Complement:Terminal Pathway 32-415 of human C and of nding of MAC Nucleated cells exposed to sublytic amounts of MAC C5b-8.C5b-9)which may diffuse away from the immediate undergo responses that promote removal of MAC and protec of from atack (Morgan. by an CD5 host cells fron lysis should MAC form on thei ote K ace.factor (DAF:CD55) membrane cofactor protein (MCP;CD46)are two other Depending on membrane-associated compo sthat inhibit arly cell type, to protection by downregulatine formation of cs con the shedding of MAC-enriched vesicles from the cell vertases surface. itronectin(M pro 二75o0samnltimiomladheo on and in with inte eptors Vitr ctin ex The Membrane Attack Complex the uid phe and 4 MAC performs slytic fun bind one C&and three sition molecules to form soluble SC5b-8 and SC5b-9,res ultrastruct ure and size of the lesion is dependent on the tively.All of thes nature of the target membrane (i.e.natural membranes ane ey are lyti active and the amo Clusterin is a multifun tional heterodimer comn sed of a single MAC are ty le each therea the am unt of C9 varies considerably.When C9 input is culates in gh MAC formed erythrocytes or synt es may contain 2容Cy s.It inhibits MAC formation by binding to fluid bling that of consists of a with c5b-8a 1S 2 rane Protection of host or'self cells from MAC-mediated lysis is provided primarily by the inhibitory protein CD59 for poly C9.Thus,C5b-8 functions much like Zn2 b nhibitory CIF) C9 unfolding and Co a well CD59 (M=20000)is expressed as a glyco required for MAC-mediated lysis (Esser,1994).MAC dylinositol(GPI)-linked protein on the membrane surface any cell types including blood cells(u ophils an average of molecules l inhibits MAC by mo less th n the average nm interacting with C8a and C9 during ass of the structures.Other studies indicate that MAC complexes complex on the same cell to which it is attached.This containing as little as one C9 are haemolytically active even Csh-8 nolecules t onal and by they lackth ultrastructural characteristics of poly of the C9 available.Pore diameters are only 1-3nm bu hence the term homologous restriction of lysis'is some sufficient to promote osmotic lysis.Thus,the pore size of the MAC is variable human ENCYCLOPEDIA OF LIFE SCIENCES/e 2001 Nature Publishing Group /www.els.net clusterin (SP40-40), and the membrane-associated protein CD59. The first two are soluble inhibitors that function to block membrane binding of MACcomplexes (e.g. C5b-7, C5b-8, C5b-9) which may diffuse away from the immediate site of complement activation. Thus, their primary role is to protect ‘bystander’ cells from MACattack. As a mem￾brane-associated inhibitor, CD59 functions primarily to protect host cells from lysis should MACform on their surface. Decay-accelerating factor (DAF; CD55) and membrane cofactor protein (MCP; CD46) are two other membrane-associated components that inhibit early reac￾tions in the complement activation cascade and contribute to protection by downregulating formation of C5 con￾vertases. Vitronectin (Mr= 75 000) is a multifunctional adhesion protein found in the circulation and in different tissues. It mediates cell–matrix and cell–cell adhesion through interaction with integrin receptors. Vitronectin exerts its MACinhibitory activity primarily by binding to C5b-7 in the fluid phase and blocking its interaction with cell membranes. The result is formation of a soluble form of C5b-7 (SC5b-7) containing an estimated 3–4 molecules of vitronectin. SC5b-7 can bind one C8 and three C9 molecules to form soluble SC5b-8 and SC5b-9, respec￾tively. All of these complexes are unable to bind to membranes, therefore they are lytically inactive and are cleared from the circulation. Clusterin is a multifunctional heterodimer composed of disulfide-linked a and b subunits each with an Mr of 35 000–40 000. It is a hydrophobic protein that circulates in a complex with lipid and apolipoprotein A-1. Clusterin is normally found along with vitronectin in soluble MAC complexes. It inhibits MACformation by binding to fluid￾phase C5b-7, C5b-8 and C5b-9, and it also can inhibit Zn2+-induced polymerization of C9. Binding studies suggest it interacts directly with C7, C8b and C9 within the MAC. Protection of host or ‘self’ cells from MAC-mediated lysis is provided primarily by the inhibitory protein CD59, also known as MACinhibitory factor (MACIF), mem￾brane inhibitor of reactive lysis (MIRL), protectin, homologous restriction factor 20 (HRF20) or p18. Human CD59 (Mr= 20 000) is expressed as a glycosylphosphati￾dylinositol (GPI)-linked protein on the membrane surface of many cell types including blood cells (e.g. neutrophils, erythrocytes, lymphocytes, platelets), epithelial cells, en￾dothelial cells and spermatozoa. It inhibits MACby interacting with C8a and C9 during assembly of the complex on the same cell to which it is attached. This interaction limits the number of C9 molecules bound by C5b-8 and restricts formation of a fully functional MAC. The inhibitory activity of CD59 is species-selective and is most effective towards C8 and C9 from the same species, hence the term ‘homologous restriction of lysis’ is some￾times used to describe CD59 function. Binding sites for human CD59 have been localized to a segment within amino acids 320–415 of human C8a and 359–411 of human C9. Nucleated cells exposed to sublytic amounts of MAC undergo responses that promote removal of MACand recovery of the cells from attack (Morgan, 1992). These responses are triggered by an increase in intracellular calcium that results from an influx of Ca2+ through channels formed by theMAC. This can promote K+ efflux, activation of protein kinases and increased production of cAMP and lipid-derived signal messengers. Depending on the cell type, removal of MACgenerally occurs by endocytosis or more commonly by ectocytosis, which is the shedding of MAC-enriched vesicles from the cell surface. The Membrane Attack Complex MACperforms its lytic function by restructuring lipid organization within its immediate environment and there￾by altering membrane permeability. The composition, ultrastructure and size of the lesion is dependent on the nature of the target membrane (i.e. natural membranes, vesicles, liposomes, etc.) and the amount of C9 available when MAC is formed. The amounts of C5b, C6, C7 and C8 in a single MACare typically one molecule each whereas the amount of C9 varies considerably. When C9 input is high, MACformed on simple membranes such as erythrocytes or synthetic vesicles may contain 12–18 C9 molecules per complex and have an ultrastructure resem￾bling that of Zn2+-induced poly C9. This form of MAC consists of a tubular-shaped, pore-like structure of polymerized C9 with C5b-8 attached as an appendage. The pore itself is a transmembrane channel lined with amphipathic helices contributed largely by C9. The pore diameter of this MACis
10 nm, which agrees with that for poly C9. Thus, C5b-8 functions much like Zn2+ by promoting C9 unfolding and formation of a well￾organized structure composed of polymerized C9. Formation of tubular-shaped poly C9 is not absolutely required for MAC-mediated lysis (Esser, 1994). MAC formed on erythrocytes at typical serum concentrations of C9 contains an average of 3–6 C9 molecules. Some complexes contain more or less than the average number and accordingly may or may not appear as tubular structures. Other studies indicate that MACcomplexes containing as little as one C9 are haemolytically active even though they lack the ultrastructural characteristics of poly C9. These nontubular complexes form smaller transmem￾brane channels or pores because of the limited amount of C9 available. Pore diameters are only
1–3 nm but sufficient to promote osmotic lysis. Thus, the pore size of the MACis variable and depends on the number of C9 molecules incorporated. Complement: Terminal Pathway 4 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net