Complement: Terminal Pathways


A central event in the complement activation is the cleavage of C5 by C5 convertases to form C5a and C5b. Nascent C5b can join sequentially with C6 through C9, and the assembly may proceed through one of the two pathways. If occurring near a phospholipid membrane, the membrane attack complex is formed; this is a circular transmembrane pore of 100 Å. If distal from a membrane, the late acting complement proteins associate with vitronectin and clusterin‐high density lipoprotein generating soluble complexes collectively referred to as SC5b‐9. Advancements in X‐ray crystallography and electron microscopy have revealed as to how the monomers transform in conformation to become protomers in the polymers. Components C6 through C9 contain a central domain (MACPF) flanked and extended by cysteine‐rich modules that perform binding functions and hold the monomers in a state of potentiation. Upon joining with C5b, several modules displace enabling assembly. Experimental evidence has suggested that the folding of subunits within the MAC is inverted to that of the bacterial cholesterol‐dependent cytolysins.

Key Concepts:

  • Complement C6 through C9 consist of a single central domain, the MACPF, flanked by auxiliary modules related to those found in thrombospondin, the LDL receptor, the epidermal growth factor, complement control proteins and factor I.

  • In their soluble monomer state the terminal components of complement (C6 through C9) are held in a state of potentiation by their auxiliary modules, which get displaced in the process of subunit association, thereby exposing their core β‐sheets enabling protomer association in the polymer.

  • The terminal components of complement through their MACPF domains share a similar tertiary structure to the equivalent domains of the bacterial cholesterol‐dependent cytolysins (CDCs), yet little to no sequence homology is evident between the vertebrate MACPFs and the analogous bacterial domains.

  • Although similar in tertiary structure, perforin (and probably the components of the MAC) fold within their respective polymers in a manner inverted to those of the bacterial CDCs.

  • Heterogeneity characterises the soluble terminal complement complexes (SC5b‐9) as these are associated with vitronectin, clusterin and lipoprotein.

  • The soluble terminal complement complexes may function in vivo for clearance and wound healing.

Keywords: complement; MAC; MACPF; vitronectin; C5; C9; poly(C9); clusterin; SC5b‐9

Figure 1.

MAC forming components of complement: (Left) SDS‐PAGE patterns of the late acting components of complement where the samples alternate between those that were unreduced (−), and those that were reduced (+) with dithiothreitol. Component C5 consists of two chains linked by a disulfide bond, and after cleavage by C5 convertase, the activation peptide, C5a, is released from the N‐terminus of the α‐chain. The larger fragment, C5b, can initiate the assembly of the MAC. C6, C7, C8β and C9 are single‐chain proteins, whereas C8α–γ consists of two chains: C8α, a homologue of the others, is tethered by a disulfide bridge to C8γ, a member of the lipocalin family. (Right) An erythrocyte membrane patch that is a remnant of a cell lysed by complement. The circular lesions are transmembrane pores of channel size ∼100 Å.

Figure 2.

Pathways of Complement Activation. The classical, lectin and alternative pathways of complement activation all converge on the specific cleavage of component C3 by complex enzymes referred to as C3 convertases. The activation peptide, C3a (Mr ∼9000), has chemotactic and inductive effects on mesenchymal stem cells. The larger fragment C3b along with its proteolytic derivatives (iC3b and C3d) serve immune adherence functions with granulocytes, monocytes and dendritic cells. An elaboration of C3 convertase, caused by covalent coupling of C3b to C3 convertase, reconfigures this multicomponent protease into C5 convertase that now has the capacity to cut with specificity component C5. The activation peptide, C5a (Mr ∼11 000) is proinflammatory, and it can recruit granulocytes as well as evoking degranulation and an oxidative burst of these cells. The larger fragment, C5b (Mr ∼185 000), can complex with C6 followed by C7 through C9 to form the MAC, which is a transmembrane circular complex with an inner diameter of ∼100 Å.

Figure 3.

Covalent structures of members of the C9 Family. Components C6 through C9 are related to perforin through the MACPF domain. In addition, the late acting complement components have modules homologous to those found in thrombospondin (TS), the LDL receptor (LDL), the epidermal growth factor (EDGF), complement control proteins (CCP) and factor I modules (FIMs). In this figure, CHO refers to asparaginyl‐linked carbohydrate. Within the MACPF is an Ω‐loop of 17–32 amino acids. C8α is linked by a disulfide bridge to C8γ a lipocalin homologue. Perforin contains a unique calcium ion‐binding domain related to phospholipase C‐δ, labelled C2, which enables peripheral membrane binding.

Figure 4.

A comparison of MACPF domains. Shown as ribbon diagrams are the MACPF domains of complement C6, C8α, perforin and PFO. Basically this domain consists of a central bent β‐sheet of four strands flanked by clusters of α‐helices. Studies performed on the CDCs exemplified by PFO have elucidated that the mechanism as to how this domain integrates into phospholipid membranes is by unfurling the clusters of α‐helices into amphipathic β‐hairpins. Although similar in fold, the vertebrate MACPF domains may function very differently for membrane incorporation.

Figure 5.

The formation of the C5b‐6 complex. The upper row exhibits the tertiary structures of C5 and C6 along with the quaternary structure of C5b‐6. The lower row showing the block diagram form the domain and modular organisations of the two proteins. After cleavage by C5 convertase, the α‐helical‐rich D‐domain of C5 transits away from the main body of the protein providing a hydrophobic groove in which a linker in C6 between TS3 and CCP1 inserts itself. Panels in this figure were originally published in Aleshin et al..

Figure 6.

Electron micrograph images of the MAC. The upper panel shows EM images of the MAC embedded in phospholipid (PL) vesicles, and the lower panel exhibits for clarity the same images windowed and contrast enhanced. The arrowhead points to the upper rim of the tubule, and the arrows are directed at the leaflet(s) consisting of C5b‐7. It is noted that the MAC is associated with either one or two leaflets, and that these project from within the ring. This arrangement is consistent with a CDC‐inverted folding.



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Further Reading

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DiScipio, Richard G(Jun 2013) Complement: Terminal Pathways. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000511.pub3]