component of surfactant. Homozygous genetic deficiency of proSP-B causes lethal respiratory distress soon after birth and is associated with aberrant processing of the precursor of SP-C. This review focuses on the chemical composition, structures and interactions of the pulmonary surfactant, in particular the associated proteins.

Phospholipid headgroup dynamics in DOPG-d5-cytochrome c complexes as revealed by 2H and 31P NMR: the effects of a peripheral protein on collective lipid fluctuations

The dynamics of the glycerol headgroup of dioleoylphosphatidylglycerol (DOPG) in hydrated bilayers were studied by 2H and 31P NMR spectroscopy, and the effects of binding a peripheral protein, cytochrome c, were evaluated. The fast headgroup segmental motions (tau c, 10(-10)-(-13) s) of DOPG in fully hydrated bilayers were not affected upon binding of cytochrome c, as evaluated by the spin-lattice (T1) relaxation of deuterons in the DOPG glycerol headgroup. In contrast, the spin-spin (T2e) relaxation is strongly affected, indicating that slow cooperative bilayer motions (tau c, 10(-3)-10(-6) s) are enhanced upon the interaction with cytochrome c, 2H and 31P NMR spectral lineshape analysis reveal details of the nature of these motions. The importance of these effects are discussed in terms of a possible mechanism for modulating membrane-associated processes.

Spontaneous formation of interfacial lipid-protein monolayers during adsorption from vesicles

Spread and adsorbed monolayers of lipid-protein mixtures have served as models for biomembranes and pulmonary surfactant, but their similarity was unclear. Epifluorescence microscopy of monolayers spontaneously adsorbed from vesicles of dipalmitoylphosphatidylcholine or dipalmitoylphosphatidylcholine plus surfactant protein C (SP-C) showed gas, liquid expanded, and liquid condensed (LC) domains. The shapes and distribution of LC domains in the adsorbed and solvent-spread monolayers were quite similar. Labeled SP-C adsorbed into the air-water interface in the company of the lipids. In both forms of monolayers, SP-C occupied the fluid phase and reduced the size and amount of the LC domains. The properties suggest that these adsorbed and spread monolayers are analogous to one another.

Acylation of pulmonary surfactant protein-C is required for its optimal surface active interactions with phospholipids

This study investigates the importance of thioester-linked acyl groups in lung surfactant protein C (SP-C) in facilitating interactions with phospholipids that yield functionally important surface active behaviors. Native SP-C, palmitoylated at cysteine residues at positions 5 and 6, was isolated from bovine lung surfactant by liquid chromatography. Deacylated SP-C (dSP-C), unchanged in composition and sequence from SP-C but having a decreased alpha-helical content in films with dipalmitoyl phosphatidylcholine (DPPC) of 52 versus 70%, was obtained by treatment with 0.1 M sodium carbonate buffer at pH 10. Surface activity was studied for SP-C and dSP-C combined with column-purified phospholipids (PPL) from calf lung surfactant or with synthetic phospholipids (DPPC or a synthetic phospholipid mixture (SPL) containing 50:35:15, DPPC:egg phosphatidylcholine:egg phosphatidylglycerol). Interfacial measurements included surface pressure time adsorption isotherms for dispersed surfactants with diffusion minimized, dynamic surface pressure area isotherms and respreading for films in the Wilhelmy balance, and overall surface tension lowering at physiologic cycling rate in oscillating bubble experiments. Dispersions of PPL:SP-C and SPL:SP-C rapidly adsorbed to high equilibrium surface pressures of 47-48 mN/m, significantly better than corresponding dispersions containing dSP-C. The adsorption of PPL:dSP-C was essentially unchanged from that of PPL alone, and the adsorption of SPL:dSP-C was improved only slightly over SPL alone. In Wilhelmy balance studies, dynamic respreading was significantly improved over phospholipids alone in films of SP-C plus PPL, SPL, or DPPC. Respreading was improved less markedly by dSP-C in corresponding films with SPL or DPPC and not at all in films with PPL. Maximum surface pressures were also higher in cycled films of SP-C versus dSP-C combined with PPL or SPL. In bubble experiments (37 degrees C, 20 cycles/min), dispersions of PPL:SP-C and SPL:SP-C reached low minimum surface tensions of <1 and 5 mN/m, respectively, whereas PPL:dSP-C and SPL:dSP-C only reached minima of approximately 20 mN/m as did PPL and SPL alone. Acylation in SP-C is crucial for its interactions with phospholipids over the full spectrum of adsorption and dynamic surface behaviors important for lung surfactant.

Fluorescently labeled pulmonary surfactant protein C in spread phospholipid monolayers

Pulmonary surfactant, a lipid-protein complex, secreted into the fluid lining of lungs prevents alveolar collapse at low lung volumes. Pulmonary surfactant protein C (SP-C), an acylated, hydrophobic, alpha-helical peptide, enhances the surface activity of pulmonary surfactant lipids. Fluorescein-labeled SP-C (F-SP-C) (3, 6, 12 wt%) in dipalmitoylphosphatidylcholine (DPPC), and DPPC:dipalmitoylphosphatidylglycerol (DPPG) [DPPC:DPPG 7:3 mol/mol] in spread monolayers was studied by epifluorescence microscopy. Mass spectometry of F-SP-C indicated that the protein is partially deacylated and labeled with 1 mol fluorescein/1 mol protein. The protein partitioned into the fluid, or liquid expanded, phase. Increasing amounts of F-SP-C in DPPC or DPPC:DPPG monolayers decreased the size and total amounts of the condensed phase at all surface pressures. Calcium (1.6 mM) increased the amount of the condensed phase in monolayers of DPPC:DPPG but not of DPPC alone, and such monolayers were also perturbed by F-SP-C. The study indicates that SP-C perturbs the packing of neutral and anionic phospholipid monolayers even when the latter systems are condensed by calcium, indicating that interactions between SP-C and the lipids are predominantly hydrophobic in nature.

Glycosphingolipid headgroup orientation in fluid phospholipid/cholesterol membranes: similarity for a range of glycolipid fatty acids

Galactosyl ceramide (GalCer) was labeled for nuclear magnetic resonance (NMR) spectroscopy by replacement of a hydrogen atom at C6 of the galactose residue with deuterium. Wideline 2H NMR of [d1]GalCer permitted consideration of a mechanism traditionally entertained for cell surface recognition site modulation: that the nature of the fatty acid attached to the sphingosine backbone of glycosphingolipids (GSLs) importantly influences carbohydrate headgroup orientation. Comparison was made among various glycolipid fatty acids by altering hydroxylation, saturation, and chain length. Studies were carried out in unsonicated bilayer membranes mimicking several important characteristics of cell plasma membranes: fluidity, low GSL content, predominant [sn-2]monounsaturated phosphatidylcholine (PC) (1-palmitoyl-2-oleoyl PC), and the presence of cholesterol. Spectroscopy was performed on samples over a range of temperatures, which included the physiological. 2H NMR spectra of [d1]GalCer having 18-carbon saturated fatty acid (stearic acid), cis-9-unsaturated fatty acid (oleic acid), D- and L-stereoisomers of alpha-OH stearic acid, or 24-carbon saturated fatty acid (lignoceric acid) were importantly similar. This argues that for GSLs dispersed as minor components in fluid membranes, variation of the glycolipid fatty acid does not provide as much potential for direct conformational modulation of the carbohydrate portion as has sometimes been assumed. However, there was some evidence of motional differences among the species studied. The 2H NMR spectra that were obtained proved to be more complex than was anticipated. Their features could be approximated by assuming a combination of axially symmetric and axially asymmetric glycolipid motions. Presuming the appropriateness of such a analysis, at a magnetic field of 3.54 T (23.215 MHz), the experimental spectra suggested predominantly asymmetric motional contributions. At the higher field of 11.7 T (76.7 MHz, equivalent to a proton frequency of 500 MHz), spectra indicated dominance by axially symmetric rotational modes. There was also evidence of some bilayer orientation in the stronger magnetic field. The unusual observation of spectral differences between the two magnetic field strengths may involve a diamagnetic response to high field on the part of some liposome physical characteristics.

Exclusion of SP-C, but not SP-B, by gel phase palmitoyl lipids

The interactions of the hydrophobic pulmonary surfactant proteins, SP-C and SP-B, with lipid bilayers were assessed by fluorescence energy transfer. SP-C and SP-B were labeled with the fluorescent probe, succinimidyl nitrobenzoxadiazolyl amino hexanoate (NBD). Fluorescence energy transfer from NBD-SP-C and NBD-SP-B to four distinct indocarbocyanine probes (CnDiI) was utilized to determine the association of the surfactant proteins with various lipid acyl chains. In lipid mixtures including DPPC and DPPG, SP-C was associated with shorter chain and unsaturated lipids below the bulk lipid phase transition. Longer chain saturated CnDiI were excluded from SP-C aggregates. In contrast, SP-B demonstrated little acyl chain preference. The association of SP-C with shorter chain and unsaturated lipids below the bulk phase transition is interpreted to arise from a mismatch in the length of the hydrophobic region of the SP-C alpha-helix relative to the length of the hydrophobic region of dipalmitoyl lipids in the gel phase.

Pulmonary surfactant-associated polypeptide SP-C in lipid micelles: CD studies of intact SP-C and NMR secondary structure determination of depalmitoyl-SP-C(1-17)

The surfactant-associated polypeptide C (SP-C) is a 35-residue lipopolypeptide which is essential for the function of surfactants used for therapy of infant respiratory distress. Modeling based on the recently determined nuclear magnetic resonance (NMR) structure of native SP-C in an organic solvent showed that SP-C could readily insert into fluid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine bilayers. The present paper describes further physical-chemical studies of intact SP-C and its N-terminal 17-residue polypeptide fragment, depalmitoyl-SP-C(1-17), in the presence of dodecylphosphocholine micelles. The results obtained provide a link between the NMR solution structure and the behaviour of SP-C in an ordered lipid environment, and thus present new insights for rational design of SP-C analogs for therapeutic purposes.

Interactions of hydrophobic lung surfactant proteins SP-B and SP-C with dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylglycerol bilayers studied by electron spin resonance spectroscopy

Hydrophobic surfactant-associated proteins SP-B and SP-C have been isolated from porcine lungs and reconstituted in multilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC) or dipalmitoylphosphatidylglycerol (DPPG) containing different phospholipid spin probes, in order to characterize the lipid--protein interactions by electron spin resonance (ESR) spectroscopy. Both proteins caused a significant increase in the outer hyperfine splittings of all the ESR spectra, indicating that SP-B and SP-C reduce the mobility of the phospholipid acyl chains. The more hydrophobic SP-C had greater effects on phospholipid bilayers than did SP-B. The effect was saturated at protein/lipid ratios of 20% and 30% (w/w) for SP-B and SP-C, respectively, in bilayers of DPPC. SP-B and SP-C increased the ordering and decreased the mobility of the lipid acyl chains in both DPPC and DPPG bilayers in the fluid phase, without affecting the gel phase on the convention ESR time scale. On the other hand, both proteins induced a more homogeneous distribution of the phospholipid spin probes in the gel phase of DPPC. The selectivity of the interaction of SP-B and SP-C with different phospholipid species was determined from the ESR spectra of spin-labeled phospholipids with different headgroups in host bilayers of either DPPC or DPPG. SP-B showed a general preference to interact with negatively charged phospholipids, which was modulated in an ionic strength-dependent manner. At near-physiological ionic strength, SP-B showed selectivity for phosphatidylglycerol.(ABSTRACT TRUNCATED AT 250 WORDS)

The NMR structure of the pulmonary surfactant-associated polypeptide SP-C in an apolar solvent contains a valyl-rich alpha-helix

The nuclear magnetic resonance (NMR) structure of the pulmonary surfactant-associated lipoplypeptide C (SP-C) was determined in a mixed solvent of C2H3Cl/C2H3OH/ 1 M HCl 32:64:5 (v/v). Sequence-specific 1H NMR assignments and the collection of conformational constraints were achieved with two-dimensional 1H NMR, and the structure was calculated with the distance geometry program DIANA. The root mean square deviations for the well-defined polypeptide segment of residues 9-34 calculated for the 20 best energy-minimized DIANA conformers relative to their mean are 0.5 and 1.3 A for the polypeptide backbone atoms N, C alpha, and C', and for all heavy atoms, respectively. The 35-residue polypeptide chain of SP-C forms an alpha-helix between positions 9 and 34, which includes two segments of seven and four consecutive valyls that are separated by a single leucyl residue. The N-terminal hexapeptide segment, which includes two palmitoylcysteinyls, is flexibly disordered. The length of the alpha-helix is about 37 A, and the helical segment of residues 13-28, which contains exclusively aliphatic residues with branched side chains, is 23-A long and about 10 A in diameter. The alpha-helix is outstandingly regular, with virtually identical chi 1 angles for all valyl residues. The observation of a helical structure of SP-C was rather unexpected, considering that Val is generally underrepresented in alpha-helices, and it provides intriguing novel insights into the structural basis of SP-C functions as well as into general structural aspects of protein-lipid interactions in biological membranes.

Pulmonary lung surfactant synthetic peptide concentration-dependent modulation of DPPC and POPG acyl chain order in a DPPC:POPG:palmitic acid lipid mixture

Lung surfactant-associated protein interaction with lipid matrices and the effects on lipid thermotropic phase behavior are areas of active research. Many studies limit the lipids to a single or two-component system. The current investigation utilizes a three-lipid component matrix (DPPC:POPG:palmitic acid) to investigate the impact of a synthetic surfactant protein B fragment (SP-B 53-78 DiACM) on the dynamic surface activity of the lipid admixture as measured by a Wilhelmy surface balance. Also, the modulation of the individual lipid acyl chain order by the peptide within the lipid matrix is studied through the use of thermal perturbation FTIR spectroscopy. The data clearly demonstrate a concentration-dependent effect of the peptide on the surface activity with an improvement in the dynamic surface tension diagram characteristics (decreased surface tension and increased collapse plateau) especially at low, 0.36 M%, peptide concentrations. These effects are diminished upon further addition of the peptide. FTIR spectral data demonstrate that the peptide addition results in a significant increase in the acyl chain order of the DPPC and POPG components as measured by the position of the methylene stretching vibrational bands. DPPC is most sensitive to the peptide presence, while the palmitic acid is least affected. The transition temperatures of the individual lipids are also increased with the addition of the peptide. The presence of POPG in the matrix achieves the surface activity similarly seen with natural lung surfactant relative to a DPPC/palmitic acid lipid matrix alone. Its presence increases the sensitivity of the DPPC acyl chains to the presence of the peptide. These effects on the chain order are most probably related to the increased acyl chain fluidity which POPG imparts to the lipid matrix because of the presence of the cis double bond. The phosphatidylglycerol headgroup also adds a negative charge to the lipid matrix which enhances the peptide-lipid interaction. Although the palmitic acid is minimally affected by the peptide, its presence, as suggested by surface balance measurements, results in the establishment of a stable lipid film with DPPC, capable of achieving low surface tension values.

Pulmonary surfactant proteins SP-B and SP-C in spread monolayers at the air-water interface: II. Monolayers of pulmonary surfactant protein SP-C and phospholipids

The interaction of the hydrophobic pulmonary surfactant protein SP-C with dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylglycerol (DPPG) and DPPC:DPPG (7:3, mol:mol) in spread monolayers at the air-water interface has been studied. At low concentrations of SP-C (about 0.5 mol% or 3 weight%protein) the protein-lipid films collapsed at surface pressures of about 70 mN.m-1, comparable to those of the lipids alone. At initial protein concentrations higher than 0.8 mol%, or 4 weight%, the isotherms displayed kinks at surface pressures of about 50 mN.m-1 in addition to the collapse plateaux at the higher pressures. The presence of less than 6 mol%, or 27 weight%, of SP-C in the protein-lipid monolayers gave a positive deviation from ideal behavior of the mean areas in the films. Analyses of the mean areas in the protein-lipid films as functions of the monolayer composition and surface pressure showed that SP-C, associated with some phospholipid (about 8-10 lipid molecules per molecule of SP-C), was squeezed out from the monolayers at surface pressures of about 55 mN.m-1. The results suggest a potential role for SP-C to modify the composition of the monolayer at the air-water interface in the alveoli.

Pulmonary surfactant proteins SP-B and SP-C in spread monolayers at the air-water interface: I. Monolayers of pulmonary surfactant protein SP-B and phospholipids

The effects of pulmonary surfactant protein SP-B on the properties of monolayers of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG), and a mixture of DPPC:DPPG (7:3, mol:mol) were studied using spread films at the air-water interface. The addition of SP-B to the phospholipid monolayers gave positive deviations from additivity of the mean areas in the films. At low protein concentrations (less than 45% amino acid residues which corresponds to 0.5 mol% or 10 weight% SP-B) monolayers of SP-B/DPPC, SP-B/DPPG and SP-B/(DPPC:DPPG) collapsed at surface pressures of about 70 mN.m-1, comparable to those of the lipids alone. At higher concentrations of SP-B in the protein-lipid monolayers, kink points appeared in the isotherms at about 40-45 mN.m-1, implying possible exclusion of material from the films, hence, changes in the original monolayer compositions. Calculated analyses of the monolayer compositions as a function of surface pressure indicated that nearly pure SP-B, associated with small amounts of phospholipid (2-3 lipid molecules per SP-B dimer), was lost from SP-B/DPPC, SP-B/DPPG, and SP-B/(DPPC:DPPG) films at surface pressures higher than 40-45 mN.m-1. The results are consistent with a low effectiveness of SP-B in removing saturated phospholipids, DPPC or DPPG, from the spread SP-B/phospholipid films.

Protein-phospholipid interactions in pulmonary surfactant. The Parker B. Francis Lectureship

SP-B is the protein in pulmonary surfactant with the greatest capacity to augment the phospholipids, ability to resist surface tension, and capability to prevent collapse of pulmonary alveoli. Synthetic peptides derived from the structure of SP-B and simplified analogues of these SP-B-derived peptides were found by tryptophan fluorescence to partition within the phospholipid layer in contact with both polar head groups and acyl side chains of the phospholipids. The intermittent hydrophilic basic residues were found to be essential for full activity, probably because of electrostatic interactions formed with phosphates of the polar head groups. The hydrophobic stretches of residues in SP-B and the related peptides supplement the activity through interaction with the phospholipid acyl side chains. By increasing intermolecular and intramolecular order of the phospholipid layer and thereby stability of the layer, the SP-B analogues provide strong surfactant activity. Simplified peptide analogues of SP-B, dispersed in DPPC and POPG, provide strong surfactant activity in vitro and in the lungs of premature infant rabbits, rhesus monkeys, and humans.

Human surfactant protein-C: genetic homogeneity and expression in RDS; comparison with other species

Human surfactant protein C (SP-C) mRNA is detected early during fetal lung development before the differentiation of the type II cell and the need for surfactant. Later in life SP-C contributes to the surface-lowering properties of surfactant, as shown by several investigators. In this study we sequenced both coding and noncoding regions of 12 genomic SP-C clones from several human groups including RDS, whites, and black Nigerians, and examined the expression of SP-C in tissues from RDS and from non-RDS. The data showed that all clones had identical DNA sequences, not only within coding regions, consistent with previous observations, but also within intervening, 5' flanking, and 3' untranslated regions. Some differences from the previously published sequence were noted. The expression of SP-C in tissues from RDS and non-RDS as determined by tissue in situ hybridization was comparable between the two groups, suggesting that altered SP-C expression, the result of pretranslational regulatory abnormalities, is an unlikely contributor to the pathogenesis of RDS. In addition the results show, using genomic blot analysis, that a remarkable conservation within coding and 5' flanking but not within 3' untranslated sequences exists in all mammalian species examined. These data taken together suggest that strong evolutionary pressures have been exerted on SP-C to maintain conservation, not only among humans but also among species, which may underscore important roles of SP-C in as yet unknown developmental/functional lung processes.

Lipid effects on aggregation of pulmonary surfactant protein SP-C studied by fluorescence energy transfer

The self-association of pulmonary surfactant protein SP-C in lipid vesicles was studied using fluorescence energy transfer. Bovine SP-C was labeled with two fluorescent probes, succinimidyl 6-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]hexanoate and eosin isothiocyanate, on the amino terminus of the protein, producing NBD-SP-C and EITC-SP-C, respectively. The N-terminus of SP-C was relatively immobile between 20 and 37 degrees C, as demonstrated by high fluorescence anisotropy of NBD-SP-C and EITC-SP-C. The mobility increased at the transition of the lipid to the fluid phase. Using fluorescence energy transfer, with NBD-SP-C as the donor and EITC-SP-C as the acceptor, a high degree of SP-C/SP-C association was found below 25 degrees C, decreasing to very little self-association above 42 degrees C in 7:1 1,2-dipalmitoylphosphatidylcholine-1,2-dipalmitoylphosphatidylglycerol (DPPC-DPPG) vesicles. The fraction of SP-C aggregated below 37 degrees C in 7:1 DPPC-DPPG was estimated from the observed energy transfer to be more than 70% of total SP-C. In various lipid mixtures, self-association of SP-C was dependent on the presence of at least some gel-phase lipids. In a lipid mixture resembling pulmonary surfactant, gradually increasing self-association was observed below 38 degrees C. The relation of the present data to the state of aggregation of SP-C in pulmonary surfactant is discussed.

Solubility of hydrophobic surfactant proteins in organic solvent/water mixtures. Structural studies on SP-B and SP-C in aqueous organic solvents and lipids

The solubility of hydrophobic pulmonary surfactant proteins in different organic solvents and organic solvent/water combinations has been analyzed. Three organic solvents have been selected: methanol (MetOH), acetonitrile (ACN) and trifluoroethanol (TFE). Porcine SP-B showed very similar calculated secondary structure when dissolved in methanol, 60% ACN or 70% TFE and reconstituted in lysophosphatidylcholine (LPC) micelles or dipalmitoylphosphatidylcholine (DPPC) vesicles, as deduced from circular dichroism studies. SP-B was calculated to possess around 45% of alpha-helix in all these systems. The fluorescence emission spectrum of SP-B has been also characterized in aqueous solvents and lipids. It always showed a splitting of the tryptophan contribution into two components with different emission maxima. SP-C had a very different structure in 80% ACN or 70% TFE. While alpha-helix was the main secondary structure of SP-C in ACN/water mixtures--around 50%--, it had almost exclusively beta-structure when dissolved in 70% TFE. The CD spectrum of SP-C in TFE showed dependence on the protein concentration, suggesting that protein-protein interactions could be important in this beta-conformation. SP-C reconstituted in LPC micelles or DPPC vesicles had a CD spectrum qualitatively similar to that one in aqueous ACN, with a dominant alpha-helical structure. The alpha-helical content of SP-C in micelles of LPC and vesicles of DPPC, 60 and 70%, respectively, was calculated to be higher than the alpha-helical content of the protein dissolved in any aqueous organic solvent.

Lung surfactant proteins, SP-B and SP-C, alter the thermodynamic properties of phospholipid membranes: a differential calorimetry study

The ability of the low molecular weight lung surfactant-associated proteins, SP-B and SP-C, to alter the thermotropic properties of synthetic multilamellar vesicles was tested using differential scanning calorimetry (DSC). The presence of either SP-B or SP-C in dipalmitoylphosphatidylcholine (DPPC) or dipalmitoylphosphatidylglycerol (DPPG) multilamellar vesicles broadened the DSC thermogram and reduced the enthalpy of transition in a concentration-dependent manner. With both proteins, the temperature at which the peak of the phase transition (Tm) was detected was shifted to a higher value. The increase in Tm caused by both proteins was greater with DPPG than DPPC. We have interpreted these results as implying the presence of a protein-perturbed domain of lipid. Both SP-B and SP-C were found to influence the surface activity of the phospholipids in a concentration-dependent fashion. We speculate that instability of lipid packing predicted to occur at protein-created lipid domain boundaries may be important for the expression of surface activity in pulmonary surfactant.