Sequence-specific 1H-NMR assignments and folding topology of human CD59

Organization, evolution and functions of the human and mouse Ly6/uPAR family genes

Members of the lymphocyte antigen-6 (Ly6)/urokinase-type plasminogen activator receptor (uPAR) superfamily of proteins are cysteine-rich proteins characterized by a distinct disulfide bridge pattern that creates the three-finger Ly6/uPAR (LU) domain. Although the Ly6/uPAR family proteins share a common structure, their expression patterns and functions vary. To date, 35 human and 61 mouse Ly6/uPAR family members have been identified. Based on their subcellular localization, these proteins are further classified as GPI-anchored on the cell membrane, or secreted. The genes encoding Ly6/uPAR family proteins are conserved across different species and are clustered in syntenic regions on human chromosomes 8, 19, 6 and 11, and mouse Chromosomes 15, 7, 17, and 9, respectively. Here, we review the human and mouse Ly6/uPAR family gene and protein structure and genomic organization, expression, functions, and evolution, and introduce new names for novel family members.

Branch-specific sialylation of IgG-Fc glycans by ST6Gal-I

Sialylated forms of the Fc fragment of immunoglobulin G, produced by the human alpha2-6 sialyltransferase ST6Gal-I, were identified as potent anti-inflammatory mediators in a mouse model of rheumatoid arthritis and are potentially the active components in intravenous IgG anti-inflammatory therapies. The activities and specificities of hST6Gal-I are, however, poorly characterized. Here MS and NMR methodology demonstrates glycan modification occurs in a branch-specific manner with the alpha1-3Man branch of the complex, biantennary Fc glycan preferentially sialylated. Interestingly, this substrate preference is preserved when using a released glycan, suggesting that the apparent occlusion of glycan termini in Fc crystal structures does not dominate specificity.

Cloning of a CD59-like gene in rainbow trout

CD59, the major inhibitor of the complement membrane attack complex, is an 18-20 kDa glycoprotein, linked to the membrane via a glycosylphosphatidylinositol (GPI)-anchor. It restricts binding of C9 to the C5b-8 complex, preventing the formation of the complement membrane attack complex C5b-9. In this study we report the cloning of a second CD59-like gene in the rainbow trout, Oncorhynchus mykiss (referred to as CD59-2 and the previously deposited trout CD59-like gene as CD59-1). Trout CD59-2 is 56% identical to CD59-1 at the amino acid level. Both of trout CD59s show the highest identity score (54%) with putative CD59-like molecules from other teleost, and the overall identity with their mammalian orthologs is less than 30%. Trout CD59s are expressed in brain, heart, intestine, kidney, liver and spleen. Particularly, CD59-2 is abundant in trout brain, while CD59-1 seems to be absent in the trout spleen. Moreover, both of trout CD59 genes seems to be present as a single copy in trout genome.

Insights into the Human CD59 Complement Binding Interface Toward Engineering New Therapeutics

CD59 is a 77-amino acid membrane glycoprotein that plays an important role in regulating the terminal pathway of complement by inhibiting formation of the cytolytic membrane attack complex (MAC or C5b-9). The MAC is formed by the self assembly of C5b, C6, C7, C8, and multiple C9 molecules, with CD59 functioning by binding C5b-8 and C5b-9 in the assembling complex. We performed a scanning alanine mutagenesis screen of residues 16-57, a region previously identified to contain the C8/C9 binding interface. We have also created an improved NMR model from previously published data for structural understanding of CD59. Based on the scanning mutagenesis data, refined models, and additional site-specific mutations, we identified a binding interface that is much broader than previously thought. In addition to identifying substitutions that decreased CD59 activity, a surprising number of substitutions significantly enhanced CD59 activity. Because CD59 has significant therapeutic potential for the treatment of various inflammatory conditions, we investigated further the ability to enhance CD59 activity by additional mutagenesis studies. Based on the enhanced activity of membrane-bound mutant CD59 molecules, clinically relevant soluble mutant CD59-based proteins were prepared and shown to have up to a 3-fold increase in complement inhibitory activity.

Identity of the residues responsible for the species-restricted complement inhibitory function of human CD59

The membrane-anchored glycoprotein CD59 inhibits assembly of the C5b-9 membrane attack complex (MAC) of human complement. This inhibitory function of CD59 is markedly selective for MAC assembled from human complement components C8 and C9, and CD59 shows little inhibitory function toward MAC assembled from rabbit and many other non-primate species. We have used this species selectivity of CD59 to identify the residues regulating its complement inhibitory function: cDNA of rabbit CD59 was cloned and used to express human/rabbit CD59 chimeras in murine SV-T2 cells. Plasma membrane expression of each CD59 chimera was quantified by use of a 5'-TAG peptide epitope, and each construct was tested for its ability to inhibit assembly of functional MAC from human versus rabbit C8 and C9. These experiments revealed that the species selectivity of CD59 is entirely determined by sequence contained between residues 42 and 58 of the human CD59 polypeptide, whereas chimeric substitution outside this peptide segment has little effect on the MAC inhibitory function of CD59. Substitution of human CD59 residues 42-58 into rabbit CD59 resulted in a molecule that was functionally indistinguishable from native human CD59, whereas the complementary construct (corresponding residues of rabbit CD59 substituted into human CD59) was functionally indistinguishable from rabbit CD59. Based on the solved solution structure of CD59, these data suggest that selectivity for human C8 and C9 resides in a cluster of closely spaced side chains on the surface of CD59 contributed by His44, Asn48, Asp49, Thr51, Thr52, Arg55, and Glu58 of the polypeptide.

Molecular evolution of snake toxins: is the functional diversity of snake toxins associated with a mechanism of accelerated evolution?

Recent studies revealed that animal toxins with unrelated biological functions often possess a similar architecture. To tentatively understand the evolutionary mechanisms that may govern this principle of functional prodigality associated with a structural economy, two complementary approaches were considered. One of them consisted of investigating the rates of mutations that occur in cDNAs and/or genes that encode a variety of toxins with the same fold. This approach was largely adopted with phospholipases A2 from Viperidae and to a lesser extent with three-fingered toxins from Elapidae and Hydrophiidae. Another approach consisted of investigating how a given fold can accommodate distinct functional topographies. Thus, a number of topologies by which three-fingered toxins exert distinct functions were investigated either by making chemical modifications and/or mutational analyses or by studying the three-dimensional structure of toxin-target complexes. This review shows that, although the two approaches are different, they commonly indicate that most if not all the surface of a snake toxin fold undergoes natural engineering, which may be associated with an accelerated rate of evolution. The biochemical process by which this phenomenon occurs remains unknown.

NMR investigations of the role of the sugar moiety in glycosylated recombinant human granulocyte-colony-stimulating factor

Human granulocyte-colony-stimulating factor (G-CSF) is a hematopoietic growth factor that plays a major role in the stimulation of the proliferation and maturation of granulocyte neutrophil cells. With the recent increased understanding of its biological properties in vivo together with available preparations of recombinant human G-CSF, this growth factor has become an essential agent for clinical applications. The presence of an O-linked carbohydrate chain at position 133 greatly improves the physical stability of the protein. To clarify the molecular basis for the stabilisation effect of saccharide moieties on human G-CSF the whole glycoprotein expressed in CHO cells has been investigated by means of two 1H-NMR-spectroscopy and two 1H-detected-heteronuclear 1H-13C experiments at natural abundance, and compared with the non-glycosylated form. The present NMR study reports assignments of 1H and 13C resonances of the bound saccharidic chain NeuNAc(alpha2-3)Gal(beta1-3)[NeuNAc(alpha2-6)]GalNAc, where NeuNAc represents N-acetylneuraminic acid, and demonstrates the alpha-anomeric configuration of the N-acetylgalactosamine-threonine linkage. It also provides results suggesting that the carbohydrate moiety reduces the local mobility around the glycosylation site, which could be responsible for the stabilising effect observed on the glycoprotein.

Binding of human and rat CD59 to the terminal complement complexes

CD59-antigen (protectin) is a widely distributed glycolipid-anchored inhibitor of complement lysis. CD59 interacts with complement components C8 and C9 during assembly of the membrane attack complex (MAC). To evaluate species specificity of these interactions we have in the present study examined cross-species binding of isolated human and rat CD59 to the terminal complement components C8 and C9. By using primarily soluble CD59 isolated from urine (CD59U) potentially non-specific binding interactions of the phospholipid portion of the membrane forms of CD59 could be avoided. Sucrose density gradient ultracentrifugation analysis showed that human CD59U bound to both human and rat C8 in the SC5b-8 complexes. Similar binding occurred when rat CD59U was used. The degree of binding did not significantly differ between the heterologous and homologous CD59-C8 combinations. C9 from both species inhibited the binding of CD59 to soluble SC5b-8. In ligand blotting analysis human and rat CD59U bound to human and rat C8 alpha gamma-subunit and C9. Binding of human and rat CD59U was stronger to human than rat C9. In plate binding assays the erythrocyte form of CD59 (CD59E) bound to both human and rat C8. Binding of CD59E to heterologous C9 was considerably weaker than to homologous C9. Our results imply that the reciprocal binding sites between C8 and CD59 and to a lesser degree between CD59 and C9 are conserved between human and rat. Interactions of CD59 with the terminal C components are thus species selective but not 'homologously restricted'.

Resistance of ovarian teratocarcinoma cell spheroids to complement-mediated lysis

We have shown previously that it is possible to target complement-mediated killing against cultured ovarian tumour cells in vitro. As malignant ovarian cells usually grow in solid nodules in vivo, we have in the present study examined the effectiveness of complement killing against ovarian teratocarcinoma cells (PA-1) growing in three-dimensional tumour microspheroids (TMSs). Our study shows that PA-1 cells growing in TMSs are less susceptible to complement-mediated killing than cells growing in monolayer cultures, even after neutralization of protectin (CD59), the main inhibitor of complement lysis. Cells in suspension and cells growing in TMSs showed a similar expression of membrane co-factor protein (MCP, CD46) and CD59. Decay-accelerating factor (DAF, CD55) was not detected on the surface of cells in suspension, but appeared focally on the outermost cell layers of the TMSs. Complement-activating antibodies bound to all PA-1 cells in suspension but only to the most peripherally located cells in TMSs, even though the target antigens were similarly expressed in the two systems. Antibody-induced complement activation on PA-1 cells in suspension led to C3 and C5b-9 deposition on most cells, while C3 and C5b-9 were only found on the outermost layers of the TMSs. The increased complement resistance of tumour cells growing in three-dimensional spheroids is partly because of an insufficient penetration of antibodies and complement into the TMSs. TMSs are a useful model for the development of more efficient ways to kill malignant cells in micrometastases with monoclonal antibodies and complement.

Expression of recombinant CD59 with an N-terminal peptide epitope facilitates analysis of residues contributing to its complement-inhibitory function

CD59 is a plasma membrane-anchored glycoprotein that serves to protect human cells from lysis by the C5b-9 complex of complement. The immunodominant epitopes of CD59 are known to be sensitive to disruption of native tertiary structure, complicating immunological measurement of expressed mutant constructs for structure function analysis. In order to quantify cell-surface expression of wild-type and mutant forms of this complement inhibitor, independent of CD59 antigen, an 11-residue peptide (TAG) recognized by monoclonal antibody (mAb) 9E10 was inserted before the N-terminal codon (L1) of mature CD59, in a pcDNA3 expression plasmid. SV-T2 cells were transfected with this plasmid, yielding cell lines expressing 0 to > 10(5) CD59/cell. The TAG-CD59 fusion protein was confirmed to be GPI-anchored, N-glycosylated and showed identical complement-inhibitory function to wild-type CD59, lacking the TAG peptide sequence. Using this construct, the contribution of each of four surface-localized aromatic residues (4Y, 47F, 61Y, and 62Y) to CD59's complement-inhibitory function was examined. These assays revealed normal surface expression with complete loss of complement-inhibitory function in the 4Y --> S, 47F --> G and 61Y --> S mutants. By contrast, 62Y --> S mutants retained approximately 40% of function of wild-type CD59. These studies confirmed the utility of the TAG-CD59 construct for quantifying CD59 surface expression and activity, and implicate surface aromatic residues 4Y, 47F, 61Y and 62Y as essential to maintenance of CD59's normal complement-regulatory function.

Elimination of potential sites of glycosylation fails to abrogate complement regulatory function of cell surface CD59

CD59 is a glycosylphosphatidylinositol-anchored membrane glycoprotein that serves as the principle cellular inhibitor of the C5b-9 membrane attack complex (MAC) of human complement. Approximately 50% of the total apparent mass of CD59 is attributable to glycosylation of a single Asn (Asn18). The deduced amino acid sequences of CD59 homologues identified in Old and New World primates as well as in rat reveal that the motif for N-linked glycosylation at the residue corresponding to Asn18 of human CD59 is invariably conserved, despite considerable sequence divergence elsewhere in the protein. Such conservation suggests that the post-translational modification at Asn18 has importance for either expression or normal function of CD59 at the cell surface. In this study, we specifically examined how deletion or transposition of the site of N-linked glycosylation in the CD59 polypeptide affects its MAC inhibitory function. Our data demonstrate that the inhibitory potency of CD59 is unaffected when glycosylation is transposed from Asn18 to another site in the polypeptide. Furthermore, we show that CD59 retains normal MAC regulatory function when mutated to eliminate all potential sites for N-linked glycosylation. These data suggest that the MAC inhibitory function of CD59 is entirely provided by residues exposed at the surface of the core polypeptide and that this core structure is not influenced by glycosylation at Asn18.

The structural role of sugars in glycoproteins

To date, high resolution X-ray structures of about 30 glycoproteins have been reported that provide some structural information on glycans. Four solution structures of glycoproteins have been described over the past three years. In all four of these cases, it was shown that glycosylation is stabilizing the glycoprotein structures, indicating that this may be a general glycan function.

Shedding and enrichment of the glycolipid-anchored complement lysis inhibitor protectin (CD59) into milk fat globules

Protectin (CD59) is a glycolipid-anchored inhibitor of the membrane attack complex (MAC) of human complement (C) that protects blood cells, endothelial cells and various epithelial cells from C-mediated lysis. Because of its activities protectin is a candidate molecule for use in the treatment of paroxysmal nocturnal haemoglobinuria or conditions where MAC causes tissue damage. Soluble, phospholipid-free forms of protectin have been isolated from human urine and produced in recombinant form, but they have only a relatively weak C lysis-inhibiting activity. In the present study we have looked for functionally active protectin in human breast milk. Milk is rich in fat droplets, milk fat globules (MFG), that are enveloped in a plasma membrane derived from secretory cells of the mammary gland. The membranes of MFG contain a variety of glycoproteins expressed by the mammary epithelial cells. Both immunofluorescence and immunoblotting analysis demonstrated that protectin was strongly expressed on human MFG. In sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis, MFG protectin (CD59M) appeared as distinct bands with apparent molecular weights of 19,000-23,000 MW, similar to protectin extracted from MCF7 breast carcinoma cells. CD59M in breast milk was functionally active and had a glycophospholipid anchor, as judged by its ability to incorporate into guinea-pig erythrocytes and inhibit their lysis by human complement. These results indicate that functionally active protectin becomes enriched in MFG and imply that secretion of glycophospholipid-anchored molecules, e.g. into cow milk and colostrum, could be exploited as a means of producing bioactive molecules that need to be targeted into cell membranes.

Urinary excretion of protectin (CD59), complement SC5b-9 and cytokines in membranous glomerulonephritis

Protectin (CD59) is a low molecular weight glycophosphoinositol-anchored inhibitor of the membrane attack complex of complement (MAC) that is present, for example, on the membranes of endothelial cells and on epithelial cells of glomeruli and distal tubuli. To examine for the possibility that CD59 becomes detached from cell surfaces following cell injury, this study evaluated renal excretion of CD59 in patients with idiopathic membranous glomerulonephritis (MGN; N = 21), diabetic nephropathy (DNP; N = 15) and in healthy control subjects (N = 13). CD59 in human urine was quantitated by a competitive solid-phase radioimmunoassay having approximately 13 kDa soluble urinary CD59 as a standard. Immunofluorescence microscopy demonstrated a decreased expression of CD59 in the glomeruli of MGN patients. Using a Triton X-114 phase separation method 91 to 97% of urinary CD59 was found to be in a soluble form without anchor-associated phospholipid. The mean (+/- SEM) level of urinary CD59 was 5.6 +/- 0.2 micrograms/ml in MGN patients, 3.7 +/- 0.4 micrograms/ml in healthy controls (P < 0.001) and 2.6 +/- 0.1 in DNP patients (P < 0.001). When related to urinary creatinine (UCr) the corresponding values were 11.9 +/- 5.6, 4.8 +/- 0.3 (P = 0.021) and 4.4 +/- 0.2 (P < 0.002), respectively. The amount of CD59 in urine correlated with the urinary excretion of soluble terminal complement complexes, SC5b-9 (r = 0.594, P < 0.006) in MGN patients. The excretion of CD59 also correlated with the excretion of the inflammatory mediator IL-1 beta (r = 0.671, P = 0.001) but not with TNF-alpha (r = 0.314, P = 0.178). No correlation of CD59 excretion was observed with duration of the disease level of proteinuria, serum albumin concentration or serum creatinine level. Based on these findings we speculate that the increased excretion of CD59 into urine in MGN patients is due to complement activation and inflammation induced shedding of CD59 from glomerular cells.

Determination of the active site of CD59 with synthetic peptides

CD59 inhibits the formation of membrane attack complex (MAC) of human complement by binding to C8 and C9 in the nascent membrane attack complex and inhibiting C9 binding to C8 in C5b-8 and C9 polymerization. Considering five disulfide bridges of CD59, we divided the molecule into two portions and synthesized the two peptides. One represented an amino-terminal half, P1-41, consisting of residues 1-41, while another represented a carboxyl-terminal half, P42-77, consisting of residues 42-77. P1-41 inhibited the MAC formation much more strongly than P42-77, indicating that the amino-terminal half contained the active site. We further synthesized P4-18 that consisted of residues 4-18 and P19-41 that consisted of residues 19-41. The activity of P4-18 was less than that of P19-41. Surprisingly, P19-41 showed higher activity than P1-41 and was comparable to urine CD59. Residues 19-41 were further divided into two portions: P20-25 which consisted of residues 20-25 and P27-38 which consisted of residues 27-38. Although their activities were significantly less than the activity of P19-41, P27-38 showed higher activity than P20-25. Residues 27-38 were further divided into three portions: P27-32 which consisted of residues 27-32, P30-34 which consisted of residues 30-34 and P33-38 which consisted of residues 33-38. When these peptides were assayed for the activities, all of them showed significant activities, even though they needed 10-fold more concentrations than P19-41. These data suggest that the portion made up of residues 27-38 is the active site constituting the binding site to C8 and C9.

Molecular cloning of the rat analogue of human CD59: structural comparison with human CD59 and identification of a putative active site

We have previously described the purification and partial characterization of the rat analogue of the human complement regulatory molecule CD59 [Hughes, Piddlesden, Williams, Harrison and Morgan (1992) Biochem. J. 284, 169-176]. We present here the molecular cloning and full sequence analysis of this molecule. A PCR-based approach utilizing primers designed from the amino-terminal protein sequence was used to isolate a full-length cDNA clone from a rat kidney cDNA library. This clone encoded a 92 bp 5'-flanking sequence, a 66 bp signal peptide and a 315 bp coding region containing putative glycosylation and GPI-anchor signals. The 3' untranslated flanking region was approximately 1.1 kbp long and included the poly-A tail and a CATA repeating sequence. The coding region was 58% identical with the human cDNA at the nucleotide level and 44% identical at the amino acid level. Despite this relatively low overall sequence conservation, several highly conserved stretches were apparent, particularly in the N-terminal portion of the molecule, in the cysteine-rich region immediately preceding the site of glycolipid attachment and in the C-terminal peptide removed during glycolipid attachment. An N-glycosylation site was identified at Asn-16 and a putative glycosylphosphatidylinositol anchor addition site at Asn-79, indicating that the mature processed protein was two residues longer than human CD59. Comparison of the sequences of rat and human CD59, together with consideration of the published three-dimensional structure of human CD59 and functional data, implicates specific regions of the protein in interactions with C-8 and/or C-9.

Cell surface adhesion receptors

Several new structural motifs found in cell surface adhesion receptors have been described in the past few years. Also, several two-domain structures of extracellular portions of cell surface proteins have been reported. Structural models for complexes between receptors and counter-receptors have been proposed. The first reports on carbohydrate conformation in intact glycoprotein domains have recently appeared. These new data are presented within a more general review of the field of cell adhesion receptors.

Structure-function relationships in the receptor for urokinase-type plasminogen activator. Comparison to other members of the Ly-6 family and snake venom alpha-neurotoxins

Plasminogen activation is regulated by the interaction between urokinase-type plasminogen activator (uPA) and its specific glycolipid-anchored cell surface receptor (uPAR). uPAR is composed of three homologous domains and is the only multi-domain member of the Ly-6 family of glycolipid-anchored membrane proteins. Recent evidence has highlighted similarities between the individual domains of uPAR and the large family of secreted, single domain snake venom alpha-neurotoxins, suggesting that uPAR may adopt the same gross folding pattern as these structurally well characterized proteins. Structural aspects of the binding between alpha-neurotoxins and the acetylcholine receptor may have a major influence on future studies of the interaction between uPA and uPAR.

Structure of a soluble, glycosylated form of the human complement regulatory protein CD59

BACKGROUND

CD59 is a cell-surface glycoprotein that protects host cells from complement-mediated lysis by binding to and preventing the normal functioning of the complement proteins C8 and/or C9 which form part of a membrane penetrating assembly called the membrane attack complex. CD59 has no structural similarity to other complement proteins, but is an example of a plasma protein domain type found also in murine Ly-6 proteins and the urokinase-type plasminogen activator receptor.

RESULTS

CD59 was purified from human urine, retaining the N-glycan and at least some of the non-lipid component of the glycosylphosphatidylinositol membrane anchor. The three-dimensional structure of the protein component has been determined in the presence of the carbohydrate groups using two-dimensional NMR spectroscopy. The protein structure is well defined by the NMR data (root mean square deviation from the mean structure of 0.65 A for backbone atoms and no distance constraint violations greater than 0.4 A). Structure calculations were also carried out to model the orientation of the N-acetylglucosamine residue that is directly linked to Asn18.

CONCLUSIONS

The main features of the protein structure are two antiparallel beta-sheets (a central one with three strands and another with two), a short helix that packs against the three-stranded beta-sheet, and a carboxy-terminal region that, although lacking regular secondary structure, is well defined and packs against the three-stranded beta-sheet, on the opposite face to the helix. We have used the structure, in combination with existing biochemical data, to identify residues that may be involved in C8 binding.