Complement:Alternative Pathway (CR1 CR2.CR3 and CR4 with the expressed on phagocytic cells.Receptor interaction results common evolution of the individual components of the way by common structu a al ami and C5a.These anaphylatoxins are potent inducers of locations of the protein domains that provide unique inflammatory reactions.Pro-inflammatory reactions mus properties for substrate binding.protein interaction and articles and prevented on hostc membrane insertion. tions of the alternative pathway at the level of C3bBb,has to be tightly controlled.Formation,stability Effector proteins and dissociation of the plasma proteins acto d by r I(CRI/CD35).me prane cofact rotein(MCP/ ntral ele CD46)and decay-accelerating factor (DAF/CD55).The activation cascade.It is the centre where the three ound regul pathways(ie.,the ne le With a concentr cells.Due to their ins The ofaction ofthese proteins is restricted to thecell surface.In C3 allows the molecule to form covalent bonds with the widely biomolecules,cell surface components and immune com- reg plexes. has the potential to attach to a variety of d hi and the native pr bolvanions and other components matrix,factor H and reconectin/FHL-I polypeptide chai large h(Figure I .1988).Central to the fun nts of the ystems.C4 and macroglobulin.also have internal surface. lytic but un te bat memb .com tions,spo f the labelled cells and cause cell lysis This pathway in itself is than1%make the thi attack by nearby nucleophilic groups These reactions tightly controlled:again.several fluid-phase and surface ow the glutamine residues to eac und regu limit activation and pore formation bound xyl or amino gr oups to an c ther mo all three In the alter complement pathway, a spontaneously formed C3(H2O)Bb protein is the first convertase and provides Components on sof particles.In its default setting The complement proteins are a prime example of proteir families with diverse,but overlapping,functional proper this reaction results in an unrestricted and excessive structurale amplincation of the complement cascade. ments. at山 genes Activation of C3 Spontaneous conformational changes in C3 directly ification and exon shuffling.Most of the complement expose the internal thioester bond,making it accessible ENCYCLOPEDIA OF LIFE SCIENCES/2001 Nature Publishing Group /www.els.net of and coating of C3b molecules on foreign cells or particles (opsonization) that allows binding and interaction of several cellular C3 receptors (CR1, CR2, CR3 and CR4) expressed on phagocytic cells. Receptor interaction results in rapid and efficient phagocytosis of these particles. (2) The initiation of the lytic pathway by formation of the membrane attack complex. (3) Pro-inflammatory reac￾tions induced by the release of the cleavage products C3a and C5a. These anaphylatoxins are potent inducers of inflammatory reactions. Pro-inflammatory reactions must be restricted to foreign particles and prevented on host cells and tissues. Thus, activation, in particular the initial reactions of the alternative pathway at the level of C3b and C3bBb, has to be tightly controlled. Formation, stability and dissociation of the complexes is regulated by the plasma proteins factor H and reconectin/FHL-1 and by three membrane-bound proteins called complement re￾ceptor 1 (CR1/CD35), membrane cofactor protein (MCP/ CD46) and decay-accelerating factor (DAF/CD55). The membrane-bound regulators are expressed by a wide range of cells and tissues and CR1/CD35,MCP/CD46 and DAF/ CD55, are present on almost all human peripheral blood cells. Due to their insertion into the cell membrane, the site of action of these proteins is restricted to the cell surface. In contrast, action of the widely distributed fluid-phase regulators (factor H and reconectin/FHL-1) is not limited to the cell membrane, but occurs in the fluid phase and at the extracellular matrix. By interacting with sialic acids, polyanions and other components of the extracellular matrix, factor H and reconectin/FHL-1 can control a larger area surrounding the cell surface. Thereby, these plasma proteins control C3b formation and reduce the number of C3b molecules that attach directly to the cell surface. Alternative complement activation initiates the lytic pathway, i.e. the formation of membrane attack complexes that incorporate holes in the plasma membrane of the labelled cells and cause cell lysis. This pathway in itself is tightly controlled; again, several fluid-phase and surface bound regulators control the individual reaction steps of this pathway and limit activation and pore formation to foreign cells and particles. Components The complement proteins are a prime example of protein families with diverse, but overlapping, functional proper￾ties defined by conserved common structural elements. The structure and organization of the genes coding for the complement proteins emphasize that the evolution of new protein function occurs via gene duplication, gene mod￾ification and exon shuffling. Most of the complement proteins show a modular, multidomain structure and several of the genes are tightly linked in gene clusters (Hourcade et al., 1992). The modular composition of the exons and the conservation of structural domains in proteins with related functions is consistent with the common evolution of the individual components of the complement system. The conserved protein domains display common three-dimensional structures which are defined by conserved and structurally essential amino acid residues and amino acid substitutions at surface-exposed locations of the protein domains that provide unique properties for substrate binding, protein interaction and membrane insertion. Effector proteins C3 The plasma glycoprotein C3 (183 kDa) is the pivotal complement component and the central element of the activation cascade. It is the centre where the three complement activation pathways (i.e. the alternative, the classical and the lectin pathway) merge. With a concentra￾tion of 1.0–1.5 mg mL 2 1 , C3 is one of the most abundant human plasma proteins. The intramolecular thioester of C3 allows the molecule to form covalent bonds with biomolecules, cell surface components and immune com￾plexes. Thus, C3 has the potential to attach to a variety of molecules and bind to various targets. C3 is synthesized as single-chain precursor and the native protein is composed of two disulfide-bonded polypeptide chains: the 115-kDa a chain and the 75-kDa b chain (Figure 2; Lambris, 1988). Central to the function of C3 is the internal thioester bond formed between Cys988 and Gln991. Two other components of the complement systems, C4 and a2-macroglobulin, also have internal thioester bonds. In the native C3 protein, this site is not accessible, but under physiological conditions, sponta￾neous conformational changes occurring at a rate of less than 1% make the thioester reactive and accessible to attack by nearby nucleophilic groups. These reactions allow the glutamine residues to react covalently via hydroxyl or amino groups to any other molecule. C3 activation is initiated by the C3 convertases formed in all three complement activation pathways. In the alter￾native complement pathway, a spontaneously formed C3(H2O)Bb protein is the first convertase and provides the source for an amplification reaction leading to formation of further convertases and deposition of C3b molecules on the surfaces of particles. In its default setting, this reaction results in an unrestricted and excessive amplification of the complement cascade. Activation of C3 Spontaneous conformational changes in C3 directly expose the internal thioester bond, making it accessible for nucleophilic attack by either H2O or nucleophiles, such as amine or hydroxyl groups. In a newly formed C3b Complement: Alternative Pathway ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 3