Complement binding of C3 Regulatory Proteins s C4 exists as two isoforms.C4A and C4B Serum-soluble protein Although the two molecules have very few amino acid es the classical f CI way ains Asp instead components ofCl.It is partof the serine p eas inhibitor hydroxyl gro structure. molecule of teleost fish is encoded by regulates eiomat8nPortaimCbpAandC46pe various gene C3 protein present in C4b by factor I.There are three isoforn ificities to a variety of complement-activatn surfaces major form is composed of seven chains (each 70kDa) It has been hypothesized that this functional and structural ndone Bchain(75kDa)that are linked by disulfide bonds C3 diversity allows these nals to expand their innate 01 2 m contains SCRs The and 6 chains are composed of eight rtases to produe and three SCRs,respectively and both C3 and C4 are inactivated by factor I in the acto polypeptide chain of 150kDa gene ate rmina egulates ampl cep or fluid-phase C3b and accelerating the decay of the um proteinase inhibito C3b.Bb convertase,or by acting as a cofactor for factor macroglobulin.suggesting that the ancestral molecule '器eeompemetpCopeaipicatio Properdin is required fo the stabilization of the C3b,B e 1 (TSPD n ong e at type Factor I is an 88-kDa serine protease with an unusual structure:from N-tern inus to its C-terminus itis Late components factor /mem comp ich ass A(LDLRA)modules and a serine protease domain. with the C5b initiating the self assembly of the membrane attack complex (MAC). za that co 0 oluble plasma inhibitors of MAC formation that bind to be for creating the transmembrane channels that lead to the C5b-7 complexes and prevent their insertion into cell cell lysis.The assembly of MAC and its insertion into the nembr -protein glycoprotein th contains m-l nd e indin on and hat a alad u hin th thrombospondin-like N-terminal domain insertion of the C5b-C7 complex into a discrete site on Carboxypeptidase N.also termed anaphylatoxin inact target membrane and it now serves as a receptor for C8. C5 C8 complex can then bind multiple in the MAC.Although polymerization of C9 is not esse Membrane-bound proteins lysis of erythrocytes and nucleated cells,it is believed to be Decay-accelerating factor(DAF or CD55)and membrane killing of bacteria.Like the classical and ve path hanne gctoLPoteinMCporCD4,haeasimilartnucture ent acu and membrane regulatory proteins. cont lvcoprotein that contains four SCRs followed by a serine threonine/proline(STP)domain.The C-terminal portion ends with tha pecomes anchore serve as the cell ENCYCLOPEDIA OF LIFE SCIENCES/2001 Nature Publishing Group/www.els.net binding of C3 and C4 to hydroxyl groups. This binding preference is clearly illustrated in the case of mammalian C4; in humans C4 exists as two isoforms, C4A and C4B. Although the two molecules have very few amino acid differences (13 substitutions in 1722 residues), C4A (which contains an Asp instead of the His residue) binds preferentially to surfaces carrying amino groups, whereas C4B (which contains the His) binds to those containing hydroxyl groups. It is noteworthy that, unlike any other species, the C3 molecule of teleost fish is encoded by various genes and that the multiple C3 proteins present in an individual fish have the ability to bind with different specificities to a variety of complement-activating surfaces. It has been hypothesized that this functional and structural C3 diversity allows these animals to expand their innate immune recognition capabilities. C3, C4 and C5 are cleaved by their respective convertases to produce C3a, C4a and C5a anaphylotoxins, and both C3 and C4 are inactivated by factor I in the presence of cofactors to generate similar cleavage pro￾ducts. Except for the C-terminal region, the overall structure and gene organization of C3, C4 and C5 very much resemble that of the serum proteinase inhibitor a2- macroglobulin, suggesting that the ancestral molecule from which the three complement components were derived might have been originated by gene duplication from an ancestral a2-macroglobulin-like molecule. Late components All three pathways of activation converge to the produc￾tion of a C5 convertase which cleaves C5 to C5b and C5a, with the C5b initiating the self assembly of the membrane attack complex (MAC). The MAC is a supramolecular organization of molecules that contains C5b, C6, C7, C8, together with numerous molecules of C9, and is respon￾sible for creating the transmembrane channels that lead to cell lysis. The assembly of MAC and its insertion into the cell membrane occurs by the following sequence of events. The C5b–C6 complex binds C7 and exposes hydrophobic sites that are concealed within the molecule. This allows insertion of the C5b–C7 complex into a discrete site on target membrane and it now serves as a receptor for C8. The C5b–C8 complex can then bind multiple molecules of C9(n= 1–18) and the polymerization of C9molecules results in the membrane lesions that are characteristic of MAC. Although polymerization of C9is not essential for lysis of erythrocytes and nucleated cells, it is believed to be required for the killing of bacteria. Like the classical and alternative pathways of complement activation, channel formation by the MAC is also under the control of serum and membrane regulatory proteins. Regulatory Proteins Serum-soluble proteins C1 inhibitor (C1-inh) regulates the classical pathway by blocking the proteolytic activity of C1r and C1s sub￾components of C1. It is part of the serine protease inhibitor or serpin superfamily and lacks any characteristic modular structure. C4b-binding protein (C4bpA and C4bpB) regulates the formation of the classical pathway C3 convertase by serving as a cofactor in the inactivation of C4b by factor I. There are three isoforms of C4bp. The major form is composed of seven a chains (each 70 kDa) and one b chain (75 kDa) that are linked by disulfide bonds to form a spider-like structure. As is true of many of the regulatory C3b- and C4b-binding proteins, C4bp also contains SCRs. The a and b chains are composed of eight and three SCRs, respectively. Factor H is a single polypeptide chain of 150 kDa composed of 20 SCRs. It downregulates amplification of the alternative pathway by binding either to surface-bound or fluid-phase C3b and accelerating the decay of the C3b,Bb convertase, or by acting as a cofactor for factor I. Properdin is required for the stabilization of the C3b,Bb convertase, allowing rapid amplification of surface-bound C3b. It contains six thrombospondin repeat type 1 (TSP1) domains. Factor I is an 88-kDa serine protease with an unusual structure; from its N-terminus to its C-terminus it is composed of a factor I/membrane attack complex (FIMAC) module, a scavenger receptor cysteine-rich (SRCR) module, two low-density lipoprotein receptor class A (LDLRA) modules and a serine protease domain. Factor I cleaves C3b and C4b (in the presence of various cofactor molecules) at several positions in the molecules. S-protein (vitronectin) and clusterin (SP-40-40) are soluble plasma inhibitors of MAC formation that bind to the C5b-7 complexes and prevent their insertion into cell membranes. S-protein is a glycoprotein that contains a fibronectin-like integrin-binding region and a heparan￾binding domain. Clusterin is a heterodimeric protein with a thrombospondin-like N-terminal domain. Carboxypeptidase N, also termed anaphylatoxin inacti￾vator, inactivates C3a, C4a and C5a by removing their C￾terminal arginyl residue. Membrane-bound proteins Decay-accelerating factor (DAF or CD55) and membrane cofactor protein (MCP or CD46) have a similar structure, and both downregulate complement activation to protect self tissue from attack by complement. DAF is a 75-kDa glycoprotein that contains four SCRs followed by a serine/ threonine/proline (STP) domain. The C-terminal portion ends with 24 hydrophobic residues that serve as a membrane anchor. DAF becomes anchored to the cell Complement 6 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net