Hydrophobic 3.7 kDa surfactant polypeptide: structural characterization of the human and bovine forms

Lung transcriptome of a COVID-19 patient and systems biology predictions suggest impaired surfactant production which may be druggable by surfactant therapy

An incomplete understanding of the molecular mechanisms behind impairment of lung pathobiology by COVID-19 complicates its clinical management. In this study, we analyzed the gene expression pattern of cells obtained from biopsies of COVID-19-affected patient and compared to the effects observed in typical SARS-CoV-2 and SARS-CoV-infected cell-lines. We then compared gene expression patterns of COVID-19-affected lung tissues and SARS-CoV-2-infected cell-lines and mapped those to known lung-related molecular networks, including hypoxia induced responses, lung development, respiratory processes, cholesterol biosynthesis and surfactant metabolism; all of which are suspected to be downregulated following SARS-CoV-2 infection based on the observed symptomatic impairments. Network analyses suggest that SARS-CoV-2 infection might lead to acute lung injury in COVID-19 by affecting surfactant proteins and their regulators SPD, SPC, and TTF1 through NSP5 and NSP12; thrombosis regulators PLAT, and EGR1 by ORF8 and NSP12; and mitochondrial NDUFA10, NDUFAF5, and SAMM50 through NSP12. Furthermore, hypoxia response through HIF-1 signaling might also be targeted by SARS-CoV-2 proteins. Drug enrichment analysis of dysregulated genes has allowed us to propose novel therapies, including lung surfactants, respiratory stimulants, sargramostim, and oseltamivir. Our study presents a distinct mechanism of probable virus induced lung damage apart from cytokine storm.

Comprehensive characterization and proteoform analysis of the hydrophobic surfactant proteins B and C in calf pulmonary surfactant

Calf pulmonary surfactant (CPS), which contains about 98% lipids and 2% hydrophobic surfactant proteins B (SP-B) and C (SP-C), has been used as a surfactant preparation for the clinical replacement therapy of respiratory distress syndrome (RDS). Characterization of SP-B and SP-C in CPS is informative for quality control and the evaluation of their biological activities. However, analysis of SP-B and SP-C is impeded by the high content of lipids in CPS. Here, we describe an integrated method by combining size exclusion chromatography (SEC)-based delipidation, SDS-PAGE separation, in-gel digestion and mass spectrometric analysis for comprehensive characterization and proteoform analysis of the extremely hydrophobic SP-B and SP-C in CPS. This study has shown that 30 proteoforms of SP-C with different truncations and modifications were identified and SP-B was found to be existed as a dimer form in the CPS.

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.

A unique story in neonatal research: the development of a porcine surfactant

Surfactant deficiency was identified as the cause of respiratory distress syndrome (RDS) as long ago as 1959. Trials of surfactant replacement in the 1960s were unsuccessful because the preparations used contained only phospholipids and they were administered inefficiently by nebulization. In the 1970s Bengt Robertson and Göran Enhörning showed that natural surfactant, containing both phospholipids and proteins, could ameliorate the signs of RDS in immature rabbits. In the 1980s Bengt Robertson and Tore Curstedt developed a porcine surfactant, Curosurf (named after their surnames), which was effective in immature animals and was used in a pilot clinical trial beginning in 1983. Subsequent randomized clinical trials were planned a year later by Bengt Robertson, Tore Curstedt and Henry Halliday, and the first trial was begun in 1985. This showed that Curosurf reduced pulmonary air leaks and neonatal mortality in preterm infants with severe RDS. A second trial, coordinated by Christian Speer, demonstrated that multiple doses of Curosurf were more effective than a single dose. Subsequent trials conducted by the Collaborative European Multicenter Study Group, which included among others Guilio Bevilacqua, Janna Koppe, Ola Saugstad, Nils Svenningsen and Jean-Pierre Relier, showed that early treatment was more effective than later administration and that infants treated at birth had similar neurodevelopmental status to untreated controls at a corrected age of 2 years. Members of the Collaborative European Multicenter Study Group in Denmark and Sweden performed studies to demonstrate the benefits of a combination of surfactant treatment and early continuous positive airway pressure. Curosurf has also been compared with several synthetic and natural surfactants, and at a dose of 200 mg/kg Curosurf has been shown to be superior to either Survanta or Curosurf used at a dose of 100 mg/kg. Recently, new-generation synthetic surfactants containing both phospholipids and proteins have been developed. After preclinical testing, CHF5633 (developed by Tore Curstedt and Jan Johansson in collaboration with Chiesi Farmaceutici) has undergone a preliminary first study in humans under the guidance of Christian Speer. If effective, this new surfactant preparation could revolutionize the treatment of preterm infants worldwide as it could be made consistently and safely in almost unlimited quantities. This story of a porcine surfactant preparation has been truly remarkable, and many thousands of preterm babies worldwide are now alive and well because of it.

Characterization of bovine surfactant proteins B and C by electrospray ionization mass spectrometry

Bovine surfactant proteins B (SP-B) and C (SP-C) were analyzed by nano-electrospray ionization mass spectrometry (nano-ESI-MS). The observed molecular masses showed discrepancies compared to the calculated molecular masses using the published amino acid sequences. The number of cysteine residues in the published bovine SP-B amino acid sequences also failed to match the observed mass shift upon reduction of the SP-B dimer. To determine the amino acid sequences of two proteins, SP-B was first digested with trypsin and analyzed by liquid chromatography/tandem mass spectrometry (LC/MS/MS), while SP-C was analyzed by MS/MS in its intact form. The amino acid sequence of bovine SP-B determined here matches the observed molecular mass. The sequence is almost identical to the sheep SP-B except for two amino acid residues, consistent with the proximity of the two species. The correct sequence contains seven cysteine residues. Bovine SP-B exists as dimers and all cysteines are oxidized to form disulfide bonds in physiological conditions, which is in agreement with the observed mass shift upon reduction of the SP-B dimer. These cysteine residues are completely conserved across all species indicating their importance for the biological functions of this surfactant protein. The sequence of SP-C determined here also reveals an L to V substitution at its position 22 compared with the published bovine SP-B sequence.

Surfactant protein C biosynthesis and its emerging role in conformational lung disease

Surfactant protein C (SP-C) is a hydrophobic 35-amino acid peptide that co-isolates with the phospholipid fraction of lung surfactant. SP-C represents a structurally and functionally challenging protein for the alveolar type 2 cell, which must synthesize, traffic, and process a 191-197-amino acid precursor protein through the regulated secretory pathway. The current understanding of SP-C biosynthesis considers the SP-C proprotein (proSP-C) as a hybrid molecule that incorporates structural and functional features of both bitopic integral membrane proteins and more classically recognized luminal propeptide hormones, which are subject to post-translational processing and regulated exocytosis. Adding to the importance of a detailed understanding of SP-C biosynthesis has been the recent association of mutations in the proSP-C sequence with chronic interstitial pneumonias in children and adults. Many of these mutations involve either missense or deletion mutations located in a region of the proSP-C molecule that has structural homology to the BRI family of proteins linked to inherited degenerative dementias. This review examines the current state of SP-C biosynthesis with a focus on recent developments related to molecular and cellular mechanisms implicated in the emerging role of SP-C mutations in the pathophysiology of diffuse parenchymal lung disease.

Palmitoylation and processing of the lipopeptide surfactant protein C

Pulmonary surfactant, a mixture of lipids and proteins, reduces the surface tension at the air-water interface of the lung alveoli by forming a surface active film. This way, it prevents alveoli from collapsing and facilitates the work of breathing. Surfactant protein C (SP-C) plays an important role in this surfactant function. SP-C is expressed as a proprotein (proSP-C), which becomes posttranslationally modified with palmitate and undergoes several rounds of proteolytical cleavage. This results in the formation of mature SP-C, which is stored in the lamellar bodies (LB) and finally secreted into the alveolar space. Recently, new insights into the sorting, processing and palmitoylation of proSP-C have been obtained by mutagenesis studies. Moreover, reports on the association of development of lung disease with SP-C deficiency have led to new insights into the importance of SP-C for proper surfactant homeostasis. In addition, new information has become available on the role of the palmitoyl chains of SP-C in surface activity. This review summarizes these recent developments in the processing and function of SP-C, with particular emphasis on the signals for and role of palmitoylation of SP-C.

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.

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.

Secondary structure and biophysical activity of synthetic analogues of the pulmonary surfactant polypeptide SP-C

Native pulmonary-surfactant-associated lipopolypeptide SP-C, its chemically depalmitoylated form and several synthetic analogues lacking the palmitoylcysteine residues were analysed for secondary structure in phospholipid micelles and for biophysical activity in 1,2-dipalmitoyl-sn-glycero-3- phosphocholine/phosphatidylglycerol/palmitic acid (68:22:9, by wt.). Compared with the native molecule, with the entire poly-valyl part in a known alpha-helical conformation, depalmitoylated SP-C was found to be still mainly alpha-helical, but with an approx. 20% decrease in the helical content. A synthetic hybrid polypeptide where the entire poly-valyl alpha-helical part of native SP-C had been replaced with the amino acid sequence of a transmembrane helix of bacteriorhodopsin is also predominantly alpha-helical. In contrast, synthetic SP-C analogues lacking only the palmitoyl groups, by replacement of the palmitoylcysteine residues with cysteine, phenylalanine or serine, or lacking the positively charged amino acids by replacement with alanine, are considerably less alpha-helical than both native and depalmitoylated SP-C. The data indicate that the SP-C palmitoyl groups are important for maintenance of the alpha-helical conformation in parts of the polypeptide, and that the poly-valyl alpha-helical conformation is not fully formed in synthetic SP-C polypeptides. Furthermore, the helical structure of both native and depalmitoylated SP-C in dodecylphosphocholine micelles is very resistant to thermal denaturation, exhibiting ordered structure at 90 degrees C. The alpha-helical content grossly parallels the peptide-induced acceleration of the spreading of phospholipids at an air/water interface and the increase of surface pressure. The data suggest that the alpha-helical conformation itself, rather than just the covalent structure, is of prime importance for the biological function of synthetic pulmonary-surfactant peptides.

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.

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.

Structure and functions of a dimeric form of surfactant protein SP-C: a Fourier transform infrared and surfactometry study

Surfactant proteins SP-B (M(r) = 8700, reduced) and SP-C (M(r) = 3000-6000, major form, non-reduced) interact with surfactant phospholipids to enhance their surface active properties. In the present study, we describe the structural and functional characteristics of a novel dimeric form of bovine SP-C (M(r) = 9000, non-reduced), which is identified as [SP-C]2. Dimeric SP-C exhibits surface tension-lowering properties differing from those of monomeric SP-C and enhances the surface properties of bovine SP-B/phospholipid mixtures. Chemical analysis indicated that [SP-C]2 was not acylated at the cysteinyl residues. Fourier transform-infrared spectroscopy (FT-IR) was utilized to determine the secondary structures of [SP-C]2 in DPPC films. Relative percentages of alpha-helical, beta-sheet, beta-turn and random coil structures were calculated by peak fit analysis of the amide I band of the FT-IR spectra indicating that, in contrast to the helical structure of monomeric SP-C, [SP-C]2 exhibits almost exclusively beta-sheet structure. In addition, only 10% of the amide (backbone) hydrogens exchanged with deuterium of D2O, indicating that the remaining 90% of amide hydrogens were not accessible to D2O due to strong hydrogen bonding or their location in a hydrophobic environment. Dimerization of SP-C effects a major change in secondary structure, a factor which may play a role in the interaction of SP-C with phospholipids in pulmonary surfactant.

An antibody with specificity for surfactant protein C precursors: identification of pro-SP-C in rat lung

Surfactant protein C (SP-C) is a lung-specific, hydrophobic peptide found in organic extracts of pulmonary surfactant. Alveolar SP-C (3.5 kD) is produced from proteolytic cleavage of a larger precursor molecule (pro-SP-C; 21 kD). While SP-C is synthesized by type II cells, the pathways for processing and secretion have remained elusive due, in part, to the lack of monospecific antibodies against SP-C or its precursors. This report describes production and characterization of a new antibody directed against pro-SP-C epitopes. Polyclonal antisera (anti-CPRO-SP-C) was prepared using a synthetic peptide corresponding to a portion of rat SP-C cDNA sequence (Ile26-Ser72). This contained amino acids 3-35 of mature SP-C plus additional C-terminal residues (His59-Ser72). On Western blots, anti-CPRO-SP-C competitively reacted to CPRO-SP-C but not to mature SP-C. Immunoblots of in vitro synthesized pro-SP-C confirmed that the antisera also recognized native protein. Immunocytochemistry with anti-CPRO-SP-C demonstrated staining for pro-SP-C peptides in isolated type II cells as well as in alveolar epithelial cells of rat lung sections. Pro-SP-C preferentially co-localized to cells that stained positive for Maclura pomifera antigen. Anti-CPRO-SP-C staining was not observed in lung interstitium, pulmonary vasculature, or several control tissues (brain, heart, and liver were negative). Western blotting of subcellular fractions demonstrated pro-SP-C peptides in plasma membrane (20 kD) and microsomal (20 and 21 kD) fractions with a 16 kD peptide present in lamellar bodies. No pro-SP-C peptides were detected in purified surfactant. These results demonstrate the use of a synthetic peptide to generate specific antiserum against more hydrophilic domains of pro-SP-C sequences and confirm that SP-C propeptides are unique to the lung.

Secondary structure and orientation of the surfactant protein SP-B in a lipid environment. A Fourier transform infrared spectroscopy study

Attenuated total reflection Fourier transform infrared spectroscopy was used to investigate the secondary structure of the surfactant protein SP-B. Nearly half of the polypeptide chain is folded in an alpha-helical conformation. No significant change of the secondary structure content was observed when the protein is associated to a lipid bilayer of dipalmitoylphosphatidylcholine (DPPC)/phosphatidylglycerol (PG) or of dipalmitoylphosphatidylglycerol (DPPG). The parameters related to the gamma w(CH2) vibration of the saturated acyl chains reveal no modification of the conformation or orientation of the lipids in the presence of SP-B. A model of orientation of the protein at the lipid/water interface is proposed. In this model, electrostatic interactions between charged residues of SP-B and polar headgroups of PG, and the presence of small hydrophobic alpha-helical peptide stretches slightly inside the bilayers, would maintain SP-B at the membrane surface.

cDNA sequence and alternative mRNA splicing of surfactant-associated protein C (SP-C) in rabbit lung

An 784 base pair (bp) copy DNA (cDNA) for the low molecular weight hydrophobic surfactant-associated protein C (SP-C) has been isolated from a lambda gt11 cDNA library constructed from fetal rabbit lung mRNA. The cDNA, which coded for a 193 amino-acid proprotein with 6 bp 5' and 193 bp 3' untranslated segments, possesses considerable nucleic acid and predicted amino-acid homology with previously reported SP-C cDNAs. The predicted amino-acid sequence of the 35 amino-acid mature polypeptide shares 94-97% identity with human, rat and mouse SP-C and is 88-91% homologous to the mature proteins from bovine, porcine and canine lung. The last 12 amino acids of mature SP-C are highly hydrophobic and invariant. Alignment of the rabbit and human nucleic acid sequences required introduction of a 27 bp gap in the rabbit sequence at a site corresponding to the exon-intron junction of the 5th exon of the human genomic sequence. Since previous studies have identified differential splicing at the 5' and 3' ends of the human 5th exon, we investigated the potential existence of alternative splicing of rabbit SP-C mRNA. Reverse transcription (RT) of total RNA followed by polymerase chain reaction (PCR) was used to establish the relative abundance of alternative splicing products from fetal and adult lung and from rabbit kidney, placenta and liver. The relative abundance of the 250, 280 and 350 bp bands observed was the same in lung and other tissues. PCR amplification of genomic rabbit DNA indicated that the 350 bp fragment corresponds to the unspliced nascent transcript. The lack of developmental or tissue-specific abundance patterns implies the absence of secondary influences on SP-C mRNA polymorphism. Indeed, free energy of formation calculations predicted the presence of hairpin structures favouring formation of the more abundant 250 bp form. These observations plus the absence of any effect of alternative splicing on SP-C protein structure led us to conclude a physiological role is unlikely.

Effects of lung surfactant proteolipid SP-C on the organization of model membrane lipids: a fluorescence study

Lipid-protein interactions of pulmonary surfactant-associated protein SP-C in model DPPC/DPPG and DPPC/DPPG/eggPC vesicles were studied using steady-state and time-resolved fluorescence measurements of two fluorescent phospholipid probes, NBD-PC and NBD-PG. These fluorescent probes were utilized to determine SP-C-induced lipid perturbations near the bilayer surface, and to investigate possible lipid headgroup-specific interactions of SP-C. The presence of SP-C in DPPC/DPPG membrane vesicles resulted in (1) a dramatic increase in steady-state anisotropy of NBD-PC and NBD-PG at gel phase temperatures, (2) a broadening of the gel-fluid phase transition, (3) a decrease in self-quenching of NBD-PC and NBD-PG probes, and (4) a slight increase in steady-state anisotropy of NBD-PG at fluid phase temperatures. Time-resolved measurements, as well as steady-state intensity measurements indicate that incorporation of SP-C into DPPC/DPPG or DPPC/DPPG/eggPC vesicles results in a increase in the fraction of the long-lifetime species of NBD-PC. The results presented here indicate that SP-C orders the membrane bilayer surface, disrupts acyl chain packing, and may increase the lateral pressure within the bilayer.

Human surfactant polypeptide SP-B. Disulfide bridges, C-terminal end, and peptide analysis of the airway form

Human hydrophobic surfactant polypeptide, SP-B, purified from lung tissue by exclusion chromatography in organic solvents, has been characterized. The polypeptide is 79 residues long, has a C-terminal methionine, and contains seven Cys residues. Native human SP-B lacks free thiol groups. Three intrachain disulfide bridges were defined, linking Cys8 to Cys77, Cys11 to Cys71 and Cys35 to Cys46. The remaining Cys48 is concluded to link the protein chains into homodimers via an interchain disulfide to its counterpart in a second SP-B polypeptide. These SS bridges are identical to those in the porcine form and confirm a consestant and unique disulfide pattern for SP-B polypeptides in general.

Surfactant protein C precursor is palmitoylated and associates with subcellular membranes

Surfactant protein C (SP-C) is a 3.7 kDa, hydrophobic protein that enhances the adsorption of phospholipids in pulmonary surfactant. SP-C is generated by proteolytic processing of a 21 kDa precursor. Murine fetal lung explant cultures and a Chinese hamster ovary cell line expressing recombinant human SP-C gene (CHO/SPC) were used to determine the subcellular location and post-translational modification(s) of proSP-C. After in vitro translation, proSP-C of Mr = 21,000 was generated. ProSP-C was associated with canine pancreatic microsomes during co-translation and was partially protected from digestion with proteinase K, supporting the concept that proSP-C enters but does not completely traverse the membrane of the endoplasmic reticulum (ER). Association of proSP-C isoforms of 21 and 26 kDa with intracellular membranes was demonstrated by subcellular fractionation of CHO/SPC cells. Pulse/chase experiments demonstrated that the 21 kDa SP-C proprotein was synthesized first and after 15 min was modified to produce a 26 kDa isoform in CHO/SPC cells or a 24 kDa isoform in murine fetal lung. Both the 21 and 26 kDa proSP-C isoforms were detected after labelling CHO/SPC cells with [3H]palmitic acid. The formation of the 26 kDa proSP-C isoform in CHO/SPC cells and the 24 kDa proSP-C isoform in murine fetal lung was blocked by cerulenin, an inhibitor of fatty acid synthesis. In conclusion, proSP-C is associated with subcellular membranes. ProSP-C is palmitoylated and undergoes additional post-translational modification that is blocked by an inhibitor of fatty acid synthesis.

Structure and orientation of the surfactant-associated protein C in a lipid bilayer

The secondary structure of native and depalmitoylated porcine surfactant-associated protein C (SP-C) was studied by attenuated total reflection Fourier-transform infrared spectroscopy. Both forms of porcine SP-C adopt mainly an alpha-helical conformation. These two forms of the protein were reconstituted in a lipid bilayer. The insertion of the protein in a membrane is associated with an increase of the alpha-helical content. Dichroic measurements show that, in both cases, the long axis of the alpha-helix is oriented parallel to the lipid acyl chains.

Surfactant protein B: disulfide bridges, structural properties, and kringle similarities

The disulfide bridges in porcine hydrophobic surfactant protein B (SP-B) were determined. Results show that three intrachain bridges link half-cystine residues 8 and 77, 11 and 71, and 35 and 46, respectively. This gives SP-B an appearance of three loops, a central big loop surrounded by two smaller ones. In the major form of SP-B, the remaining half-cystine, Cys-48, is probably interchain-linked to its counterpart in another molecule, compatible with the existence of dimeric molecules. A minor fraction, with monomeric SP-B but also lacking free thiols, could be due to polypeptides having Cys-57 (instead of Leu in the major form) and hence an additional intrachain bond (Cys-48-Cys-57). Notably, one of the three intrachain bonds common to all SP-B molecules is analogous to one of the disulfide linkages in the kringle structure of complex serine proteases. SP-B and kringles are also similar in size and in positions of half-cystine residues. SP-B and the kringle of coagulation factor XII exhibit 26% residue identity. This structural similarity of SP-B to a binding domain could reflect functional homology, compatible with the notion that SP-B interacts with surfactant anionic phospholipids, which is also in agreement with an SP-B excess of basic residues. Finally, weak similarities between the perform of SP-B and complex serine proteases are also found. This has implications on further possible relationships between kringles, serine proteases, and antiproteases.

Canine hydrophobic surfactant polypeptide SP-C. A lipopeptide with one thioester-linked palmitoyl group

The amino acid sequence and the posttranslational modification of the hydrophobic surfactant polypeptide SP-C from canine, rabbit and bovine lungs were established by direct sequence analysis and plasma-desorption time-of-flight mass spectrometry. The results reveal that canine SP-C has only one cysteine residue which, however, is palmitoylated, like the two Cys residues in other characterized SP-C molecules. In addition, canine SP-C is N-terminally truncated, with only 34 amino acid residues in its longest form. Thus, SP-C molecules can apparently vary to some extent in the N-terminal lipid-modified part, whereas the extremely hydrophobic middle and C-terminal parts are well conserved.

Staining properties of bovine low molecular weight hydrophobic surfactant proteins after polyacrylamide gel electrophoresis

Reports describing polyacrylamide gel electrophoresis patterns of bovine hydrophobic surfactant proteins are not consistent. In this study, we found unusual staining characteristics of these proteins that may explain some of these inconsistencies. Low molecular weight surfactant proteins extracted from bronchoalveolar lavage with organic solvent are partially delipidated with Sephadex LH-20 chromatography using chloroform and methanol. Fractions from the first protein peak are dried under nitrogen then subjected to SDS electrophoresis on 20% polyacrylamide gels. Under nonreducing conditions, silver staining identifies 5- and 26-kDa bands, and Coomassie blue identifies 6-, 12-, and 26-kDa bands. When gels are stained with Coomassie blue then silver, the 5- and 26-kDa bands stain with silver and 6- and 12-kDa bands remain stained with Coomassie blue. If gels are first stained with silver then Coomassie blue, similar results occur. We modified the silver staining protocol by treating gels with dithiothreitol or 2-mercaptoethanol after electrophoresis. With this modification, 5-, 6-, 12-, 26-, and also 17-kDa bands are identifiable. Using the modified protocol and restaining gels previously stained with silver, 6-, 12-, and 17-kDa bands that were not identified previously all became visible. In further experiments, protein bands of 6-, 12-, and 26-kDa that were identified by Coomassie blue were electroeluted under nonreducing conditions. After electrophoresis of the eluted 26-kDa protein, bands of 17-, and 26-kDa under nonreducing, and 8-kDa only under reducing conditions, were apparent by using the modified silver protocol.(ABSTRACT TRUNCATED AT 250 WORDS)

Hydrophobic surfactant-associated polypeptides: SP-C is a lipopeptide with two palmitoylated cysteine residues, whereas SP-B lacks covalently linked fatty acyl groups

Pulmonary surfactant contains two hydrophobic polypeptides, SP-B and SP-C, with known amino acid sequences and with truncated subforms lacking the N-terminal residues. Treatment of SP-C with KOH releases fatty acids (palmitic acid to more than 85%) in molar ratios of 1.8-2.0 relative to the polypeptide. Furthermore, plasma-desorption mass spectrometry shows native SP-C of both the intact and truncated types to be monomers with masses about 500 units higher than those expected for the polypeptide chains. After treatment with KOH, trimethylamine, or dithioerythritol, the polypeptide masses are obtained. These results prove that native SP-C is a lipopeptide with two palmitoyl groups covalently linked to the polypeptide chain. The deacylation conditions, the presence of two cysteine residues in the polypeptide, and the absence of other possible attachment sites establish that the palmitoyl groups are thioester-linked to the two adjacent cysteine residues. In contrast, the major form of porcine SP-B is a dimer without fatty acid components. That SP-C is a true lipopeptide with covalently bound palmitoyl groups suggests possibilities for functional interactions. It gives a direct physical link between SP-C and surfactant phospholipid components. Long-chain acylation may constitute a means for association of proteins with membranes and could conceivably modulate the stability and biological activity of surfactant films.

Hydrophobic homopolymeric peptides enhance the biophysical activity of synthetic lung phospholipids

The effects of homopolymeric amino acids (molecular weight 2300 to 14,000) on the surface activity of dipalmitoyl phosphatidylcholine (DPPC) and DPPC/egg-phosphatidylglycerol (PG) were characterized by adsorption and dynamic surface tension lowering measurements at 37 degrees C. Homopolyamino acids studied included poly-L-leucine (poly-Leu) and poly-L-valine (poly-Val), since Leu and Val are known to be prominent in the structure of hydrophobic lung surfactant apoprotein SP-B and SP-C. In addition, several other homopolyamino acids with varying hydrophobicity index were also investigated, including poly-L-phenylalanine (poly-Phe), poly-L-serine (poly-Ser), poly-L-lysine (poly-Lys) and poly-L-glutamic acid (poly-Glu). Results showed that hydrophobic poly-Leu and poly-Phe at 1 and 10 weight percent greatly increased the adsorption facility of DPPC and DPPC/PG mixtures, with maximum surface pressures (up to 49 mN/m) near the equilibrium limit for phospholipid systems. In oscillating bubble studies, 1% mixture of poly-Leu or poly-Phe with DPPC or 8:2 DPPC/PG lowered surface tension into the range (near 1 mN/m) associated with active lung surfactant. In contrast, mixtures of DPPC and DPPC/PG with the more hydrophilic peptides poly-Ser, poly-Lys and poly-Glu showed little or no enhancement of surface activity over the phospholipids alone. Mixtures of poly-Val and phospholipids did not combine well with the simple co-sonication procedure used, and also exhibited little improvement in surface activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural relationship between the two small hydrophobic apoproteins in bovine pulmonary surfactant

Lipid extracts of bovine pulmonary surfactant contain two very hydrophobic surfactant-associated proteins (SP) designated SP-B (15 kDa nonreduced) and SP-C (3.5 kDa). These two low molecular weight apoproteins were delipidated and purified on silica SEP-PAK cartridges using various reagents. Dansylation studies revealed that the 15 kDa apoprotein has three N-termini: Phe, Leu and Ile, while the 3.5 kDa apoprotein has two N-termini: Leu and Ile. In either protein, only a very small amount of N-Ile is present. Quantitative N-terminal dansylation analysis of the 15 kDa protein indicated that Phe and Leu (plus Ile) are present in a 1:1 ratio. Carboxy-terminal analysis showed that the 15 kDa protein contains C-terminal Gly, and the 3.5 kDa protein contains C-terminal Leu. Gas-phase amino terminal sequencing of the 15 kDa protein revealed almost exclusively the Phe-polypeptide (SP-B). These results suggest that the 15 kDa apoprotein is not an oligomer of SP-B and SP-C. The reason that analysis of SP-B reveals N-terminal Leu and Ile by dansylation which cannot be confirmed by amino acid sequencing is not known.