Conformational flexibility of pulmonary surfactant proteins SP-B and SP-C, studied in aqueous organic solvents

Exploring protein-protein interactions and oligomerization state of pulmonary surfactant protein C (SP-C) through FRET and fluorescence self-quenching

Pulmonary surfactant (PS) is a lipid-protein complex that forms films reducing surface tension at the alveolar air-liquid interface. Surfactant protein C (SP-C) plays a key role in rearranging the lipids at the PS surface layers during breathing. The N-terminal segment of SP-C, a lipopeptide of 35 amino acids, contains two palmitoylated cysteines, which affect the stability and structure of the molecule. The C-terminal region comprises a transmembrane α-helix that contains a ALLMG motif, supposedly analogous to a well-studied dimerization motif in glycophorin A. Previous studies have demonstrated the potential interaction between SP-C molecules using approaches such as Bimolecular Complementation assays or computational simulations. In this work, the oligomerization state of SP-C in membrane systems has been studied using fluorescence spectroscopy techniques. We have performed self-quenching and FRET assays to analyze dimerization of native palmitoylated SP-C and a non-palmitoylated recombinant version of SP-C (rSP-C) using fluorescently labeled versions of either protein reconstituted in different lipid systems mimicking pulmonary surfactant environments. Our results reveal that doubly palmitoylated native SP-C remains primarily monomeric. In contrast, non-palmitoylated recombinant SP-C exhibits dimerization, potentiated at high concentrations, especially in membranes with lipid phase separation. Therefore, palmitoylation could play a crucial role in stabilizing the monomeric α-helical conformation of SP-C. Depalmitoylation, high protein densities as a consequence of membrane compartmentalization, and other factors may all lead to the formation of protein dimers and higher-order oligomers, which could have functional implications under certain pathological conditions and contribute to membrane transformations associated with surfactant metabolism and alveolar homeostasis.

Synchrotron radiation circular dichroism spectroscopy reveals that gold and silver nanoparticles modify the secondary structure of a lung surfactant protein B analogue

Inhaled nanoparticles (NPs) depositing in the alveolar region of the lung interact initially with a surfactant layer and in vitro studies have demonstrated that NPs can adversely affect the biophysical function of model pulmonary surfactants (PS), of which surfactant protein B (SP-B) is a key component. Other studies have demonstrated the potential for NPs to modify the structure and function of proteins. It was therefore hypothesised that NPs may affect the biophysical function of PS by modifying the structure of SP-B. Synchrotron radiation circular dichroism (SRCD) spectroscopy was used to explore the effect of various concentrations of gold nanoparticles (AuNPs) (5, 10, 20 nm), silver nanoparticles (AgNPs) (10 nm) and silver citrate on the secondary structure of surfactant protein B analogue, SP-B_1-25, in a TFE/PB dispersion. For Au and Ag NPs the SRCD spectra indicated a concentration dependent reduction in the α-helical structure of SP-B_1-25 (5 nm AuNP ≈ 10 nm AgNP ≫ 10 nm AuNP > 20 nm AuNP). For AuNPs the effect was greater for the 5 nm size, which was not fully explained by consideration of surface area. The impact of the 10 nm AgNPs was greater than that of the 10 nm AuNPs and the effect of AgNPs was greater than that of silver citrate at equivalent Ag mass concentrations. For 10 nm AuNPs, SRCD spectra for dispersions in, the more physiologically relevant, DPPC showed a similar concentration dependent pattern. The results demonstrate the potential for inhaled NPs to modify SP-B_1-25 structure and thus potentially adversely impact the physiological function of the lung, however, further studies are necessary to confirm this.

About TFE: Old and New Findings

The fluorinated alcohol 2,2,2-Trifluoroethanol (TFE) has been implemented for many decades now in conformational studies of proteins and peptides. In peptides, which are often disordered in aqueous solutions, TFE acts as secondary structure stabilizer and primarily induces an α -helical conformation. The exact mechanism through which TFE plays its stabilizing roles is still debated and direct and indirect routes, relying either on straight interaction between TFE and molecules or indirect pathways based on perturbation of solvation sphere, have been proposed. Another still unanswered question is the capacity of TFE to favor in peptides a bioactive or a native-like conformation rather than simply stimulate the raise of secondary structure elements that reflect only the inherent propensity of a specific amino-acid sequence. In protein studies, TFE destroys unique protein tertiary structure and often leads to the formation of non-native secondary structure elements, but, interestingly, gives some hints about early folding intermediates. In this review, we will summarize proposed mechanisms of TFE actions. We will also describe several examples, in which TFE has been successfully used to reveal structural properties of different molecular systems, including antimicrobial and aggregation-prone peptides, as well as globular folded and intrinsically disordered proteins.

Synthetic surfactants with SP-B and SP-C analogues to enable worldwide treatment of neonatal respiratory distress syndrome and other lung diseases

Treatment of neonatal respiratory distress syndrome (RDS) using animal-derived lung surfactant preparations has reduced the mortality of handling premature infants with RDS to a 50th of that in the 1960s. The supply of animal-derived lung surfactants is limited and only a part of the preterm babies is treated. Thus, there is a need to develop well-defined synthetic replicas based on key components of natural surfactant. A synthetic product that equals natural-derived surfactants would enable cost-efficient production and could also facilitate the development of the treatments of other lung diseases than neonatal RDS. Recently the first synthetic surfactant that contains analogues of the two hydrophobic surfactant proteins B (SP-B) and SP-C entered clinical trials for the treatment of neonatal RDS. The development of functional synthetic analogues of SP-B and SP-C, however, is considerably more challenging than anticipated 30 years ago when the first structural information of the native proteins became available. For SP-B, a complex three-dimensional dimeric structure stabilized by several disulphides has necessitated the design of miniaturized analogues. The main challenge for SP-C has been the pronounced amyloid aggregation propensity of its transmembrane region. The development of a functional non-aggregating SP-C analogue that can be produced synthetically was achieved by designing the amyloidogenic native sequence so that it spontaneously forms a stable transmembrane α-helix.

Altered gene expression in the lower respiratory tract of Car6 (-/-) mice

From birth, the respiratory tract mucosa is exposed to various chemical, physical, and microbiological stress factors. Efficient defense mechanisms and strictly regulated renewal systems in the mucosa are thus required. Carbonic anhydrase VI (CA VI) is the only secreted isoenzyme of the α-CA gene family. It is transported in high concentrations in saliva and milk into the alimentary tract where it contributes to optimal pH homeostasis. Earlier study of transcriptomic responses of Car6 (-/-) mice has shown changes in the response to oxidative stress and brown fat cell differentiation in the submandibular gland. It has been suggested that CA VI delivered to the mucosal surface of the bronchiolar epithelium is an essential factor in defense and renewal of the lining epithelium. In this study, the transcriptional effects of CA VI deficiency were investigated in both trachea and lung of Car6 (-/-) mice using a cDNA microarray analysis. Functional clustering of the results indicated significant changes of gene transcription in the lower airways. The altered biological processes included antigen transport by M-cells, potassium transport, muscle contraction, and thyroid hormone synthesis. Immunohistochemical staining confirmed the absence of CA VI in the submandibular gland of Car6 (-/-) mice. Immunostaining of the trachea and lung samples revealed no differences between the knockout and wild type groups nor were any morphological changes observed. The present findings can help us to recognize novel functions for CA VI-one of the major protein constituents of saliva and milk.

Electrostatically driven lipid-protein interaction: Answers from FRET

Electrostatics govern the association of a large number of proteins with cellular membranes. In some cases, these proteins present specialized lipid-binding modules or membrane targeting domains while in other cases association is achieved through nonspecific interaction of unstructured clusters of basic residues with negatively charged lipids. Given its spatial resolution in the nanometer range, Förster resonance energy transfer (FRET) is a powerful tool to give insight into protein-lipid interactions and provide molecular level information which is difficult to retrieve with other spectroscopic techniques. In this review we present and discuss the basic formalisms of both hetero- and homo-FRET pertinent to the most commonly encountered problems in lipid-protein interaction studies and highlight some examples of implementations of different FRET methodologies to characterize lipid/protein systems in which electrostatic interactions play a crucial role. This article is part of a Special Issue entitled: Lipid-protein interactions.

Direct simulation of protein-mediated vesicle fusion: lung surfactant protein B

We simulated spontaneous fusion of small unilamellar vesicles mediated by lung surfactant protein B (SP-B) using the MARTINI force field. An SP-B monomer triggers fusion events by anchoring two vesicles and facilitating the formation of a lipid bridge between the proximal leaflets. Once a lipid bridge is formed, fusion proceeds via a previously described stalk - hemifusion diaphragm - pore-opening pathway. In the absence of protein, fusion of vesicles was not observed in either unbiased simulations or upon application of a restraining potential to maintain the vesicles in close proximity. The shape of SP-B appears to enable it to bind to two vesicles at once, forcing their proximity, and to facilitate the initial transfer of lipids to form a high-energy hemifusion intermediate. Our results may provide insight into more general mechanisms of protein-mediated membrane fusion, and a possible role of SP-B in the secretory pathway and transfer of lung surfactant to the gas exchange interface.

Critical structural and functional roles for the N-terminal insertion sequence in surfactant protein B analogs

Background

Surfactant protein B (SP-B; 79 residues) belongs to the saposin protein superfamily, and plays functional roles in lung surfactant. The disulfide cross-linked, N- and C-terminal domains of SP-B have been theoretically predicted to fold as charged, amphipathic helices, suggesting their participation in surfactant activities. Earlier structural studies with Mini-B, a disulfide-linked construct based on the N- and C-terminal regions of SP-B (i.e., ∼residues 8–25 and 63–78), confirmed that these neighboring domains are helical; moreover, Mini-B retains critical in vitro and in vivo surfactant functions of the native protein. Here, we perform similar analyses on a Super Mini-B construct that has native SP-B residues (1–7) attached to the N-terminus of Mini-B, to test whether the N-terminal sequence is also involved in surfactant activity.

Methodology/Results

FTIR spectra of Mini-B and Super Mini-B in either lipids or lipid-mimics indicated that these peptides share similar conformations, with primary α-helix and secondary β-sheet and loop-turns. Gel electrophoresis demonstrated that Super Mini-B was dimeric in SDS detergent-polyacrylamide, while Mini-B was monomeric. Surface plasmon resonance (SPR), predictive aggregation algorithms, and molecular dynamics (MD) and docking simulations further suggested a preliminary model for dimeric Super Mini-B, in which monomers self-associate to form a dimer peptide with a “saposin-like” fold. Similar to native SP-B, both Mini-B and Super Mini-B exhibit in vitro activity with spread films showing near-zero minimum surface tension during cycling using captive bubble surfactometry. In vivo, Super Mini-B demonstrates oxygenation and dynamic compliance that are greater than Mini-B and compare favorably to full-length SP-B.

Conclusion

Super Mini-B shows enhanced surfactant activity, probably due to the self-assembly of monomer peptide into dimer Super Mini-B that mimics the functions and putative structure of native SP-B.

Molecular dynamics simulation study of a pulmonary surfactant film interacting with a carbonaceous nanoparticle

This article reports an all-atom molecular dynamics simulation to study a model pulmonary surfactant film interacting with a carbonaceous nanoparticle. The pulmonary surfactant is modeled as a dipalmitoylphosphatidylcholine monolayer with a peptide consisting of the first 25 residues from surfactant protein B. The nanoparticle model with a chemical formula C188H53 was generated using a computational code for combustion conditions. The nanoparticle has a carbon cage structure reminiscent of the buckyballs with open ends. A series of molecular-scale structural and dynamical properties of the surfactant film in the absence and presence of nanoparticle are analyzed, including radial distribution functions, mean-square displacements of lipids and nanoparticle, chain tilt angle, and the surfactant protein B peptide helix tilt angle. The results show that the nanoparticle affects the structure and packing of the lipids and peptide in the film, and it appears that the nanoparticle and peptide repel each other. The ability of the nanoparticle to translocate the surfactant film is one of the most important predictions of this study. The potential of mean force for dragging the particle through the film provides such information. The reported potential of mean force suggests that the nanoparticle can easily penetrate the monolayer but further translocation to the water phase is energetically prohibitive. The implication is that nanoparticles can interact with the lung surfactant, as supported by recent experimental data by Bakshi et al.

Characterization of the in situ structural and interfacial properties of the cationic hydrophobic heteropolypeptide, KL4, in lung surfactant bilayer and monolayer models at the air-water interface: implications for pulmonary surfactant delivery

This study examines the various equilibrium in situ secondary structures of the pharmaceutical heteropolypeptide, KL 4, in the solid state, in solution, and in the monolayer state alone and mixed with dipalmitoylphosphatidylcholine (DPPC) and palmitoyloleoylphosphatidylglycerol (POPG). In situ surface circular dichroism spectroscopy, using a method first reported by Damodaran (Damodaran, S. Anal. Bioanal. Chem. 2003, 376, 182-188), of equilibrated KL 4, DPPC/KL 4, POPG/KL 4, and DPPC/POPG/KL 4 monolayers at the air-water interface was used to examine the in situ two-dimensional conformation of KL 4. Gravimetric vapor sorption by solid KL 4 was used to analyze the effects of water molecules on the conformation of KL 4 when confined as a monolayer at the surface of water. Solid-state KL 4 conformation was determined by X-ray powder diffraction (XRPD). The equilibrium interfacial and spreading properties were measured at 25 degrees C, 37 degrees C, and 45 degrees C using the Wilhelmy plate method and Langmuir film balance. Equilibrium phase transition temperatures were measured using differential scanning calorimetry (DSC). It was found that solid-state KL 4, which takes up very little water, exhibits beta-sheet and alpha-helix secondary structures, whereas KL 4 in solution appears to exist only as an alpha-helix. KL 4 forms a stable, insoluble monolayer, exhibiting beta-sheet and aperiodic structures. These structures provide KL 4, when confined in two-dimensions, the structural flexibility to maximize favorable cationic lysine-water interactions and favorable leucine-leucine hydrophobic and van der Waals interactions; while effectively "shielding" the leucine residues away from water. In DPPC/KL 4 monolayers, KL 4 retains its native beta-sheet and aperiodic structures, consistent with phase separation of DPPC and KL 4 in bilayers and monolayers. In POPG/KL 4 monolayers, KL 4 exhibits an increase in aperiodic secondary structures (loss of beta-sheet) to maximize favorable electrostatic interactions, consistent with the observed negative deviations from ideal monolayer mixing.

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.

Structure of pulmonary surfactant membranes and films: the role of proteins and lipid-protein interactions

The pulmonary surfactant system constitutes an excellent example of how dynamic membrane polymorphism governs some biological functions through specific lipid-lipid, lipid-protein and protein-protein interactions assembled in highly differentiated cells. Lipid-protein surfactant complexes are assembled in alveolar pneumocytes in the form of tightly packed membranes, which are stored in specialized organelles called lamellar bodies (LB). Upon secretion of LBs, surfactant develops a membrane-based network that covers rapidly and efficiently the whole respiratory surface. This membrane-based surface layer is organized in a way that permits efficient gas exchange while optimizing the encounter of many different molecules and cells at the epithelial surface, in a cross-talk essential to keep the whole organism safe from potential pathogenic invaders. The present review summarizes what is known about the structure of the different forms of surfactant, with special emphasis on current models of the molecular organization of surfactant membrane components. The architecture and the behaviour shown by surfactant structures in vivo are interpreted, to some extent, from the interactions and the properties exhibited by different surfactant models as they have been studied in vitro, particularly addressing the possible role played by surfactant proteins. However, the limitations in structural complexity and biophysical performance of surfactant preparations reconstituted in vitro will be highlighted in particular, to allow for a proper evaluation of the significance of the experimental model systems used so far to study structure-function relationships in surfactant, and to define future challenges in the design and production of more efficient clinical surfactants.

Identification of a segment in the precursor of pulmonary surfactant protein SP-B, potentially involved in pH-dependent membrane assembly of the protein

In the present work, the hydrophobic properties of proSP-B, the precursor of pulmonary surfactant protein SP-B, have been analyzed under different pH conditions, and the sequence segment at position 111-135 of the N-terminal domain of the precursor has been detected as potentially possessing pH-dependent hydrophobic properties. We have studied the structure and lipid-protein interactions of the synthetic peptides BpH, with sequence corresponding to the segment 111-135 of proSP-B, and BpH-W, bearing the conservative substitution F127W to use the tryptophan as an intrinsic fluorescent probe. Peptide BpH-W interacts with both zwitterionic and anionic phospholipid vesicles at neutral pH, as monitored by the blue-shifted maximum emission of its tryptophan reporter. Insertion of tryptophan into the membranes is further improved at pH 5.0, especially in negatively-charged membranes. Peptides BpH and BpH-W also showed pH-dependent properties to insert into phospholipid monolayers. We have also found that the single sequence variation F120K decreases substantially the interaction of this segment with phospholipid surfaces as well as its pH-dependent insertion into deeper regions of the membranes. We hypothesize that this region could be involved in pH-triggered conformational changes occurring in proSP-B along the exocytic pathway of surfactant in type II cells, leading to the exposure of the appropriate segments for processing and assembly of SP-B within surfactant lipids.

Hydrophobic surfactant proteins and their analogues

Lung surfactant is a complex mixture of phospholipids and four surfactant-associated proteins (SP-A, SP-B, SP-C and SP-D). Its major function in the lung alveolus is to reduce surface tension at the air-water interface in the terminal airways by the formation of a surface-active film enriched in surfactant lipids, hence preventing cellular collapse during respiration. Surfactant therapy using bovine or porcine lung surfactant extracts, which contain only polar lipids and native SP-B and SP-C, has dramatically improved the therapeutic outcomes of preterm infants with respiratory distress syndrome (RDS). One important goal of surfactant researchers is to replace animal-derived therapies with fully synthetic preparations based on SP-B and SP-C, produced by recombinant technology or peptide synthesis, and reconstituted with selected synthetic lipids. Here, we review recent research developments with peptide analogues of SP-B and SP-C, designed using either the known primary sequence and three-dimensional (3D) structure of the native proteins or, alternatively, the known 3D structures of closely homologous proteins. Such SP-B and SP-C mimics offer the possibility of studying the mechanisms of action of the respective native proteins, and may allow the design of optimized surfactant formulations for specific pulmonary diseases (e.g., acute lung injury (ALI) or acute respiratory distress syndrome (ARDS)). These synthetic surfactant preparations may also be a cost-saving therapeutic approach, with better quality control than may be obtained with animal-based treatments.

The role of charged amphipathic helices in the structure and function of surfactant protein B

Surfactant protein B (SP-B) is essential for normal lung surfactant function. Theoretical models predict that the disulfide cross-linked, N- and C-terminal domains of SP-B fold as charged amphipathic helices, and suggest that these adjacent helices participate in critical surfactant activities. This hypothesis is tested using a disulfide-linked construct (Mini-B) based on the primary sequences of the N- and C-terminal domains. Consistent with theoretical predictions of the full-length protein, both isotope-enhanced Fourier transform infrared (FTIR) spectroscopy and molecular modeling confirm the presence of charged amphipathic alpha-helices in Mini-B. Similar to that observed with native SP-B, Mini-B in model surfactant lipid mixtures exhibits marked in vitro activity, with spread films showing near-zero minimum surface tensions during cycling using captive bubble surfactometry. In vivo, Mini-B shows oxygenation and dynamic compliance that compare favorably with that of full-length SP-B. Mini-B variants (i.e. reduced disulfides or cationic residues replaced by uncharged residues) or Mini-B fragments (i.e. unlinked N- and C-terminal domains) produced greatly attenuated in vivo and in vitro surfactant properties. Hence, the combination of structure and charge for the amphipathic alpha-helical N- and C-terminal domains are key to SP-B function.

Physical properties and surface activity of surfactant-like membranes containing the cationic and hydrophobic peptide KL4

Surfactant-like membranes containing the 21-residue peptide KLLLLKLLLLKLLLLKLLLLK (KL4), have been clinically tested as a therapeutic agent for respiratory distress syndrome in premature infants. The aims of this study were to investigate the interactions between the KL4 peptide and lipid bilayers, and the role of both the lipid composition and KL4 structure on the surface adsorption activity of KL4-containing membranes. We used bilayers of three-component systems [1,2-dipalmitoyl-phosphatidylcholine/1-palmitoyl-2-oleoyl-phosphatidylglycerol/palmitic acid (DPPC/POPG/PA) and DPPC/1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC)/PA] and binary lipid mixtures of DPPC/POPG and DPPC/PA to examine the specific interaction of KL4 with POPG and PA. We found that, at low peptide concentrations, KL4 adopted a predominantly alpha-helical secondary structure in POPG- or POPC-containing membranes, and a beta-sheet structure in DPPC/PA vesicles. As the concentration of the peptide increased, KL4 interconverted to a beta-sheet structure in DPPC/POPG/PA or DPPC/POPC/PA vesicles. Ca2+ favored alpha<-->beta interconversion. This conformational flexibility of KL4 did not influence the surface adsorption activity of KL4-containing vesicles. KL4 showed a concentration-dependent ordering effect on POPG- and POPC-containing membranes, which could be linked to its surface activity. In addition, we found that the physical state of the membrane had a critical role in the surface adsorption process. Our results indicate that the most rapid surface adsorption takes place with vesicles showing well-defined solid/fluid phase co-existence at temperatures below their gel to fluid phase transition temperature, such as those of DPPC/POPG/PA and DPPC/POPC/PA. In contrast, more fluid (DPPC/POPG) or excessively rigid (DPPC/PA) KL4-containing membranes fail in their ability to adsorb rapidly onto and spread at the air-water interface.

Production and characterisation of recombinant forms of human pulmonary surfactant protein C (SP-C): Structure and surface activity

Surfactant protein C (SP-C) is an essential component for the surface tension-lowering activity of the pulmonary surfactant system. It contains a valine-rich alpha helix that spans the lipid bilayer, and is one of the most hydrophobic proteins known so far. SP-C is also an essential component of various surfactant preparations of animal origin currently used to treat neonatal respiratory distress syndrome (NRDS) in preterm infants. The limited supply of this material and the risk of transmission of infectious agents and immunological reactions have prompted the development of synthetic SP-C-derived peptides or recombinant humanized SP-C for inclusion in new preparations for therapeutic use. We describe herein the recombinant production in bacterial cultures of SP-C variants containing phenylalanines instead of the palmitoylated cysteines of the native protein, as fusions to the hydrophilic nuclease A (SN) from Staphylococcus aureus. The resulting chimerae were partially purified by affinity chromatography and subsequently subjected to protease digestion. The SP-C forms were recovered from the digestion mixtures by organic extraction and further purified by size exclusion chromatography. The two recombinant SP-C variants so obtained retained more than 50% alpha-helical content and showed surface activity comparable to the native protein, as measured by surface spreading of lipid/protein suspensions and from compression pi-A isotherms of lipid/protein films. Compared to the protein purified from porcine lungs, the recombinant SP-C forms improved movement of phospholipid molecules into the interface (during adsorption), or out from the interfacial film (during compression), suggesting new possibilities to develop improved therapeutic preparations.

Critical structure-function determinants within the N-terminal region of pulmonary surfactant protein SP-B

Surfactant protein SP-B is absolutely required for the surface activity of pulmonary surfactant and postnatal lung function. The results of a previous study indicated that the N-terminal segment of SP-B, comprising residues 1-9, is specifically required for surface activity, and suggested that prolines 2, 4, and 6 as well as tryptophan 9, may constitute essential structural motifs for protein function. In this work, we assessed the role of these two motifs in promoting the formation and maintenance of surface-active films. Three synthetic peptides were synthesized including a peptide corresponding to the N-terminal 37 amino acids of native SP-B and two variants in which prolines 2, 4, 6, or tryptophan 9 were substituted by alanines. All three synthetic peptides were surface-active, as expected from their amphipathic structure. The peptides were also able to insert into dipalmitoylphosphatidylcholine/palmitoyloleoylphosphatidylglycerol (7:3 w/w ratio) monolayers preformed at pressures >30 mN/m, indicating that they perturb and insert into membranes. Substitution of alanine for tryptophan at position 9 significantly decreased both the rate of adsorption/insertion of the peptide into the interface and reinsertion of surface-active material excluded from the film during successive compression-expansion cycles. Substitution of alanines for prolines at positions 2, 4, and 6 did not produce substantial changes in the rate of adsorption/insertion; however, reinsertion of surface-active material into the expanding interface film was not as effective as in the presence of the nativelike peptide. These results suggest that W9 is critical for optimal interface affinity, whereas prolines may promote a conformation that facilitates rapid insertion of the peptide into phospholipid monolayers compressed to the highest pressures during compression-expansion cycling.

Structure and properties of phospholipid-peptide monolayers containing monomeric SP-B(1-25) II. Peptide conformation by infrared spectroscopy

The conformation and orientation of synthetic monomeric human sequence SP-B(1-25) (mSP-B(1-25)) was studied in films with phospholipids at the air-water (A/W) interface by polarization modulation infrared reflectance absorption spectroscopy (PM-IRRAS). Modified two-dimensional infrared (2D IR) correlation analysis was applied to PM-IRRAS spectra to define changes in the secondary structure and rates of reorientation of mSP-B(1-25) in the monolayer during compression. PM-IRRAS spectra and 2D IR correlation analysis showed that, in pure films, mSP-B(1-25) had a major alpha-helical conformation plus regions of beta-sheet structure. These alpha-helical regions reoriented later during film compression than beta structural regions, and became oriented normal to the A/W interface as surface pressure increased. In mixed films with 4:1 mol:mol acyl chain perdeuterated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/1,2-dioleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (sodium salt) (DPPC-d(62):DOPG), the IR spectra of mSP-B(1-25) showed that a significant, concentration-dependent conformational change occurred when mSP-B(1-25) was incorporated into a DPPC-d(62):DOPG monolayer. At an mSP-B(1-25) concentration of 10 wt.%, the peptide assumed a predominantly beta-sheet conformation with no contribution from alpha-helical structures. At lower, more physiological peptide concentrations, 2D IR correlation analysis showed that the propensity of mSP-B(1-25) to form alpha-helical structures was increased. In phospholipid films containing 5 wt.% mSP-B(1-25), a substantial alpha-helical peptide structural component was observed, but regions of alpha and beta structure reoriented together rather than independently during compression. In films containing 1 wt.% mSP-B(1-25), peptide conformation was predominantly alpha-helical and the helical regions reoriented later during compression than the remaining beta structural components. The increased alpha-helical structure of mSP-B(1-25) demonstrated here by PM-IRRAS and 2D IR correlation analysis in monolayers of 4:1 DPPC:DOPG containing 1 wt.% (and, to a lesser extent, 5 wt.%) peptide may be relevant for the formation of the intermediate order 'dendritic' surface phase observed in similar surface films by epi-fluorescence.

Effect of pulmonary surfactant protein SP-B on the micro- and nanostructure of phospholipid films

Monolayers of dipalmitoylphosphatidylcholine (DPPC) and DPPC/dipalmitoylphosphatidylglycerol (DPPG) (7:3, w/w) in the absence or in the presence of 2, 5, 10, or 20 weight percent of porcine surfactant protein SP-B were spread at the air-liquid interface of a surface balance, compressed up to surface pressures in the liquid-expanded/liquid-condensed (LE-LC) plateau of the isotherm, transferred onto mica supports, and analyzed by scanning force microscopy. In the absence of protein, the films showed micrometer-sized condensed domains with morphology and size that were analogous to those observed in situ at the air-liquid interface by epifluorescence microscopy. Scanning force microscopy permits examination of the coexisting phases at a higher resolution than previously achieved with fluorescent microscopy. Both LE and LC regions of DPPC films were heterogeneous in nature. LC microdomains contained numerous expanded-like islands whereas regions apparently liquid-expanded were covered by a condensed-like framework of interconnected nanodomains. Presence of increasing amounts of pulmonary surfactant protein SP-B affected the distribution of the LE and LC regions of DPPC and DPPC/DPPG films both at the microscopic and the nanoscopic level. The condensed microdomains became more numerous but their size decreased, resulting in an overall reduction of the amount of total LC phase in both DPPC and DPPC/DPPG films. At the nanoscopic level, SP-B also caused a marked reduction of the size of the condensed-like nanodomains in the LE phase and an increase in the length of the LE/LC interface. SP-B promotes a fine nanoscopic framework of lipid and lipid-protein nanodomains that is associated with a substantial mechanical resistance to film deformation and rupture as observed during film transference and manipulation. The effect of SP-B on the nanoscopic structure of the lipid films was greater in DPPC/DPPG than in pure DPPC films, indicating additional contributions of electrostatic lipid-protein interactions. The alterations of the nanoscopic structures of phospholipid films by SP-B provide the structural framework for the protein simultaneously sustaining structural stability as well as dynamical flexibility in surfactant films at the extreme conditions imposed by the respiratory mechanics. SP-B also formed segregated two-dimensional clusters that were associated with the boundaries between LC microdomains and the LE regions of DPPC and DPPC/DPPG films. The presence of these clusters at protein-to-lipid proportions above 2% by weight suggests that the concentration of SP-B in the surfactant lipid-protein complexes may be close to the solubility limit of the protein in the lipid films.

Conformational changes in SP-B as a function of surface pressure

X-ray reflectivity of bovine and sheep surfactant-associated protein B (SP-B) monolayers is used in conjunction with pressure-area isotherms and protein models to suggest that the protein undergoes changes in its tertiary structure at the air/water interface under the influence of surface pressure, indicating the likely importance of such changes to the phenomena of protein squeeze out as well as lipid exchange between the air-water interface and subphase structures. We describe an algorithm based on the well-established box- or layer-models that greatly assists the fitting of such unknown scattering-length density profiles, and which takes the available instrumental resolution into account. Scattering-length density profiles from neutron reflectivity of bovine SP-B monolayers on aqueous subphases are shown to be consistent with the exchange of a large number of labile protons as well as the inclusion of a significant amount of water, which is partly squeezed out of the protein monolayer at elevated surface pressures.

Topographical organization of the N-terminal segment of lung pulmonary surfactant protein B (SP-B(1-25)) in phospholipid bilayers

The location and depth of each residue of lung pulmonary surfactant protein B (SP-B(1-25)) in a phospholipid bilayer (PB) was determined by fluorescence quenching using synthesized single-residue-substituted peptides that were reconstituted into 1,2-dipalmitoyl phosphatidylcholine (DPPC)-enriched liposomes. The single-residue substitutions in peptides were either aspartate or tryptophan. The aspartate was subsequently labeled with the N-cyclohexyl-N'-(4-(dimethylamino)naphthyl)carbodiimide (NCD-4) fluorophore, whereas tryptophan is autofluorescent. Spin-labeled compounds, 5-doxylstearic acid (5-DSA), 7-doxylstearic acid (7-DSA), 12-doxylstearic acid (12-DSA), 4-(N,N-dimethyl-N-hexadecyl)ammonium-2,2,6,6-tetramethylpiperidine-1-oxyl iodide (CAT-16), and 4-trimethylammonium-2,2,6,6-tetramethylpiperidine-1-oxy iodide (CAT-1), were used in the quenching experiments. The effective quenching order is determined by the accessibility of the quencher to a fluorescent group on the peptide. The order of quenching efficiency provides information about the relative locations of individual residues in the PB. Our data indicate that residues Phe1-Pro6 are located at the surface of PB, residues Tyr7-Trp9 are embedded in PB, and residues Leu10-Ile22 are involved in an amphipathic alpha-helix with its axis parallel to the surface of PB; residues Pro23-Gly25 reside at the surface. The effects of intermolecular disulfide bond formation in the SP-B(1-25) dimer were also investigated. The experiments suggest that the SP-B helix A has to rotate at an angle to form a disulfide bond with the neighboring cysteine, which makes the hydrophobic sides of the amphipathic helices face each other, thus forming a hydrophobic domain. The detailed topographical mapping of SP-B(1-25) and its dimer in PB provides new insights into the conformational organization of the lung pulmonary surfactant proteins in the environment that mimics the native state. The environment-specific conformational flexibility of the hydrophobic domain created by SP-B folding may explain the key functional properties of SP-B including their impact on phospholipid transport between the lipid phases and in modulating the cell inflammatory response during respiratory distress syndrome.

Effects of oligomerization and secondary structure on the surface behavior of pulmonary surfactant proteins SP-B and SP-C

The relationship among protein oligomerization, secondary structure at the interface, and the interfacial behavior was investigated for spread layers of native pulmonary surfactant associated proteins B and C. SP-B and SP-C were isolated either from butanol or chloroform/methanol lipid extracts that were obtained from sheep lung washings. The proteins were separated from other components by gel exclusion chromatography or by high performance liquid chromatography. SDS gel electrophoresis data indicate that the SP-B samples obtained using different solvents showed different oligomerization states of the protein. The CD and FTIR spectra of SP-B isolated from all extracts were consistent with a secondary structure dominated by alpha-helix. The CD and FTIR spectra of the first SP-C corresponded to an alpha-helical secondary structure and the spectra of the second SP-C corresponded to a mixture of alpha-helical and beta-sheet conformation. In contrast, the spectra of the third SP-C corresponded to antiparallel beta-sheets. The interfacial behavior was characterized by surface pressure/area (pi-A) isotherms. Differences in the oligomerization state of SP-B as well as in the secondary structure of SP-C all produce significant differences in the surface pressure/area isotherms. The molecular cross sections determined from the pi-A isotherms and from dynamic cycling experiments were 6 nm(2)/dimer molecule for SP-B and 1.15 nm(2)/molecule for SP-C in alpha-helical conformation and 1.05 nm(2)/molecule for SP-C in beta-sheet conformation. Both the oligomer ratio of SP-B and the secondary structure of SP-C strongly influence organization and behavior of these proteins in monolayer assemblies. In addition, alpha-helix --> beta-sheet conversion of SP-C occurs simply by an increase of the summary protein/lipid concentration in solution.

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.

Surfactant-associated proteins: functions and structural variation

Pulmonary surfactant is a barrier material of the lungs and has a dual role: firstly, as a true surfactant, lowering the surface tension; and secondly, participating in innate immune defence of the lung and possibly other mucosal surfaces. Surfactant is composed of approximately 90% lipids and 10% proteins. There are four surfactant-specific proteins, designated surfactant protein A (SP-A), SP-B, SP-C and SP-D. Although the sequences and post-translational modifications of SP-B and SP-C are quite conserved between mammalian species, variations exist. The hydrophilic surfactant proteins SP-A and SP-D are members of a family of collagenous carbohydrate binding proteins, known as collectins, consisting of oligomers of trimeric subunits. In view of the different roles of surfactant proteins, studies determining the structure-function relationships of surfactant proteins across the animal kingdom will be very interesting. Such studies may reveal structural elements of the proteins required for surface film dynamics as well as those required for innate immune defence. Since SP-A and SP-D are also present in extrapulmonary tissues, the hydrophobic surfactant proteins SP-B and SP-C may be the most appropriate indicators for the evolutionary origin of surfactant. SP-B is essential for air-breathing in mammals and is therefore largely conserved. Yet, because of its unique structure and its localization in the lung but not in extrapulmonary tissues, SP-C may be the most important indicator for the evolutionary origin of surfactant.

Quantitation of Pulmonary Surfactant Protein SP-B in the Absence or Presence of Phospholipids by Enzyme-Linked Immunosorbent Assay

We have developed an enzyme-linked immunosorbent assay (ELISA) that uses polyclonal or monoclonal anti-surfactant protein SP-B antibodies to quantitate purified SP-B in chloroform/methanol and in chloroform/methanol extracts of whole pulmonary surfactant at nanogram levels. This method has been used to explore the effect of the presence of different phospholipids on the immunoreactivity of SP-B. Both polyclonal and monoclonal antibodies produced reproducible ELISA calibration curves for methanolic SP-B solutions with protein concentrations in the range of 20-1000 ng/mL. At these protein concentrations, neither dipalmitoylphosphatidylcholine, dipalmitoylphosphatidylglycerol, nor phosphatidylcholine or phosphatidylglycerol from egg yolk had significant effects on the binding of antibodies to SP-B up to protein-to-lipid weight ratios of 1:20. Coating of ELISA plates with SP-B concentrations higher than 1 microg/mL produced a substantial decrease in the binding of antibodies to the protein that was prevented by the presence of negatively charged but not zwitterionic phospholipids. Characterization of the secondary structure of SP-B by far-UV circular dichroism showed that phospholipids induced pronounced changes on the conformation of SP-B when the solvent was evaporated and dry lipid-protein films were formed, a necessary step to expose protein to antibodies in ELISA. Under these conditions, negatively charged lipids, but not zwitterionic ones, induced a marked decrease on the ellipticity of SP-B that would be associated with a conformation that is significantly more exposed to antibodies.

Intrinsic structural differences in the N-terminal segment of pulmonary surfactant protein SP-C from different species

Predictive studies suggest that the known sequences of the N-terminal segment of surfactant protein SP-C from animal species have an intrinsic tendency to form beta-turns, but there are important differences on the probable location of these motifs in different SP-C species. Our hypothesis is that intrinsic structural determinants of the sequence of the N-terminal region of SP-C could define conformation, acylation and perhaps surface properties of the mature protein. To test this hypothesis we have synthesized peptides corresponding to the 13-residue N-terminal sequence of porcine and canine SP-C, and studied their structural behaviour in solution and in phospholipid bilayers and monolayers. In these peptides, leucine at position 1 of both sequences has been replaced by tryptophan in order to allow their study by fluorescence spectroscopy. Far-u.v. circular dichroism spectra of the peptides in aqueous and organic solutions and in phospholipid micelles or vesicles are consistent with predicted conformational differences between the porcine and the canine sequences. Both families of peptides showed changes in their fluorescence emission spectra in the presence of phospholipids that were consistent with spontaneous lipid/peptide interactions. Both canine and porcine peptides were able to form monolayers at air-liquid interfaces, the canine peptides occupying lower area/molecule and being compressible to higher pressures than the porcine sequences. The peptides also shifted the isotherms and perturbed the packing of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG) monolayers, the effects being always higher in anionic than in zwitterionic lipids, and also substantially higher in films containing canine peptide in comparison to porcine peptide. Acylation of cysteines at the N-terminal end of SP-C may modulate these intrinsic conformational features and the changes induced could be important for the development of its surface activity.

Secondary structure in lung surfactant SP-B peptides: IR and CD studies of bulk and monolayer phases

Pulmonary surfactant protein SP-B is known to facilitate adsorption and spreading of surfactant components across the air/water interface. This property appears essential for in vivo function in the alveolar subphase and at the air/alveolar surface. Three peptides with amino acid sequences based on SP-B containing predicted alpha-helical regions (SP-B(1--20), SP-B(9--36A), SP-B(40--60A)) have been synthesized to probe structure-function relationships and protein-lipid interaction in bulk phase and monolayer environments. IR and CD studies are reported along with traditional surface pressure-molecular area (pi-A) isotherms and IR reflection-absorption spectroscopy (IRRAS) investigations conducted at the air/water interface. In bulk phase, helix-promoting environments (methanol and aqueous dispersions of lipid vesicles), SP-B(1--20) and SP-B(9--36A) contained significant amounts of alpha-helical structure, whereas varying degrees of alpha-helix, random coil, and beta-sheet were observed in aqueous solutions and monolayers. The most striking behavior was observed for SP-B(9--36A), which displayed reversible surface pressure-induced beta-sheet formation. Bulk phase lipid melting curves and monolayer experiments with peptide-lipid mixtures showed subtle differences in the degree of bulk phase interaction and substantial differences in peptide surface activity. The uniqueness of IRRAS is emphasized as the importance of evaluating secondary structure in both bulk phase and monolayer environments for lung surfactant peptide mimics is demonstrated.

Functional Tests for the Characterization of Surfactant Protein B (SP-B) and a Fluorescent SP-B Analog

Surfactant protein B (SP-B) enhances lipid insertion into the alveolar air/liquid interface upon inhalation. The aim of this study was (i) to apply a palette of tests for a detailed biochemical and biophysical characterization of SP-B and (ii) to use these tests to compare native SP-B with a fluorescent (Bodipy) SP-B analog. The method of labeling was fast and resulted in a covalent fluorophore-protein bond. The ability of both proteins to spread a surfactant film on top of a buffer surface was determined in a spreading tray using the Wilhelmy plate technique to allow detection of alterations in surface tension and calculation of spreading velocities. In a captive bubble surfactometer surface tensions of spread films were measured. Similar biophysical properties were found for both native and Bodipy-labeled SP-B. It is concluded that the combination of tests used allows detection of small differences in structure and activity between the two proteins.

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.

Fluoroalcohols as structure modifiers in peptides and proteins: hexafluoroacetone hydrate stabilizes a helical conformation of melittin at low pH

The effect of hexafluoroacetone hydrate (HFA) on the structure of the honey bee venom peptide melittin has been investigated. In aqueous solution at low pH melittin is predominantly unstructured. Addition of HFA at pH approximately 2.0 induces a structural transition from the unstructured state to a predominantly helical conformation as suggested by intense diagnostic far UV CD bands. The structural transition is highly cooperative and complete at 3.6 M (50% v/v) HFA. A similar structural transition is also observed in 2,2,2 trifluoroethanol which is complete only at a cosolvent concentration of approximately 8 M. Temperature dependent CD experiments support a 'cold denaturation' of melittin at low concentrations of HFA, suggesting that selective solvation of peptide by HFA is mediated by hydrophobic interactions. NMR studies in 3.6 M HFA establish a well-defined helical structure of melittin at low pH, as suggested by the presence of strong NH/NHi+1 NOEs throughout the sequence, along with many medium range helical NOEs. Structure calculations using NOE-driven distance constraints reveal a well-ordered helical fold with a relatively flexible segment around residues T10-G11-T12. The helical structure of melittin obtained at 3.6 M HFA at low pH is similar to those determined in methanolic solution and perdeuterated dodecylphosphocholine micelles. HFA as a cosolvent facilitates helix formation even in the highly charged C-terminal segment.

Rotational dynamics of spin-labelled surfactant-associated proteins SP-B and SP-C in dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylglycerol bilayers

Pulmonary surfactant proteins SP-B and SP-C have been isolated from porcine lungs and selectively labelled with 2,2,6, 6-tetramethylpiperidine-N-oxyl (TEMPO)-isothiocyanate at their N-terminal amine ends, to analyse the mobility of both proteins on the nanosecond time scale using electron spin resonance (ESR) spectroscopy. Reconstitution of the labelled forms of these proteins in bilayers of dipalmitoylphosphatidylcholine (DPPC) or dipalmitoylphosphatidylglycerol (DPPG) results in much broader and anisotropic ESR spectra, indicating a large restriction in rotational mobility of the protein-attached probe when inserted in membranes. Distinctive differences were found between the ESR spectra of the two polypeptides, that were consistent with intrinsic differences in mode of interaction of SP-B and SP-C with phospholipid bilayers. The mobility of the protein spin probes was sensitive to temperature on the time scale of conventional spin-label ESR. Both proteins, TEMPO-SP-B and TEMPO-SP-C, showed considerable increases in mobility at temperatures above the pretransition of pure DPPC. Finally, the mobility of the spin probes attached to both SP-B and SP-C was more restricted in DPPG than in DPPC bilayers, demonstrating that electrostatic interactions of the positively charged residues at the protein surface influence the rotational dynamics of the proteins in anionic lipid bilayers. Although some residual segmental mobility of the thiourea-linked probes cannot be discounted, the results clearly reflect preferential differences in overall protein dynamics in gel and fluid phases of the two phospholipids that could be important for the biophysical properties of surfactant bilayers and monolayers.

Structure and properties of surfactant protein B

Surfactant protein B is a small homodimeric protein that is found tightly associated with surfactant lipids in the alveolar space. In this review, we discuss the actions of SP-B on phospholipid membranes using information predominantly obtained from model membrane systems. We try to correlate these model actions with current concepts of SP-B structure and proposed biological functions. These functions may include critical roles in the intracellular assembly of surfactant through a role in lamellar body organogenesis, the structural rearrangement of secreted surfactant lipids into tubular myelin, and the subsequent rapid insertion of secreted surfactant phospholipids into the surface film itself. The relevance of SP-B to human biology is emphasized by the fatal respiratory distress that is associated with a genetic deficiency of SP-B and the important role of SP-B in certain exogenous surfactant formulations in wide clinical use.

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.

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.

A model for the interaction of trifluoroethanol with peptides and proteins

The structural stabilizing property of 2,2,2-trifluoroethanol (TFE) in peptides has been widely demonstrated. More recently, TFE has been shown to enhance secondary structure content in globular proteins, and to influence quaternary interactions in protein multimers. The molecular mechanisms by which TFE exerts its influence on peptide and protein structures remain poorly understood. The present analysis integrates the known physical properties of TFE with a variety of experimental observations on the interaction of TFE with peptides and proteins and on the properties of fluorocarbons. Two features of TFE, namely the hydrophobicity of the trifluoromethyl group and the hydrogen bonding character (strong donor and poor acceptor), emerge as the most important factors for rationalising the observed effects of TFE. A model is proposed for TFE interaction with peptides which involves an initial replacement of the hydration shell by fluoroalcohol molecules, a process driven by apolar interactions and favourable entropy of dehydration. Subsequent bifurcated hydrogen-bond formation with peptide carbonyl groups, which leave intramolecular interactions unaffected, promotes secondary structure formations.