Trimeric structure of a C-type mannose-binding protein

The amino-terminal heptad repeats of the coiled-coil neck domain of pulmonary surfactant protein d are necessary for the assembly of trimeric subunits and dodecamers

Pulmonary surfactant protein D (SP-D), a lung host defense protein, is assembled as multimers of trimeric subunits. Trimerization of SP-D monomers is required for high affinity saccharide binding, and the oligomerization of trimers is required for many of its functions. A peptide containing the alpha-helical neck region can spontaneously trimerize in vitro. However, it is not known whether this sequence is necessary for the complete cellular assembly of disulfide-cross-linked, trimeric subunits and dodecamers. For the present studies, we synthesized mutant cDNAs with deletions or site-directed substitutions in the neck domain of rat SP-D, and examined the assembly of the newly synthesized proteins after transfection of CHO-K1 cells. The neck domain contains three "classical" heptad repeat motifs with leucine residues at the "d position," and a distinctive C-terminal repeat previously suggested to drive trimeric chain association. Deletion of the highly conserved core of the latter repeat (FSRYLKK) did not interfere with the secretion of dodecamers with lectin activity. By contrast, deletion of the entire neck domain or deletion of one or two amino-terminal repeats resulted in defective molecular assembly. The secreted proteins eluted in the position of monomers by gel filtration under nondenaturing conditions. In addition, the neck + carbohydrate recognition domain of SP-D was necessary and sufficient for the trimerization of a heterologous collagen sequence located amino-terminal to the trimeric coiled-coil. These studies provide strong evidence that the amino-terminal heptad repeats of the neck domain are necessary for the intracellular, trimeric association of SP-D monomers and for the assembly and secretion of functional dodecamers.

Activity of pulmonary surfactant protein-D (SP-D) in vivo is dependent on oligomeric structure

Pulmonary surfactant protein-D (SP-D) is a member of the collectin family of C-type lectins that is synthesized in many tissues including respiratory epithelial cells in the lung. SP-D is assembled predominantly as dodecamers consisting of four homotrimeric subunits each. Association of these subunits is stabilized by interchain disulfide bonds involving two conserved amino-terminal cysteine residues (Cys-15 and Cys-20). Mutant recombinant rat SP-D lacking these residues (RrSP-Dser15/20) is secreted in cell culture as trimeric subunits rather than as dodecamers. In this study, transgenic mice that express this mutant were generated to elucidate the functional importance of SP-D oligomerization in vivo. Expression of RrSP-Dser15/20 failed to correct the pulmonary phospholipid accumulation and emphysema characteristic of SP-D null (mSP-D-/-) mice. Expression of high concentrations of the mutant protein in wild-type mice reduced the abundance of disulfide cross-linked oligomers of endogenous SP-D in the bronchoalveolar lavage fluid and demonstrated a phenotype that partially overlapped with that of the SP-D-/- mice; the animals developed emphysema and foamy macrophages without the associated abnormalities in alveolar phospholipids typical of SP-D-/- mice. Development of foamy macrophages in SP-D-deficient mice is not secondary to the increased abundance of surfactant phospholipids. Disulfide cross-linked SP-D oligomers are required for the regulation of surfactant phospholipid homeostasis and the prevention of emphysema and foamy macrophages in vivo.

An extended conformation of the macrophage mannose receptor

The macrophage mannose receptor mediates phagocytosis of pathogenic microorganisms and endocytosis of potentially harmful soluble glycoproteins by recognition of their defining carbohydrate structures. The mannose receptor is the prototype for a family of receptors each having an extracellular region consisting of 8-10 domains related to C-type carbohydrate recognition domains (CRDs), a fibronectin type II repeat and an N-terminal cysteine-rich domain. Hydrodynamic analysis and proteolysis experiments performed on fragments of the extracellular region of the receptor have been used to investigate its conformation. Size and shape parameters derived from sedimentation and diffusion coefficients indicate that the receptor is a monomeric, elongated and asymmetric molecule. Proteolysis experiments indicate the presence of close contacts between several pairs of domains and exposed linker regions separating CRDs 3 and 6 from their neighboring domains. Hydrodynamic coefficients predicted for modeled receptor conformations are consistent with an extended conformation with close contacts between three pairs of CRDs. The N-terminal cysteine-rich domain and the fibronectin type II repeat appear to increase the rigidity of the molecule. The rigid, extended conformation of the receptor places domains with different functions at distinct positions with respect to the membrane.

Domains of surfactant protein A that affect protein oligomerization, lipid structure and surface tension

Surfactant protein A (SP-A) is an abundant protein found in pulmonary surfactant which has been reported to have multiple functions. In this review, we focus on the structural importance of each domain of SP-A in the functions of protein oligomerization, the structural organization of lipids and the surface-active properties of surfactant, with an emphasis on ultrastructural analyses. The N-terminal domain of SP-A is required for disulfide-dependent protein oligomerization, and for binding and aggregation of phospholipids, but there is no evidence that this domain directly interacts with lipid membranes. The collagen-like domain is important for the stability and oligomerization of SP-A. It also contributes shape and dimension to the molecule, and appears to determine membrane spacing in lipid aggregates such as common myelin and tubular myelin. The neck domain of SP-A is primarily involved in protein trimerization, which is critical for many protein functions, but it does not appear to be directly involved in lipid interactions. The globular C-terminal domain of SP-A clearly plays a central role in lipid binding, and in more complex functions such as the formation and/or stabilization of curved membranes. In recent work, we have determined that the maintenance of low surface tension of surfactant in the presence of serum protein inhibitors requires cooperative interactions between the C-terminal and N-terminal domains of the molecule. This effect of SP-A requires a high degree of oligomeric assembly of the protein, and may be mediated by the activity of the protein to alter the form or physical state of surfactant lipid aggregates.

Socket: a program for identifying and analysing coiled-coil motifs within protein structures

The coiled coil is arguably the simplest protein-structure motif and probably the most ubiquitous facilitator of protein-protein interactions. Coiled coils comprise two or more alpha-helices that wind around each other to form "supercoils". The hallmark of most coiled coils is a regular sequence pattern known as the heptad repeat. Despite this apparent simplicity and relatedness at the sequence level, coiled coils display a considerable degree of structural diversity: the helices may be arranged parallel or anti-parallel and may form a variety of oligomer states. To aid studies of coiled coils, we developed SOCKET, a computer program to identify these motifs automatically in protein structures. We used SOCKET to gather a set of unambiguous coiled-coil structures from the RCSB Protein Data Bank. Rather than searching for sequence features, the algorithm recognises the characteristic knobs-into-holes side-chain packing of coiled coils; this proved to be straightforward to implement and was able to distinguish coiled coils from the great majority of helix-helix packing arrangements observed in globular domains. SOCKET unambiguously defines coiled-coil helix boundaries, oligomerisation states and helix orientations, and also assigns heptad registers. Structures retrieved from the Protein Data Bank included parallel and anti-parallel variants of two, three and four-stranded coiled coils, one example of a parallel pentamer and a small number of structures that extend the classical description of a coiled coil. We anticipate that our structural database and the associated sequence data that we have gathered will be of use in identifying principles for coiled-coil assembly, prediction and design. To illustrate this we give examples of sequence and structural analyses of the structures that are possible using the new data bases, and we present amino acid profiles for the heptad repeats of different motifs.

Mechanism of pH-dependent N-acetylgalactosamine binding by a functional mimic of the hepatocyte asialoglycoprotein receptor

Efficient release of ligands from the Ca(2+)-dependent carbohydrate-recognition domain (CRD) of the hepatic asialoglycoprotein receptor at endosomal pH requires a small set of conserved amino acids that includes a critical histidine residue. When these residues are incorporated at corresponding positions in an homologous galactose-binding derivative of serum mannose-binding protein, the pH dependence of ligand binding becomes more like that of the receptor. The modified CRD displays 40-fold preferential binding to N-acetylgalactosamine compared with galactose, making it a good functional mimic of the asialoglycoprotein receptor. In the crystal structure of the modified CRD bound to N-acetylgalactosamine, the histidine (His(202)) contacts the 2-acetamido methyl group and also participates in a network of interactions involving Asp(212), Arg(216), and Tyr(218) that positions a water molecule in a hydrogen bond with the sugar amide group. These interactions appear to produce the preference for N-acetylgalactosamine over galactose and are also likely to influence the pK(a) of His(202). Protonation of His(202) would disrupt its interaction with an asparagine that serves as a ligand for Ca(2+) and sugar. The structure of the modified CRD without sugar displays several different conformations that may represent structures of intermediates in the release of Ca(2+) and sugar ligands caused by protonation of His(202).

Collectin structure: a review

Collectins are animal calcium dependent lectins that target the carbohydrate structures on invading pathogens, resulting in the agglutination and enhanced clearance of the microorganism. These proteins form trimers that may assemble into larger oligomers. Each polypeptide chain consists of four regions: a relatively short N-terminal region, a collagen like region, an alpha-helical coiled-coil, and the lectin domain. Only primary structure data are available for the N-terminal region, while the most important features of the collagen-like region can be derived from its homology with collagen. The structures of the alpha-helical coiled-coil and the lectin domain are known from crystallographic studies of mannan binding protein (MBP) and lung surfactant protein D (SP-D). Carbohydrate binding has been structurally characterized in several complexes between MBP and carbohydrate; all indicate that the major interaction between carbohydrate and collectin is the binding of two adjacent carbohydrate hydroxyl group to a collectin calcium ion. In addition, these hydroxyl groups hydrogen bond to some of the calcium amino acid ligands. While each collectin trimer contains three such carbohydrate binding sites, deviation from the overall threefold symmetry has been demonstrated for SP-D, which may influence its binding properties. The protein surface between the three binding sites is positively charged in both MBP and SP-D.

Affinity enhancement by multivalent lectin-carbohydrate interaction

The binding of simple carbohydrate ligands by proteins often requires affinity enhancement to attain biologically relevant strength. This is especially true for endocytotic receptors and the molecules that engage in the first-line of defense. For such purposes, nature often utilizes a mode of affinity enhancement that arises from multiple interactions between the binding proteins and the carbohydrate ligands, which we term glycoside cluster effect. In this review article we give a number of examples and describe important factors in the multi-valent interactions that govern the degree of affinity enhancement.

Mannan-binding lectin (MBL) in chickens: molecular and functional aspects

Mannan-binding lectin (MBL) is a serum collectin (i.e. mosaic protein with collagenous and lectin domains) involved in the innate immune defence against various microbes. In vitro studies indicate that MBL exerts its function by binding to the microbial surface through its carbohydrate recognition domains followed by direct opsonization or complement activation via the MBL associated serine proteases MASP-1 and MASP-2. In Aves (i.e. chickens), as in man, only one MBL form has been found, while traditional laboratory animals (i.e. mouse and rat) have two MBL forms in serum. MBL has been extensively studied in mammals but recently also in Aves. This review summarizes the present knowledge of MBL in chickens and compares it to the situation in mammals.

Collectins and innate immunity in the lung

Evidence from both in vitro and in vivo studies suggests that the collectins are important elements in host innate immune defences against infectious agents. Study of the collectins in specific disease settings now raises the prospects of developing therapies exploiting these mechanisms of innate immunity.

Mannose-binding lectin binds to a range of clinically relevant microorganisms and promotes complement deposition

Mannose-binding lectin (MBL) is a collagenous serum lectin believed to be of importance in innate immunity. Genetically determined low levels of the protein are known to predispose to infections. In this study the binding of purified MBL to pathogens isolated from immunocompromised children was investigated by flow cytometry. Diverse Candida species, Aspergillus fumigatus, Staphylococcus aureus, and beta-hemolytic group A streptococci exhibited strong binding of MBL, whereas Escherichia coli, Klebsiella species, and Haemophilus influenzae type b were characterized by heterogeneous binding patterns. In contrast, beta-hemolytic group B streptococci, Streptococcus pneumoniae, and Staphylococcus epidermidis showed low levels of binding. Bound MBL was able to promote C4 deposition in a concentration-dependent manner. We conclude that MBL may be of importance in first-line immune defense against several important pathogens.

Crystal structure of a lectin-like natural killer cell receptor bound to its MHC class I ligand

Natural killer (NK) cell function is regulated by NK receptors that interact with MHC class I (MHC-I) molecules on target cells. The murine NK receptor Ly49A inhibits NK cell activity by interacting with H-2D(d) through its C-type-lectin-like NK receptor domain. Here we report the crystal structure of the complex between the Ly49A NK receptor domain and unglycosylated H-2D(d). The Ly49A dimer interacts extensively with two H-2D(d) molecules at distinct sites. At one interface, a single Ly49A subunit contacts one side of the MHC-I peptide-binding platform, presenting an open cavity towards the conserved glycosylation site on the H-2D(d) alpha2 domain. At a second, larger interface, the Ly49A dimer binds in a region overlapping the CD8-binding site. The smaller interface probably represents the interaction between Ly49A on the NK cell and MHC-I on the target cell, whereas the larger one suggests an interaction between Ly49A and MHC-I on the NK cell itself. Both Ly49A binding sites on MHC-I are spatially distinct from that of the T-cell receptor.

A Novel LPS-Inducible C-Type Lectin Is a Transcriptional Target of NF-IL6 in Macrophages

C-type lectins serve multiple functions through recognizing carbohydrate chains. Here we report a novel C-type lectin, macrophage-inducible C-type lectin (Mincle), as a downstream target of NF-IL6 in macrophages. NF-IL6 belongs to the CCAAT/enhancer binding protein (C/EBP) of transcription factors and plays a crucial role in activated macrophages. However, what particular genes are regulated by NF-IL6 has been poorly defined in macrophages. Identification of downstream targets is required to elucidate the function of NF-IL6 in more detail. To identify downstream genes of NF-IL6, we screened a subtraction library constructed from wild-type and NF-IL6-deficient peritoneal macrophages and isolated Mincle that exhibits the highest homology to the members of group II C-type lectins. Mincle mRNA expression was strongly induced in response to several inflammatory stimuli, such as LPS, TNF-α, IL-6, and IFN-γ in wild-type macrophages. In contrast, NF-IL6-deficient macrophages displayed a much lower level of Mincle mRNA induction following treatment with these inflammatory reagents. The mouse Mincle proximal promoter region contains an indispensable NF-IL6 binding element, demonstrating that Mincle is a direct target of NF-IL6. The Mincle gene locus was mapped at 0.6 centiMorgans proximal to CD4 on mouse chromosome 6.

Residual dipolar coupling derived orientational constraints on ligand geometry in a 53 kDa protein-ligand complex

The geometric relationships between ligands and the functional groups that bind ligands in soluble ligand-protein complexes have traditionally been deduced from distance constraints between pairs of NMR active nuclei spanning the ligand-protein interface. Frequently, the steep inverse distance dependence of the nuclear Overhauser effect (NOE), from which the distance constraints are derived, makes identification of sufficient numbers of constraints difficult. In these cases the ability to supplement NOE-derived information with distance-independent angular information can be very important. Here, the observation of residual dipolar couplings from alpha-methyl mannose bound to mannose binding-protein in a dilute liquid crystalline medium has allowed the determination of a bound ligand's average orientation. The 3-fold rotational symmetry of mannose-binding protein defines its orientational tensor and obviates the need to determine experimentally the protein's average orientation. Through superimposition of ligand and protein orientational tensors we describe the binding geometry of alpha-methyl mannose bound to mannose-binding protein. This new method is of general applicability to the study of ligands bound to proteins, and it is of particular interest when neither X-ray crystallography nor NOE techniques can provide sufficient information to describe binding geometries.

Crystal structure of invasin: a bacterial integrin-binding protein

The Yersinia pseudotuberculosis invasin protein promotes bacterial entry by binding to host cell integrins with higher affinity than natural substrates such as fibronectin. The 2.3 angstrom crystal structure of the invasin extracellular region reveals five domains that form a 180 angstrom rod with structural similarities to tandem fibronectin type III domains. The integrin-binding surfaces of invasin and fibronectin include similarly located key residues, but in the context of different folds and surface shapes. The structures of invasin and fibronectin provide an example of convergent evolution, in which invasin presents an optimized surface for integrin binding, in comparison with host substrates.

C-type lectin-like domains

Carbohydrate-recognition domains of C-type (Ca2+-dependent) animal lectins serve as prototypes for an important family of protein modules. Only some domains in this family bind Ca2+ or sugars. A comparison of recent structures of C-type lectin-like domains reveals diversity in the modular fold, particularly in the region associated with Ca2+ and sugar binding. Some of this diversity reflects the changes that occur during normal physiological functioning of the domains. C-type lectin-like domains associate with each other through several different surfaces to form dimers and trimers, from which ligand-binding sites project in a variety of different orientations.

Identification of side-chain clusters in protein structures by a graph spectral method

This paper presents a novel method to detect side-chain clusters in protein three-dimensional structures using a graph spectral approach. Protein side-chain interactions are represented by a labeled graph in which the nodes of the graph represent the Cbeta atoms and the edges represent the distance between the Cbeta atoms. The distance information and the non-bonded connectivity of the residues are represented in the form of a matrix called the Laplacian matrix. The constructed matrix is diagonalized and clustering information is obtained from the vector components associated with the second lowest eigenvalue and cluster centers are obtained from the vector components associated with the top eigenvalues. The method uses global information for clustering and a single numeric computation is required to detect clusters of interest. The approach has been adopted here to detect a variety of side-chain clusters and identify the residue which makes the largest number of interactions among the residues forming the cluster (cluster centers). Detecting such clusters and cluster centers are important from a protein structure and folding point of view. The crucial residues which are important in the folding pathway as determined by PhiF values (which is a measure of the effect of a mutation on the stability of the transition state of folding) as obtained from protein engineering methods, can be identified from the vector components corresponding to the top eigenvalues. Expanded clusters are detected near the active and binding site of the protein, supporting the nucleation condensation hypothesis for folding. The method is also shown to detect domains in protein structures and conserved side-chain clusters in topologically similar proteins.

Horseshoe crab acetyl group-recognizing lectins involved in innate immunity are structurally related to fibrinogen

We have characterized and cloned newly isolated lectins from hemolymph plasma of the horseshoe crab Tachypleus tridentatus, which we named tachylectins 5A and 5B (TLs-5). TLs-5 agglutinated all types of human erythrocytes and Gram-positive and Gram-negative bacteria. TLs-5 specifically recognize acetyl group-containing substances including noncarbohydrates; the acetyl group is required and is sufficient for recognition. TLs-5 enhanced the antimicrobial activity of a horseshoe crab-derived big defensin. cDNA sequences of TLs-5 indicated that they consist of a short N-terminal Cys-containing segment and a C-terminal fibrinogen-like domain with the highest sequence identity (51%) to that of mammalian ficolins. TLs-5, however, lack the collagenous domain found in a kind of "bouquet arrangement" of ficolins and collectins. Electron microscopy revealed that TLs-5 form two- to four-bladed propeller structures. The horseshoe crab is equipped with a unique functional homologue of vertebrate fibrinogen, coagulogen, as the target protein of the clotting cascade. Our observations clearly show that the horseshoe crab has fibrinogen-related molecules in hemolymph plasma and that they function as nonself-recognizing lectins. An ancestor of fibrinogen may have functioned as a nonself-recognizing protein.

Tachylectin-2: crystal structure of a specific GlcNAc/GalNAc-binding lectin involved in the innate immunity host defense of the Japanese horseshoe crab Tachypleus tridentatus

Tachylectin-2, isolated from large granules of the hemocytes of the Japanese horseshoe crab (Tachypleus tridentatus), is a 236 amino acid protein belonging to the lectins. It binds specifically to N-acetylglucosamine and N-acetylgalactosamine and is a part of the innate immunity host defense system of the horseshoe crab. The X-ray structure of tachylectin-2 was solved at 2.0 A resolution by the multiple isomorphous replacement method and this molecular model was employed to solve the X-ray structure of the complex with N-acetylglucosamine. Tachylectin-2 is the first protein displaying a five-bladed beta-propeller structure. Five four-stranded antiparallel beta-sheets of W-like topology are arranged around a central water-filled tunnel, with the water molecules arranged as a pentagonal dodecahedron. Tachylectin-2 exhibits five virtually identical binding sites, one in each beta-sheet. The binding sites are located between adjacent beta-sheets and are made by a large loop between the outermost strands of the beta-sheets and the connecting segment from the previous beta-sheet. The high number of five binding sites within the single polypeptide chain strongly suggests the recognition of carbohydrate surface structures of pathogens with a fairly high ligand density. Thus, tachylectin-2 employs strict specificity for certain N-acetyl sugars as well as the surface ligand density for self/non-self recognition.

Identification of a pathogen-binding lectin in salmon serum

A mannose-binding lectin was isolated from the blood serum of Atlantic salmon (Salmo salar). Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing and non-reducing conditions revealed a multimeric structure composed of 17000 Mr subunits. Hexosamine analysis and glycosidase digestion showed that the lectin is not glycosylated and amino acid analysis revealed no unusual compositional features. Using ruthenium red staining, the lectin was shown to bind Ca2+ ions. N-terminal sequencing by Edman degradation gave: H2N-TGAKGAEEGVVPAETRNQXPTGWFQFGS. A database search revealed no similarity to protein sequences deposited to date. Binding experiments using biotinylated lectin revealed that it specifically recognizes and binds to mannose on the surfaces of two salmon pathogens, Vibrio anguillarum and Aeromonas salmonicida, implying an immunological role for this lectin in Atlantic salmon.

Crystal structure of the trimeric alpha-helical coiled-coil and the three lectin domains of human lung surfactant protein D

BACKGROUND

Human lung surfactant protein D (hSP-D) belongs to the collectin family of C-type lectins and participates in the innate immune surveillance against microorganisms in the lung through recognition of carbohydrate ligands present on the surface of pathogens. The involvement of this protein in innate immunity and the allergic response make it the subject of much interest.

RESULTS

We have determined the crystal structure of a trimeric fragment of hSP-D at 2.3 A resolution. The structure comprises an alpha-helical coiled-coil and three carbohydrate-recognition domains (CRDs). An interesting deviation from symmetry was found in the projection of a single tyrosine sidechain into the centre of the coiled-coil; the asymmetry of this residue influences the orientation of one of the adjacent CRDs. The cleft between the three CRDs presents a large positively charged surface.

CONCLUSIONS

The fold of the CRD of hSP-D is similar to that of the mannan-binding protein (MBP), but its orientation relative to the alpha-helical coiled-coil region differs somewhat to that seen in the MBP structure. The novel central packing of the tyrosine sidechain within the coiled-coil and the resulting asymmetric orientation of the CRDs has unexpected functional implications. The positively charged surface might facilitate binding to negatively charged structures, such as lipopolysaccharides.

Molecular determinants of oligomer formation and complement fixation in mannose-binding proteins

Rat serum mannose-binding protein (MBP-A) functions as part of the innate immune system by targetting complement toward potentially pathogenic microorganisms. In order to examine the molecular basis for complement activation, rat MBP-A has been overproduced in Chinese hamster ovary cells. Recombinant protein is post-translationally modified in the same way as the native lectin. Hydrodynamic studies indicate that MBP-A consists predominantly of covalent oligomers containing one to four copies of a subunit that comprises a trimer of polypeptides. These oligomers are non-interconverting and do not assemble into higher order structures at concentrations in excess of those normally found in serum. Disulfide bonds formed between cysteine residues at the N-terminal end of the collagen-like domain link polypeptides to form covalent oligomers. Analysis of wild-type MBP-A and MBP-A containing the substitution Cys6 --> Ser suggests that polypeptides within each trimeric structural unit are mostly linked by disulfide bonds between cysteine residues at positions 13 and 18 arranged in an asymmetrical configuration. Disulfide bonds involving Cys6 connect polypeptides within separate trimers. Analysis of chimeras between MBP-A and rat liver MBP (MBP-C) indicates that residues within the N-terminal region of the collagenous domain and the cysteine-rich domain of MBP-A enable assembly of trimers into higher order oligomers. The activity of MBP-A in a hemolytic complement fixation assay using mannan-coated sheep erythrocytes was approximately 20-fold greater than the activity of MBP-C. Analysis of the MBP chimeras and isolated oligomers of MBP-A reveals that the larger oligomers are more efficient at complement activation. These data indicate that the overall complement fixing activity of MBP-A is a function of the individual molecular activities of oligomers and their relative abundance within the serum.

Deletion mapping of N-terminal domains of surfactant protein A. The N-terminal segment is required for phospholipid aggregation and specific inhibition of surfactant secretion

The objective of the current study was to examine the functional importance of the N-terminal domains of surfactant protein A (SP-A) including the N-terminal segment from Asn1 to Ala7 (denoted domain 1), the N-terminal portion of the collagen domain from Gly8 to Gly44 (domain 2), and the C-terminal portion of the collagen-like domain from Gly45 to Pro80 (domain 3). Wild type recombinant SP-A (SP-Ahyp; where hyp indicates hydroxyproline-deficient) and truncated mutant (TM) SP-As containing deletions of domain(s) 1 (TM1), 2 (TM2), 1 and 2 (TM1-2), and 1, 2, and 3 (TM1-2-3) were synthesized in insect cells and purified by mannose-Sepharose affinity chromatography. N-terminal disulfide-dependent dimerization was preserved at near wild type levels in the TM1-2 (at Cys-1) and TM2 proteins (at Cys-1 and Cys6), and to a lesser extent in TM1 (at Cys-1), but not in TM1-2-3. Cross-linking analyses demonstrated that the neck + CRD was sufficient for assembly of monomers into noncovalent trimers and that the N-terminal segment was required for the association of trimers to form higher oligomers. All TM proteins except TM1-2-3 bound to phospholipid, but only the N-terminal segment containing TM proteins aggregated phospholipid vesicles. The TM1, TM1-2, and TM2 but not the TM1-2-3 inhibited the secretion of surfactant from type II cells as effectively as SP-Ahyp, but the inhibitory activity of each mutant was blocked by excess alpha-methylmannoside and therefore nonspecific. TM1 and TM1-2-3 did not enhance the uptake of phospholipids by isolated type II cells, but the TM1-2 and TM2 had activities that were 72 and 83% of SP-Ahyp, respectively. We conclude the following for SP-A: 1) trimerization does not require the collagen-like region or interchain disulfide linkage; 2) the N-terminal portion of the collagen-like domain is required for specific inhibition of surfactant secretion but not for binding to liposomes or for enhanced uptake of phospholipids into type II cells; 3) N-terminal interchain disulfide linkage can functionally replace the N-terminal segment for lipid binding, receptor binding, and enhancement of lipid uptake; 4) the N-terminal segment is required for the association of trimeric subunits into higher oligomers, for phospholipid aggregation, and for specific inhibition of surfactant secretion and cannot be functionally replaced by disulfide linkage alone for these activities.

Structure, processing and properties of surfactant protein A

Surfactant protein A (SP-A) is a highly ordered, oligomeric glycoprotein that is secreted into the airspaces of the lung by the pulmonary epithelium. The in vitro activities of protein suggest diverse roles in pulmonary host defense and surfactant homeostasis, structure and surface activity. Functional mapping of SP-A using directed mutagenesis has identified domains which interact with surfactant phospholipids, alveolar type II cells and microbes. Recently developed genetically manipulated animal models are beginning to clarify the critical physiological roles for SP-A in the normal lung, and in the pathophysiology of pulmonary disease.

Supercoiled protein motifs: the collagen triple-helix and the alpha-helical coiled coil

The collagen triple-helix and the alpha-helical coiled coil represent the two basic supercoiled multistranded protein motifs. Originally they were characterized in fibrous proteins, but have been found more recently in a number of other proteins containing rod-shaped domains. Coiled-coil domains are responsible for the oligomerization of proteins, as well as other specific functions, while the triple-helix domains associate to form supramolecular structures and bind a variety of ligands. Both structures were originally solved by fiber diffraction, and recent crystallographic studies on small proteins and peptide models have confirmed the structure and provided molecular details. The differences in the molecular conformations of these two motifs and the interactions stabilizing these conformations are discussed. The molecular structures of both motifs constrain the amino acid sequence to recognizable patterns, requiring the (Gly-X-Y)n repeating sequence for the collagen triple-helix and a less stringent heptad repeat requirement (h-x-x-h-x-x-x)n for the coiled-coil domains, where h represents hydrophobic residues. The features and roles of these supercoiled domains in proteins are considered when they are found adjacent in the same protein.

Structural aspects of collectins and receptors for collectins

The collectins are oligomeric molecules composed of C-type lectin domains attached to collagen regions via alpha-coiled neck regions. Five members of the collectins have been characterized. Mannan-binding lectin (MBL), conglutinin and collectin-43 (CL-43) are serum proteins produced by the liver. Lung surfactant protein A (SP-A) and lung surfactant protein D (SP-D) are mainly found in the lung, where they are synthesized by alveolar type II cells and secreted to the alveolar surface. The collectins are believed to play an important role in innate immunity. They bind oligosaccharides on the surface of a variety of microbial pathogens. After binding of the collectins to the microbial surface effector mechanisms such as agglutination, neutralizing or opsonization of the microorganisms for phagocytosis are initiated. SP-A and SP-D stimulate chemotaxis of phagocytes and once bound to the phagocytes, the production of oxygen radicals can be induced. In the case of MBL the opsonization can be further enhanced by complement activation via the MBLectin pathway while conglutinin interacts with the complement system by binding to the complement degradation product iC3b. A number of receptors and binding molecules interacting with the collectins are found on the membrane or in association with the membrane of various cells responsible for phagocytosis and clearance of microorganisms. This paper focus on the structural aspects of the collectins and the receptors for collectins.

Collectins and pulmonary host defense

The surfactant-associated proteins SP-A and SP-D are members of a family of collagenous host defense lectins, designated collectins. There is increasing evidence that these pulmonary epithelial-derived proteins are important components of the innate immune response to microbial challenge, and that they participate in other aspects of immune and inflammatory regulation within the lung. The collectins bind to glycoconjugates and/or lipid moieties expressed by a wide variety of microorganisms and certain other organic particles in vitro. Although binding may facilitate microbial clearance through aggregation or other direct effects on the organism, SP-A and SP-D also have the capacity to modulate leukocyte function and, in some circumstances, to enhance their killing of microorganisms. The biologic activity of cell wall components, such as gram-negative bacterial polysaccharides, may be altered by interactions with collectins. Complementary or cooperative interactions between SP-A and SP-D could contribute to the efficiency of this defense system. Collectins may play particularly important roles in settings of inadequate or impaired specific immunity. Acquired or genetic alterations in the levels of active proteins within the airspaces and distal airways may increase susceptibility to infection.

90 years on: a therapy to 'stimulate the phagocytes'?

The collectins (collagenous lectins) constitute a major element of the innate immune system and mannan-binding lectin (or mannose-binding lectin, MBL) is now recognized as a serum constituent with many of the attributes of both antibody and C1q. A common opsonic deficiency recognized more than 20 years ago is known to be a functional manifestation of MBL deficiency. This is largely explained by three single point mutations in exon 1 of the MBL gene, each of which disrupts the collagenous region and probably prevents assembly of higher order oligomers. Several independent studies suggest that deficiency of the protein increases significantly the risk of infection with a range of different pathogens and also predisposes such individuals to autoimmune disease. The first description of MBL replacement therapy [Valdimarsson et al., Scand J Immunol 1998;48:116-123] provides evidence that administration of this purified plasma protein is safe, practical and possibly efficacious. These preliminary observations in two recipients now need to be extended in large-scale trials.

Mechanism of N-acetylgalactosamine binding to a C-type animal lectin carbohydrate-recognition domain

The mammalian hepatic asialoglycoprotein receptor, a member of the C-type animal lectin family, displays preferential binding to N-acetylgalactosamine compared with galactose. The structural basis for selective binding to N-acetylgalactosamine has been investigated. Regions of the carbohydrate-recognition domain of the receptor believed to be important in preferential binding to N-acetylgalactosamine have been inserted into the homologous carbohydrate-recognition domain of a mannose-binding protein mutant that was previously altered to bind galactose. Introduction of a single histidine residue corresponding to residue 256 of the hepatic asialoglycoprotein receptor was found to cause a 14-fold increase in the relative affinity for N-acetylgalactosamine compared with galactose. The relative ability of various acyl derivatives of galactosamine to compete for binding to this modified carbohydrate-recognition domain suggest that it is a good model for the natural N-acetylgalactosamine binding site of the asialoglycoprotein receptor. Crystallographic analysis of this mutant carbohydrate-recognition domain in complex with N-acetylgalactosamine reveals a direct interaction between the inserted histidine residue and the methyl group of the N-acetyl substituent of the sugar. Evidence for the role of the side chain at position 208 of the receptor in positioning this key histidine residue was obtained from structural analysis and mutagenesis experiments. The corresponding serine residue in the modified carbohydrate-recognition domain of mannose-binding protein forms a hydrogen bond to the imidazole side chain. When this serine residue is changed to valine, loss in selectivity for N-acetylgalactosamine is observed. The structure of this mutant reveals that the beta-branched valine side chain interacts directly with the histidine side chain, resulting in an altered imidazole ring orientation.

Cloning of a novel C-type lectin expressed by murine macrophages

We report the cloning of a novel macrophage-restricted C-type lectin by differential display polymerase chain reaction. This molecule, named mouse macrophage C-type lectin, is a 219-amino acid, type II transmembrane protein with a single extracellular C-type lectin domain. Northern blot analysis indicates that it is expressed in cell lines and normal mouse tissues in a macrophage-restricted manner. The cDNA and genomic sequences of mouse macrophage C-type lectin indicate that it is related to the Group II animal C-type lectins. The mcl gene locus has been mapped between the genes for the interleukin-17 receptor and CD4 on mouse chromosome 6, the same chromosome as the mouse natural killer cell gene complex.

Surfactant protein A and surfactant protein D in health and disease

Surfactant protein (SP) A and SP-D are collagenous glycoproteins with multiple functions in the lung. Both of these proteins are calcium-dependent lectins and are structurally similar to mannose-binding protein and bovine conglutinin. Both form polyvalent multimeric structures for interactions with pathogens, cells, or other molecules. SP-A is an integral part of the surfactant system, binds phospholipids avidly, and is found in lamellar bodies and tubular myelin. Initially, most research interest focused on its role in surfactant homeostasis. Recently, more attention has been placed on the role of SP-A as a host defense molecule and its interactions with pathogens and phagocytic cells. SP-D is much less involved with the surfactant system. SP-D appears to be primarily a host defense molecule that binds surfactant phospholipids poorly and is not found in lamellar inclusion bodies or tubular myelin. Both SP-A and SP-D bind a wide spectrum of pathogens including viruses, bacteria, fungi, and pneumocystis. In addition, both molecules have been measured in the systemic circulation by immunologic methods and may be useful biomarkers of disease. The current challenges are characterization of the three-dimensional crystal structure of SP-A and SP-D, molecular cloning of their receptors, and determination of their precise physiological functions in vivo.

The C-terminal domain of matrilin-2 assembles into a three-stranded alpha-helical coiled coil

Matrilin-2 is a member of von Willebrand factor A containing extracellular matrix proteins in which the cDNA-derived sequence shows similar domain organization to cartilage matrix protein/matrilin-1, but information on the protein structure is limited. Here we studied the oligomerization potential of a synthetic peptide NH2-ENLILFQNVANEEVRKLTQRLEEMTQRMEALENRLKYR-COOH corresponding to the C-terminal sequence of mouse matrilin-2. The central portion of this sequence shows a periodicity of hydrophobic residues occupying positions a and d of a heptad pattern (abcdefg)n, which is characteristic for alpha-helical coiled-coil proteins. Circular dichroism spectroscopy revealed a high alpha-helical content, and the shape of the spectra is indicative for a coiled-coil conformation. Chemical cross-linking and size exclusion chromatography suggest a homotrimeric configuration. Thermal denaturation in benign buffer shows a single cooperative transition with DeltaH0 = -375 kJ/mol. Melting temperatures Tm varied from 38 to 51 degreesC within a concentration range of 10 to 85 microM, which is about 35 degreesC lower than determined for a peptide corresponding to the C-terminal domain of matrilin-1. The data suggest that despite the low sequence identity within this region, matrilin-2 will form a homotrimer as matrilin-1 does.

The C-type lectin superfamily in the immune system

Protein-carbohydrate interactions serve multiple functions in the immune system. Many animal lectins (sugar-binding proteins) mediate both pathogen recognition and cell-cell interactions using structurally related Ca(2+)-dependent carbohydrate-recognition domains (C-type CRDs). Pathogen recognition by soluble collections such as serum mannose-binding protein and pulmonary surfactant proteins, and also the macrophage cell-surface mannose receptor, is effected by binding of terminal monosaccharide residues characteristic of bacterial and fungal cell surfaces. The broad selectivity of the monosaccharide-binding site and the geometrical arrangement of multiple CRDs in the intact lectins explains the ability of the proteins to mediate discrimination between self and non-self. In contrast, the much narrower binding specificity of selectin cell adhesion molecules results from an extended binding site within a single CRD. Other proteins, particularly receptors on the surface of natural killer cells, contain C-type lectin-like domains (CTLDs) that are evolutionarily divergent from the C-type lectins and which would be predicted to function through different mechanisms.

X-ray crystal structure of the human galectin-3 carbohydrate recognition domain at 2.1-A resolution

Galectins are a family of lectins which share similar carbohydrate recognition domains (CRDs) and affinity for small beta-galactosides, but which show significant differences in binding specificity for more complex glycoconjugates. We report here the x-ray crystal structure of the human galectin-3 CRD, in complex with lactose and N-acetyllactosamine, at 2.1-A resolution. This structure represents the first example of a CRD determined from a galectin which does not show the canonical 2-fold symmetric dimer organization. Comparison with the published structures of galectins-1 and -2 provides an explanation for the differences in carbohydrate-binding specificity shown by galectin-3, and for the fact that it fails to form dimers by analogous CRD-CRD interactions.

Analysis and design of three-stranded coiled coils and three-helix bundles

Three-stranded coiled coils and three-helix bundles are increasingly being identified in proteins. Design and engineering on the scaffolds of these motifs is a potential route towards combating associated viral infections as well as introducing novel functional sites.

Evolution of vertebrate fibrin formation and the process of its dissolution

The thrombin-catalysed conversion of fibrinogen into a fibrin gel is common to all extant vertebrates. Because fibrin formation is both temporary and risky, an effective scheme for fibrinolysis evolved concomitantly. In this regard, the fibrinogen molecule is well adapted both for network polymerization and for subsequent dismantling. The question is, has it always been so? It has long been known that the three non-identical chains that compose vertebrate fibrinogen are descended from a common ancestor, and the original molecule must have been either a homotrimer or a dimer thereof. Three-dimensional studies on core fragments of fibrinogen are revealing new insights about both fibrin formation and its destruction. These studies are also showing exactly what structural changes have accompanied changes in function for the various domains. Chief among these is the reversal of direction for the alpha chain after replacement of its C-terminal domain.

Mechanism of ligand binding to E- and P-selectin analyzed using selectin/mannose-binding protein chimeras

The mechanism of oligosaccharide binding to the selectin cell adhesion molecules has been analyzed by transferring regions of the carbohydrate-recognition domains of E- and P-selectin into corresponding sites in the homologous rat serum mannose-binding protein. Insertion of two basic regions and an adjacent glutamic acid residue leads to efficient binding of HL-60 cells and sialyl-Lewisx-conjugated serum albumin. Substitution of glycine for a histidine residue known to stabilize mannose in the binding site of wild type mannose-binding protein results in dramatic loss of affinity for mannose without decreasing binding to sialyl-Lewisx. The accumulated effect of these changes is to alter the ligand binding selectivity of the domain so that it resembles E- or P-selectin more closely than it resembles the parental mannose-binding domain. Affinity labeling using sialyl-Lewisx in which the sialic acid has been mildly oxidized has been used to verify this switch in specificity and to show that the sialic acid-containing portion of the ligand interacts near the sequence Lys-Lys-Lys corresponding to residues 111-113 of E-selectin. The binding of sialyl-Lewisx-serum albumin is inhibited dramatically at physiological and higher salt concentrations, consistent with a significant electrostatic component to the binding interaction. The binding characteristics of these gain-of-function chimeras suggest that they contain many of the selectin residues responsible for selective ligand binding.

The crystal structure of a complement-1q family protein suggests an evolutionary link to tumor necrosis factor

ACRP30--adipocyte complement-related protein of 30 kDa or AdipoQ--is an abundant serum protein, secreted exclusively from fat cells, which is implicated in energy homeostasis and obesity [1,2]. ACRP30 is a close homologue of the complement protein C1q, which is involved in the recognition of microbial surfaces [3-5] and antibody-antigen complexes [6,7] in the classical pathway of complement. We have determined the crystal structure of a homotrimeric fragment from ACRP30 at 2.1 A resolution. The structure reveals an unexpected homology to the tumor necrosis factor (TNF) family. Identical folding topologies, key residue conservations, and similarity of trimer interfaces and intron positions firmly establish an evolutionary link between the TNF and C1q families. We suggest that TNFs--which control many aspects of inflammation, adaptive immunity, apoptosis and energy homeostasis--arose by divergence from a primordial recognition molecule of the innate immune system. The evolutionary connection between C1q-like proteins and TNFs illuminates the shared functions of these two important groups of proteins.

Activation of Complement by Mannose-Binding Lectin on Isogenic Mutants of Neisseria meningitidis Serogroup B

Mannose-binding lectin (MBL) is a serum protein that has been demonstrated to activate the classical complement pathway and to function directly as an opsonin. Although MBL deficiency is associated with a common opsonic defect and a predisposition to infection, the role of the protein in bacterial infection remains unclear. We have investigated MBL binding to Neisseria meningitidis serogroup B1940 and three isogenic mutants, and the subsequent activation of the two major isoforms of C4 (C4A and C4B) by an associated serine protease, MASP. The mutants lacked expression of the capsular polysaccharide (siaD−), the lipo-oligosaccharide (LOS) outer core that prevented LOS sialylation (cpsD−), or both capsule and LOS outer core (cps−). Using flow cytometry, it was possible to detect strong MBL binding to the cps− and cpsD− mutants over a wide range of concentrations. In contrast, minimal or no MBL binding was detected on the parent organism, with binding to siaD− only at higher MBL concentrations. C4 was activated and bound by mutants that had previously bound MBL/MASP, but there was no significant difference in the amounts of C4A and C4B bound. When sialic acid residues were removed from the parent organism by neuraminidase treatment, the binding of both MBL and C4 increased significantly. Our results suggest that MBL may bind to and activate complement on these encapsulated organisms, and the major determinants of these effects are the LOS structure and sialylation.

Lectin-carbohydrate interactions: different folds, common recognition principles

Lectins can be found in many organisms and are involved in a multitude of cellular processes that depend on specific recognition of complex carbohydrates. The stereochemical principles underlying the recognition process have been the subject of extensive biochemical and structural studies. When examined from the viewpoint of the bound sugar, the structural information accumulated so far on lectins and other proteins that are specific to galactose and glucose (or mannose), provides suggestive evidence for distinct ligand-dependent distribution of hydrogen-bond partners in the combining site.

The Cys6 intermolecular disulfide bond and the collagen-like region of rat SP-A play critical roles in interactions with alveolar type II cells and surfactant lipids

Rat pulmonary surfactant protein A is an oligomer of 18 polypeptide chains which are associated by triple helix formation in the collagen-like domain and interchain disulfide bridges at the NH2 terminus. The roles of the intermolecular bond at Cys6 and the collagen-like domain (Gly8-Pro80) in the interactions of SP-A with phospholipids and alveolar type II cells were investigated using mutant forms of the protein. Wild type SP-A (SP-Ahyp), SP-A with the substitution Cys6 --> Ser to prevent disulfide formation (SP-Ahyp, C6S), and SP-A with the collagen-domain deleted (SP-ADeltaG8-P80) were synthesized in insect cells using recombinant baculoviruses. The SP-As were glycosylated and secreted from the invertebrate cells and the binding affinities of the wild type and mutant proteins for the mannose-Sepharose matrix used for purification were nearly identical. The SP-Ahyp and SP-ADeltaG8-P80 were at least nonameric in solution based on gel exclusion chromatography, and demonstrated extensive sulfhydryl-dependent oligomerization under nonreducing conditions. The SP-Ahyp,C6S was also oligomeric in solution and formed disulfide-dependent dimers, indicating the presence of at least one additional interchain disulfide bond. The SPADeltaG8-P80 but not the SP-Ahyp,C6S aggregated lipid vesicles at 20 degrees C and augmented the surface tension lowering effect of extracts of natural surfactant. The SP-ADeltaG8-P80 competed poorly with native SP-A for receptor occupancy on isolated alveolar type II cells and was a potent but nonspecific (concanavalin A-like) inhibitor of surfactant secretion. In contrast, the SP-Ahyp,C6S partially competed for receptor occupancy and weakly inhibited surfactant secretion in a specific manner. Neither the SP-ADeltaG8-P80 nor the SP-Ahyp,C6S supported the association of phospholipid liposomes with type II cells. We conclude that: 1) the Cys6 interchain disulfide bond of SP-A is required for aggregation of liposomes and for potent inhibition of surfactant secretion. 2) The collagen-like region is required for competition with 125I-SP-A for receptor occupancy and specific inhibition of surfactant secretion in the presence of competing sugars. 3) Both the NH2-terminal disulfide and the collagen-like region are required to enhance the association of phospholipid vesicles with type II cells.

Cell-surface carbohydrate recognition by animal and viral lectins

Many animal and viral lectins are specific for monosaccharides found in particular glycosidic linkages, or for larger oligosaccharide structures. Recent crystal structures of complexes between these proteins and receptor fragments have provided insights into the recognition of linkage isomers and oligosaccharide conformation.

Crystal structure of tetranectin, a trimeric plasminogen-binding protein with an alpha-helical coiled coil

Tetranectin is a plasminogen kringle 4-binding protein. The crystal structure has been determined at 2.8 A resolution using molecular replacement. Human tetranectin is a homotrimer forming a triple alpha-helical coiled coil. Each monomer consists of a carbohydrate recognition domain (CRD) connected to a long alpha-helix. Tetranectin has been classified in a distinct group of the C-type lectin superfamily but has structural similarity to the proteins in the group of collectins. Tetranectin has three intramolecular disulfide bridges. Two of these are conserved in the C-type lectin superfamily, whereas the third is present only in long-form CRDs. Tetranectin represents the first structure of a long-form CRD with intact calcium-binding sites. In tetranectin, the third disulfide bridge tethers the CRD to the long helix in the coiled coil. The trimerization of tetranectin as well as the fixation of the CRDs relative to the helices in the coiled coil indicate a demand for high specificity in the recognition and binding of ligands.

Asymmetry adjacent to the collagen-like domain in rat liver mannose-binding protein

Rat liver mannose-binding protein (MBP-C) is the smallest known member of the collectin family of animal lectins, many of which are involved in defence against microbial pathogens. It consists of an N-terminal collagen-like domain linked to C-terminal carbohydrate-recognition domains. MBP-C, overproduced in Chinese-hamster ovary cells, is post-translationally modified and processed in a manner similar to the native lectin. Analytical ultracentrifugation experiments indicate that MBP-C is trimeric, with a weight-averaged molecular mass of approx. 77 kDa. The rate of sedimentation of MBP-C and its mobility on gel filtration suggest a highly elongated molecule. Anomalous behaviour on gel filtration due to this extended conformation may explain previous suggestions that MBP-C forms a higher oligomer. The polypeptide chains of the MBP-C trimer are linked by disulphide bonds between two cysteine residues at the N-terminal junction of the collagen-like domain. Analysis of an N-terminal tryptic fragment reveals that the disulphide bonding in MBP-C is heterogeneous and asymmetrical. These results indicate that assembly of MBP-C oligomers probably proceeds in a C- to N-terminal direction: trimerization at the C-terminus is followed by assembly of the collagenous domain and finally formation of N-terminal disulphide bonds. The relatively simple organization of MBP-C provides a template for understanding larger, more complex collectins.