Crystal structure of complement protein C8γ in complex with a peptide containing the C8γ binding site on C8α: Implications for C8γ ligand binding

Gamma subunit of complement component 8 is a neuroinflammation inhibitor

The complement system is part of the innate immune system that comprises several small proteins activated by sequential cleavages. The majority of these complement components, such as components 3a (C3a) and C5a, are chemotactic and pro-inflammatory. However, in this study, we revealed an inhibitory role of complement component 8 gamma (C8G) in neuroinflammation. In patients with Alzheimer's disease, who exhibit strong neuroinflammation, we found higher C8G levels in brain tissue, CSF, and plasma. Our novel findings also showed that the expression level of C8G increases in the inflamed mouse brain, and that C8G is mainly localized to brain astrocytes. Experiments using recombinant C8G protein and shRNA-mediated knockdown showed that C8G inhibits glial hyperactivation, neuroinflammation, and cognitive decline in acute and chronic animal models of Alzheimer's disease. Additionally, we identified sphingosine-1-phosphate receptor 2 (S1PR2) as a novel interaction protein of C8G and demonstrated that astrocyte-derived C8G interacts with S1PR2 to antagonize the pro-inflammatory action of S1P in microglia. Taken together, our results reveal the previously unrecognized role of C8G as a neuroinflammation inhibitor. Our findings pave the way towards therapeutic containment of neuroinflammation in Alzheimer's disease and related neurological diseases.

Identification and expression of complement component C8α, C8β and C8γ gene in half-smooth tongue sole (Cynoglossus semilaevis) and C8α recombinant protein antibacterial activity analysis

Complement component C8, which mediates membrane attack complex formation and bacterial lysis, plays important roles in the complement system. The cDNA sequences of the C8α, C8β and C8γ genes were cloned from half-smooth tongue sole (Cynoglossus semilaevis). Full-length cDNA of CsC8α (C8α of C. semilaevis), CsC8β and CsC8γ was 1990, 2219 and 886 bp, respectively, which contained open reading frames of 1797, 1749 and 666 bp, encoding 598, 582 and 221 amino acids, respectively. The deduced proteins of CsC8α, CsC8β and CsC8γ showed the closest amino acid similarity to C8α (73%) of Siniperca chuatsi, C8β (76%) of Oryzias latipes and C8γ (72%) of Takifugu rubripes, respectively. The highest expression level of CsC8α, CsC8β and CsC8γ among the 13 normal tissues was observed in liver tissue, followed by much lower levels in other tissues. After infection with Vibrio anguillarum, CsC8α, CsC8β and CsC8γ were significantly up-regulated in all of the detected tissues, including the intestine, liver, gill, head kidney, blood and spleen. Then, a recombinant expression plasmid was constructed, and the recombinant CsC8α protein was expressed in GS115 pichia pastoris yeast. Furthermore, to investigate the biological functions of recombinant CsC8α, an antibacterial assay was performed, and the results showed that recombinant CsC8α obviously inhibited growth of V. anguillarum, Edwardsiella tarda and Vibrio parahaemolyticus. Taken together, these results suggest that CsC8α, CsC8β and CsC8γ may play important roles in the immune defense of C. semilaevis.

The making of a macromolecular machine: assembly of the membrane attack complex

The complement terminal pathway clears pathogens by generating cytotoxic membrane attack complex (MAC) pores on target cells. For more than 40 years, biochemical and cellular assays have been used to characterize the lytic nature of the MAC and to define its protein composition. Although models for pore formation have been inferred from structures of bacterial cytolysins, it was only recently that we were able to visualize how complement components come together during MAC assembly. This review highlights structural analyses of terminal pathway complexes to explore molecular mechanisms underlying MAC formation.

Structural biology of the membrane attack complex

The complement system is an intricate network of serum proteins that mediates humoral innate immunity through an amplification cascade that ultimately leads to recruitment of inflammatory cells or opsonisation or killing of pathogens. One effector arm of this network is the terminal pathway of complement, which leads to the formation of the membrane attack complex (MAC) composed of complement components C5b, C6, C7, C8 and C9. Upon formation of C5 convertases via the classical or alternative pathways of complement activation, C5b is generated from C5 by proteolytic cleavage, nucleating a series of association and polymerisation reactions of the MAC-constituting complement components that culminate in pore formation of pathogenic membranes. Recent structures of MAC components and homologous proteins significantly increased our understanding of oligomerisation, membrane association and integration, shedding light onto the molecular mechanism of this important branch of the innate immune system.

Molecular cloning of the alpha subunit of complement component C8 (CpC8α) of whitespotted bamboo shark (Chiloscyllium plagiosum)

Complement-mediated cytolysis is the important effect of immune response, which results from the assembly of terminal complement components (C5b-9). Among them, α subunit of C8 (C8α) is the first protein that traverses the lipid bilayer, and then initiates the recruitment of C9 molecules to form pore on target membranes. In this article, a full-length cDNA of C8α (CpC8α) is identified from the whitespotted bamboo shark (Chiloscyllium plagiosum) by RACE. The CpC8α cDNA is 2183 bp in length, encoding a protein of 591 amino acids. The deduced CpC8α exhibits 89%, 49% and 44% identity with nurse shark, frog and human orthologs, respectively. Sequence alignment indicates that the C8α is well conserved during the evolution process from sharks to mammals, with the same modular architecture as well as the identical cysteine composition in the mature protein. Phylogenetic analysis places CpC8α and nurse shark C8α in cartilaginous fish clade, in parallel with the teleost taxa, to form the C8α cluster with higher vertebrates. Hydrophobicity analysis also indicates a similar hydrophobicity of CpC8α to mammals. Finally, expression analysis revealed CpC8α transcripts were constitutively highly expressed in shark liver, with much less expression in other tissues. The well conserved structure and properties suggests an analogous function of CpC8α to mammalian C8α, though it remains to be confirmed by further study.

Putting the structure into complement

In a field where structure has finally begun to have a real impact, a series of new structures over the last two years have further extended our understanding of some of the critical regulatory events of the complement system. Notably, information has begun to flow from larger assemblies of components which allow insight into the often transient assemblies critical to complement regulation at the cell surface. This review will summarise the key structures determined since the last International Complement Workshop and the insights these have given us, before highlighting some questions that still require molecular frameworks to drive understanding.

The crystal structure of human α1-microglobulin reveals a potential haem-binding site

We describe the 2.3 Å (1 Å=0.1 nm) X-ray structure of α1m (α1-microglobulin), an abundant protein in human blood plasma, which reveals the β-barrel fold typical for lipocalins with a deep pocket lined by four loops at its open rim. Loop #1 harbours the residue Cys34 which is responsible for covalent cross-linking with plasma IgA. A single disulfide bond between Cys72 and Cys169 connects the C-terminal segment to the β-barrel, as in many other lipocalins. The exposed imidazole side chains of His122 and His123 in loop #4 give rise to a double Ni2+-binding site together with a crystallographic neighbour. The closest structural relatives of α1m are the complement protein component C8γ, the L-prostaglandin D synthase and lipocalin 15, three other structurally characterized members of the lipocalin family in humans that have only distant sequence similarity. In contrast with these, α1m is initially expressed as a bifunctional fusion protein with the protease inhibitor bikunin. Neither the electron density nor ESI–MS (electrospray ionization MS) provide evidence for a chromophore bound to the recombinant α1m, also known as ‘yellow/brown lipocalin’. However, the three side chains of Lys92, Lys118 and Lys130 that were reported to be involved in covalent chromophore binding appear to be freely accessible to ligands accommodated in the hydrophobic pocket. A structural feature similar to the well-known Cys–Pro haem-binding motif indicates the presence of a haem-binding site within the loop region of α1m, which explains previous biochemical findings and supports a physiological role in haem scavenging, as well as redox-mediated detoxification.

Structure of human complement C8, a precursor to membrane attack

Complement component C8 plays a pivotal role in the formation of the membrane attack complex (MAC), an important antibacterial immune effector. C8 initiates membrane penetration and coordinates MAC pore formation. High-resolution structures of C8 subunits have provided some insight into the function of the C8 heterotrimer; however, there is no structural information describing how the intersubunit organization facilitates MAC assembly. We have determined the structure of C8 by electron microscopy and fitted the C8α-MACPF (membrane attack complex/perforin)-C8γ co-crystal structure and a homology model for C8β-MACPF into the density. Here, we demonstrate that both the C8γ protrusion and the C8α-MACPF region that inserts into the membrane upon activation are accessible.

Crystal structure of the dog lipocalin allergen Can f 2: implications for cross-reactivity to the cat allergen Fel d 4

The dog lipocalin allergen Can f 2 is an important cause of allergic sensitization in humans worldwide. Here, the first crystal structure of recombinant rCan f 2 at 1.45 A resolution displays a classical lipocalin fold with a conserved Gly-Xaa-Trp motif, in which Trp19 stabilizes the overall topology of the monomeric rCan f 2. Phe38 and Tyr84 localized on the L1 and L5 loops, respectively, control access to the highly hydrophobic calyx. Although the rCan f 2 calyx is nearly identical with the aero-allergens MUP1, Equ c 1 and A2U from mouse, horse and rat, respectively, no IgE cross-reactivity was found using sera from five mono-sensitized subjects. However, clear IgE cross-reactivity was demonstrated between Can f 2 and the cat allergen Fel d 4, although they share less than 22% sequence identity. This suggests a role for these allergens in co-sensitization between cat- and dog-allergic patients.

Grading the commercial optical biosensor literature-Class of 2008: 'The Mighty Binders'

Optical biosensor technology continues to be the method of choice for label-free, real-time interaction analysis. But when it comes to improving the quality of the biosensor literature, education should be fundamental. Of the 1413 articles published in 2008, less than 30% would pass the requirements for high-school chemistry. To teach by example, we spotlight 10 papers that illustrate how to implement the technology properly. Then we grade every paper published in 2008 on a scale from A to F and outline what features make a biosensor article fabulous, middling or abysmal. To help improve the quality of published data, we focus on a few experimental, analysis and presentation mistakes that are alarmingly common. With the literature as a guide, we want to ensure that no user is left behind.

Serendipitous fatty acid binding reveals the structural determinants for ligand recognition in apolipoprotein M

Apolipoprotein M (ApoM) is a 25-kDa HDL-associated apolipoprotein and a member of the lipocalin family of proteins. Mature apoM retains its signal peptide, which serves as a lipid anchor attaching apoM to the lipoproteins, thereby keeping it in the circulation. Studies in mice have suggested apoM to be antiatherogenic, but its physiological function is yet unknown. We have now determined the 1.95 A resolution crystal structure of recombinant human apoM expressed in Escherichia coli and made the unexpected discovery that apoM, although refolded from inclusion bodies, was in complex with fatty acids containing 14, 16 or 18 carbon atoms. ApoM displays the typical lipocalin fold characterised by an eight-stranded antiparallel beta-barrel that encloses an internal ligand-binding pocket. The crystal structures of two different complexes provide a detailed picture of the ligand-binding determinants of apoM. Additional fatty acid- and lipid-binding studies with apoM and the mutants apoM(W47F) and apoM(W100F) showed that sphingosine-1-phosphate is able to displace the bound fatty acids and efficiently quenched the intrinsic fluorescence with an IC(50) of 0.90 muM. Whereas the fatty acids bound in the crystal structure could be a mere consequence of recombinant protein production, the observed binding of sphingosine-1-phosphate might provide a key to a better understanding of the physiological function of apoM.

The 1.8-A crystal structure of alpha1-acid glycoprotein (Orosomucoid) solved by UV RIP reveals the broad drug-binding activity of this human plasma lipocalin

Alpha(1)-acid glycoprotein (AGP) is an important drug-binding protein in human plasma and, as an acute-phase protein, it has a strong influence on pharmacokinetics and pharmacodynamics of many pharmaceuticals. We report the crystal structure of the recombinant unglycosylated human AGP at 1.8 A resolution, which was solved using the new method of UV-radiation-damage-induced phasing (UV RIP). AGP reveals a typical lipocalin fold comprising an eight-stranded beta-barrel. Of the four loops that form the entrance to the ligand-binding site, loop 1, which connects beta-strands A and B, is among the longest observed so far and exhibits two full turns of an alpha-helix. Furthermore, it carries one of the five N-linked glycosylation sites, while a second one occurs underneath the tip of loop 2. The branched, partly hydrophobic, and partly acidic cavity, together with the presumably flexible loop 1 and the two sugar side chains at its entrance, explains the diverse ligand spectrum of AGP, which is known to vary with changes in glycosylation pattern.

Crystal structure of the MACPF domain of human complement protein C8 alpha in complex with the C8 gamma subunit

Human C8 is one of five complement components (C5b, C6, C7, C8, and C9) that assemble on bacterial membranes to form a porelike structure referred to as the "membrane attack complex" (MAC). C8 contains three genetically distinct subunits (C8 alpha, C8 beta, C8 gamma) arranged as a disulfide-linked C8 alpha-gamma dimer that is noncovalently associated with C8 beta. C6, C7 C8 alpha, C8 beta, and C9 are homologous. All contain N- and C-terminal modules and an intervening 40-kDa segment referred to as the membrane attack complex/perforin (MACPF) domain. The C8 gamma subunit is unrelated and belongs to the lipocalin family of proteins that display a beta-barrel fold and generally bind small, hydrophobic ligands. Several hundred proteins with MACPF domains have been identified based on sequence similarity; however, the structure and function of most are unknown. Crystal structures of the secreted bacterial protein Plu-MACPF and the human C8 alpha MACPF domain were recently reported and both display a fold similar to those of the bacterial pore-forming cholesterol-dependent cytolysins (CDCs). In the present study, we determined the crystal structure of the human C8 alpha MACPF domain disulfide-linked to C8 gamma (alphaMACPF-gamma) at 2.15 A resolution. The alphaMACPF portion has the predicted CDC-like fold and shows two regions of interaction with C8 gamma. One is in a previously characterized 19-residue insertion (indel) in C8 alpha and fills the entrance to the putative C8 gamma ligand-binding site. The second is a hydrophobic pocket that makes contact with residues on the side of the C8 gamma beta-barrel. The latter interaction induces conformational changes in alphaMACPF that are likely important for C8 function. Also observed is structural conservation of the MACPF signature motif Y/W-G-T/S-H-F/Y-X(6)-G-G in alphaMACPF and Plu-MACPF, and conservation of several key glycine residues known to be important for refolding and pore formation by CDCs.