Structural requirements for targeting of surfactant protein B (SP-B) to secretory granules in vitro and in vivo

Synthetic processing of surfactant protein C by alevolar epithelial cells. The COOH terminus of proSP-C is required for post-translational targeting and proteolysis

Surfactant protein C (SP-C) is synthesized by alveolar type II cells as a 21-kDa propeptide (proSP-C21) which is proteolytically processed in subcellular compartments distal to the trans-Golgi network to yield a 35-residue mature form. Initial synthetic processing events for SP-C include post-translational cleavages of the COOH terminus of proSP-C21 yielding two intermediates (16 and 6 kDa). To test the role of specific COOH-terminal domains in intracellular targeting and proteolysis of proSP-C21, synthesis and processing of SP-C was evaluated using a lung epithelial cell line (A549) transfected with a eukaryotic expression vector containing either the full-length cDNA for rat SP-C (SP-Cwt) or one of six polymerase chain reaction (PCR)-generated COOH terminally truncated forms (SP-C1-185, SP-C1-175, SP-C1-147, SP-C1-120, SP-C1-72, and SP-C1-59). Using in vitro transcription/translation, each of the seven constructs produced a 35S-labeled product of appropriate length which could be immunoprecipitated by epitope specific proSP-C antisera. Immunoprecipitation of 35S-labeled A549 cell lysates from SP-Cwt transfectants demonstrated rapid synthesis of [35S]proSP-C21 with processing to SP-C16 and SP-C6 intermediates via cleavages of the COOH-terminal propeptide. Both the intermediates as well as the kinetics of processing in A549 cells were similar to that observed in rat type II cells. In contrast, constructs SP-C1-185, SP-C1-175, SP-C1-147, SP-C1-120, SP-C1-72, and SP-C1-59 were each translated but degraded without evidence of proteolytic processing. Fluorescence immunocytochemistry identified proSP-Cwt in cytoplasmic vesicles of A549 cells while all COOH-terminal deletional mutants were restricted to an endoplasmic reticulum/Golgi compartment identified by co-localization with fluorescein isothiocyanate-concanavalin A. We conclude that SP-Cwt expressed in A549 cells is directed to cytoplasmic vesicles where it is proteolytically processed in a manner similar to native type II cells and that amino acids Cys186-Ile194 located at the COOH terminus of proSP-C21 are necessary for correct intracellular targeting and subsequent cleavage events.

Effect of acidic pH on the structure and lipid binding properties of porcine surfactant protein A. Potential role of acidification along its exocytic pathway

Pulmonary surfactant protein A (SP-A) is synthesized by type II cells and stored intracellularly in secretory granules (lamellar bodies) together with surfactant lipids and hydrophobic surfactant proteins B and C (SP-B and SP-C). We asked whether the progressive decrease in pH along the exocytic pathway could influence the secondary structure and lipid binding and aggregation properties of porcine SP-A. Conformational analysis from CD spectra of SP-A at various pH values indicated that the percentage of alpha-helix progressively decreased and that of beta-sheet increased as the pH was reduced. The protein underwent a marked self-aggregation at mildly acidic pH in the presence of Ca2+, conditions thought to resemble those existing in the trans-Golgi network. Protein aggregation was greater as the pH was reduced. We also found that both neutral and acidic vesicles either with or without SP-B or SP-C bound to SP-A at acidic pH as demonstrated by co-migration during centrifugation. However, the binding of acidic but not neutral vesicles to SP-A led to 1) a striking change in the CD spectra of the protein, which was interpreted as a decrease of the level of SP-A self-aggregation, and 2) a protection of the protein from endoproteinase Glu-C degradation at pH 4.5. SP-A massively aggregated acidic vesicles but poorly aggregated neutral vesicles at acidic pH. Aggregation of dipalmitoylphosphatidylcholine (DPPC) vesicles either with or without SP-B and/or SP-C strongly depended on pH, being progressively decreased as the pH was reduced and markedly increased when pH was shifted back to 7.0. At the pH of lamellar bodies, SP-A-induced aggregation of DPPC vesicles containing SP-B or a mixture of SP-B and SP-C was very low, although SP-A bound to these vesicles. These results indicate that 1) DPPC binding and DPPC aggregation are different phenomena that probably have different SP-A structural requirements and 2) aggregation of membranes induced by SP-A at acidic pH is critically dependent on the presence of acidic phospholipids, which affect protein structure, probably preventing the formation of large aggregates of protein.

Surfactant proteins: molecular genetics of neonatal pulmonary diseases

Genetic and phenotypic complexity has been described for diseases of varied etiology. Groups of patients with varied phenotype can be used in association studies as an initial approach to identify contributing loci. Although association studies have limitations, their value is enhanced by using candidate genes with functions related to disease. Surfactant proteins have been studied in the etiopathogenesis of neonatal pulmonary diseases. SP-A and SP-B polymorphisms are found at a higher frequency in certain groups of patients with respiratory distress syndrome (RDS), and SP-B mutations are linked to the pathogenesis of congenital alveolar proteinosis (CAP). Phenotypic heterogeneity is observed for both CAP and RDS. The available data suggest that a number of factors contribute to the etiology of CAP and RDS and, therefore, a multidisciplinary approach of clinical, genetic, epidemiologic, and statistical considerations is necessary for an in-depth understanding of the pathophysiology of these and other pulmonary diseases.

Secondary structure and limited proteolysis give experimental evidence that the precursor of pulmonary surfactant protein B contains three saposin-like domains

The 42 kDa precursor of surfactant protein B generates the 79 residue mature SP-B polypeptide, which belongs to the family of saposin-like proteins and has unique functional roles in pulmonary surfactant. From sequence comparisons it has been suggested that proSP-B, in addition to SP-B, contains two saposin-like domains, but their existence has until now not been experimentally verified. The 381 residue human proSP-B was now fused to an N-terminal poly-His tag, expressed in Escherichia coli, and purified from inclusion bodies by resolubilisation with 2.5% (w/v) SDS and, after removal of SDS, subsequent metal affinity chromatography. Recombinant proSP-B thus obtained exhibits about 35% alpha-helical structure in sodium phosphate buffer and is proteolytically cleaved preferentially between the three saposin-like domains. These results experimentally support that proSP contains, in addition to SP-B, two further saposin-like domains.

Passive sorting in maturing granules of AtT-20 cells: the entry and exit of salivary amylase and proline-rich protein

Previous studies have suggested that salivary amylase and proline-rich protein are sorted differently when expressed in AtT-20 cells (Castle, A.M., L.E. Stahl, and J.D. Castle. 1992. J. Biol. Chem. 267:13093- 13100; Colomer, V., K. Lal, T.C. Hoops, and M.J. Rindler. 1994.EMBO (Eur. Mol. Biol. Organ.) J. 13:3711- 3719). We now show that both exocrine proteins behave similarly and enter the regulated secretory pathway as judged by immunolocalization and secretagogue- dependent stimulation of secretion. Analysis of stimulated secretion of newly synthesized proline-rich protein, amylase, and endogenous hormones indicates that the exogenous proteins enter the granule pool with about the same efficiency as the endogenous hormones. However, in contrast to the endogenous hormones, proline-rich protein and amylase are progressively removed from the granule pool during the process of granule maturation such that only small portions remain in mature granules where they colocalize with the stored hormones. The exogenous proteins that are not stored are recovered from the incubation medium and are presumed to have undergone constitutive-like secretion. These results point to a level of sorting for regulated secretion after entry of proteins into forming granules and indicate that retention is essential for efficient storage. Consequently, the critical role of putative sorting receptors for regulated secretion may be in retention rather than in granule entry.

Rescue of SP-B knockout mice with a truncated SP-B proprotein. Function of the C-terminal propeptide

The function of the 102-amino acid C-terminal propeptide of surfactant protein B (SP-B) was analyzed by characterizing the phenotype associated with loss of expression of this peptide domain in transgenic mice. A construct encoding the signal peptide, N-terminal propeptide, and mature peptide of human SP-B (hSP-BDeltac) was cloned under the control of the 3.7-kilobase human SP-C promoter and injected into fertilized eggs of the FVB/N mouse strain. Founder mice expressing the hSP-BDeltac transgene were bred with heterozygous SP-B knockout mice (SP-B +/-). Offspring containing the transgene and one allele of mouse SP-B were identified and subsequently crossed to generate a transgenic line that expressed SP-BDeltac in a null background (SP-B(-/-)/hSP-BDeltac(+/+)). Expression of hSP-BDeltac in SP-B(-/-) mice was restricted to type II cells and resulted in a 2-fold increase in mature SP-B relative to wild type littermates. These mice survived without any evidence of respiratory problems and had normal lung function, normal alveolar surfactant phospholipid pool sizes, and typical tubular myelin indicating that the 102-residue C-terminal propeptide of SP-B is not required for normal structure and function of extracellular surfactant. However, proteolytic processing of the SP-C proprotein was perturbed resulting in the accumulation of a processing intermediate, Mr = 11,000, similar to the phenotype detected in SP-B(-/-) mice; furthermore, lamellar bodies in type II cells of SP-B(-/-)/hSP-BDeltac(+/+) mice were much larger than in the wild type animal and saturated phosphatidylcholine content in lung tissue was significantly increased although the incorporation of choline into saturated phosphatidylcholine was normal. Collectively, these results demonstrate a role for the C-terminal propeptide of SP-B in SP-C proprotein processing and the maintenance of lamellar body size. The C-terminal propeptide may be an important determinant of intracellular surfactant pool size.

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.