Molecular weight of the membrane C5b-9 complex of human complement: characterization of the terminal complex as a C5b-9 monomer

Apical tubular complement activation and the loss of kidney function in proteinuric kidney diseases

Many kidney diseases are associated with proteinuria. Since proteinuria is independently associated with kidney function loss, anti-proteinuric medication, often in combination with dietary salt restriction, comprises a major cornerstone in the prevention of progressive kidney failure. Nevertheless, complete remission of proteinuria is very difficult to achieve, and most patients with persistent proteinuria slowly progress toward kidney failure. It is well-recognized that proteinuria leads to kidney inflammation and fibrosis via various mechanisms. Among others, complement activation at the apical side of the proximal tubular epithelial cells is suggested to play a crucial role as a cause of progressive loss of kidney function. However, hitherto limited attention is given to the pathophysiological role of tubular complement activation relative to glomerular complement activation. This review aims to summarize the evidence for tubular epithelial complement activation in proteinuric kidney diseases in relation to loss of kidney function.

Complement activation and regulation in preeclampsia and hemolysis, elevated liver enzymes, and low platelet count syndrome

The complement system is critical to human health owing to its central role in host defense and innate immunity. During pregnancy, the complement system must be appropriately regulated to allow for immunologic tolerance to the developing fetus and placenta. Although some degree of complement activation can be seen in normal pregnancy, the fetus seems to be protected in part through the placental expression of complement regulatory proteins, which inhibit complement activation at different steps along the complement activation cascade. In women who develop preeclampsia and hemolysis, elevated liver enzymes, and low platelet count syndrome, there is a shift toward increased complement activation and decreased complement regulation. There is an increase in placental deposition of C5b-9, which is the terminal effector of classical, lectin, and alternative complement pathways. C5b-9 deposition stimulates trophoblasts to secrete soluble fms-like tyrosine kinase-1, which sequesters vascular endothelial growth factor and placental growth factor. Pathogenic mutations or deletions in complement regulatory genes, which predispose to increased complement activation, have been detected in women with preeclampsia and hemolysis, elevated liver enzymes, and low platelet count syndrome. Before the disease, biomarkers of alternative complement pathway activation are increased; during active disease, biomarkers of terminal complement pathway activation are increased. Urinary excretion of C5b-9 is associated with preeclampsia with severe features and distinguishes it from other hypertensive disorders of pregnancy. Taken together, existing data link preeclampsia and hemolysis, elevated liver enzymes, and low platelet count syndrome with increased activation of the terminal complement pathway that, in some cases, may be influenced by genetic alterations in complement regulators. These findings suggest that the inhibition of the terminal complement pathway, possibly through C5 blockade, may be an effective strategy to treat preeclampsia and hemolysis, elevated liver enzymes, and low platelet count syndrome, but this strategy warrants further evaluation in clinical trials.

Aberrant activation of the complement system in renal grafts is mediated by cold storage

Aberrant complement activation leads to tissue damage during kidney transplantation, and it is recognized as an important target for therapeutic intervention. However, it is not clear whether cold storage (CS) triggers the complement pathway in transplanted kidneys. The goal of the present study was to determine the impact of CS on complement activation in renal transplants. Male Lewis and Fischer rats were used, and donor rat kidneys were exposed to 4 h or 18 h of CS followed by transplantation (CS + transplant). To study CS-induced effects, a group with no CS was included in which the kidney was removed and transplanted back to the same rat [autotransplantation (ATx)]. Complement proteins (C3 and C5b-9) were evaluated with Western blot analysis (reducing and nonreducing conditions) and immunostaining. Western blot analysis of renal extracts or serum indicated that the levels of C3 and C5b-9 increased after CS + transplant compared with ATx. Quite strikingly, intracellular C3 was profoundly elevated within renal tubules after CS + transplant but was absent in sham or ATx groups, which showed only extratubular C3. Similarly, C5b-9 immunofluorescence staining of renal sections showed an increase in C5b-9 deposits in kidneys after CS + transplant. Real-time PCR (SYBR green) showed increased expression of CD11b and CD11c, components of complement receptors 3 and 4, respectively, as well as inflammatory markers such as TNF-α. In addition, recombinant TNF-α significantly increased C3 levels in renal cells. Collectively, these results demonstrate that CS mediates aberrant activation of the complement system in renal grafts following transplantation.NEW & NOTEWORTHY This study highlights cold storage-mediated aberrant activation of complement components in renal allografts following transplantation. Specifically, the results demonstrate, for the first time, that cold storage functions in exacerbation of C5b-9, a terminal cytolytic membrane attack complex, in renal grafts following transplantation. In addition, the results indicated that cold storage induces local C3 biogenesis in renal proximal cells/tubules and that TNF-α promotes C3 biogenesis and activation in renal proximal tubular cells.

Cisterno-lumbar gradient of complement fractions in geriatric patients with suspected normal pressure hydrocephalus

BACKGROUND

The complement system is a functional link between the innate and adaptive immune system and present in all compartments of the body. The composition of the cerebrospinal fluid (CSF) differs between the ventricular, cisternal and lumbar space. Usually, concentrations of blood-derived CSF proteins increase from ventricular to lumbar fractions.

METHODS

In 20 geriatric patients with suspected normal pressure hydrocephalus (NPH) [13 women, 7 men, age 80.5 (75/85) years; median (25th/75th percentile)] a lumbar spinal tap of 40 ml was performed, and 10 ml of serum was drawn. CSF, sequentially collected in 8 fractions of 5 ml (1st fraction: lumbar CSF; 8th fraction: cisterna magna-near CSF), was analyzed for complement protein C3, and the activation products C3a and sC5b-9 by enzyme immunoassay.

RESULTS

The concentrations of the complement factors measured in fractions 1 and 8 of each individual patient were strongly correlated: C3 (Spearman's rank correlation coefficient r_S = 0.75, p = 0.0002); C3a (r_S = 0.93, p < 0.0001); sC5b-9 (r_S = 0.64, p = 0.002). CSF complement concentrations were lower in the cistern-near fraction 8 than in the lumbar fraction 1 (C3: p = 0.005; C3a: p = 0.0009; sC5b-9: p = 0.0003, Wilcoxon signed rank test). The concentrations of complement factors in CSF were two orders of magnitude lower than those in serum. C3 levels in the lumbar CSF strongly correlated with the lumbar CSF/serum albumin concentration quotient (Q_Alb) as a measure of the functionability of the blood-CSF barrier and the velocity of CSF flow (r_S = 0.84, p < 0.0001) suggesting diffusion of C3 from blood to CSF. The lumbar and cistern-near concentrations of C3a did not significantly correlate with Q_Alb (r_S = 0.26) pointing to a local conversion of C3 to C3a. The lumbar concentrations of sC5b-9 moderately correlated with Q_Alb (r_S = 0.62, p = 0.004). Plotting the CSF/serum quotient of C3 and sC5b-9 versus the Q_Alb revealed an approx. 50% local synthesis of C3, but a strong production of sC5b-9 in the CNS.

CONCLUSIONS

The increase of the complement concentrations from cisternal to lumbar CSF and the strong correlation of C3 with Q_Alb suggest that (1) a substantial portion of complement C3 in CSF originates from blood and (2) the complement system is mildly activated in the CSF of NPH patients.

Quantitative measurement of SC5b-9 and C5b-9(m) in infarcted areas of human myocardium

Previous immunohistochemical work has indicated that terminal C5b-9 complement complexes are selectively deposited in infarcted areas of human myocardium. In the present study, we sought to quantify C5b-9 levels in myocardial tissue, and to differentiate between the membrane-bound C5b-9 (m) and the cytolytically inactive SC5b-9 complex. Paired tissue specimens from infarcted and non-infarcted myocardium were obtained from 36 autopsies. The homogenized and washed tissues were extracted with n-octyl-beta-D-glucopyranoside (octylglucoside) detergent, and the concentrations of C5b-9 in the extracts were determined by ELISA. Membrane-derived C5b-9 (m) and SC5b-9 were differentiated from each other on the basis of their characteristic sedimentation behaviour in sucrose density gradients. It was found that infarcted myocardial tissue contained on average an approximately three-fold higher concentration of C5b-9, compared with non-infarcted tissue. This increase was due in part to an increase in levels of C5b-9 (m). The results corroborate previous immunohistochemical data and show that complement activation occurs to completion with the generation of potentially cytotoxic C5b-9 complexes in infarcted myocardial tissues.

Interaction of tetanus toxin with lipid vesicles. Effects of pH, surface charge, and transmembrane potential on the kinetics of channel formation

We investigated the interaction of tetanus toxin with small unilamellar vesicles composed of different phospholipids as a function of pH, toxin concentration, temperature, and ionic strength of the solution. Tetanus toxin increased the permeability of the vesicles to fluorescent markers of molecular weight up to 700. The time course of the permeabilization was described as the sum of two exponential components of which the faster accounts for more than 70% of the total effect. Both time constants decreased when the pH of the solution was lowered and when vesicles contained negative lipids. These results can be explained in terms of a phenomenological model based on reaction rate theory. The model assumes that tetanus toxin, after equilibrating with the local pH existing at the surface of the vesicles, inserts into the lipid bilayer forming an ionic channel through which solutes can diffuse. Trigger event for the insertion of the toxin is the protonation, and consequent neutralization of one charged group which makes the molecule more hydrophobic. The intrinsic pK of this group was found to be 3.4 +/- 0.2, suggesting that it may be a carboxyl group. Since the toxin equilibrates with the local pH, the enhancing effect of acidic phospholipids is merely explained by the creation of a negative surface potential which increases the local proton concentration. This was confirmed by the inhibitory effect of high Na+ concentration which reduced the surface charge by screening and specific binding. We found still small differences between the lipids tested and the following order of sensitivity to the action of the toxin: phosphatidylinositol greater than phosphatidylserine greater than phosphatidylcholine approximately cholesterol. The activation energy for the two time constants was found to be 19.8 and 14.8 kcal/mol, fast and slow component, respectively, i.e., slightly larger than that for pure diffusion through the bilayer. The permeabilization induced by tetanus toxin is a voltage-dependent process because vesicles bearing an inner negative potential were depolarized very quickly whereas those bearing an inner positive voltage were barely depolarized at all.

Measurement of terminal complement complexes in rheumatoid arthritis

Though complement activation is recognized as a central event in inflammation in the rheumatoid joint, little attention has been paid to the role of the cytolytic membrane attack complex of complement in the pathogenesis of this disease. The membrane attack complex causes a variety of non-lethal effects in nucleated cells, including stimulation of release of inflammatory mediators, and cell proliferation. Thus in the rheumatoid synovium, non-lethal effects of complement membrane attack may play a major role in disease pathology. In order to investigate this possibility, assays for the detection of terminal complement complexes in biological fluids have been established, and used to demonstrate membrane attack pathway activation in rheumatoid arthritis. Terminal complement complexes were present in increased levels in synovial fluid (mean, 1,334 ng/ml) and plasma (mean, 513 ng/ml) in 20 patients with rheumatoid arthritis when compared with controls (mean, 285 ng/ml and 129 ng/ml respectively). Using an assay specific for the SC5b-9 complex it was demonstrated that the raised levels of terminal complement complexes in rheumatoid synovial fluid consisted of a mixture of inactive SC5b-9 complexes and fluid-phase complement membrane attack complexes.

Monoclonal antibodies against neoantigens of the terminal C5b-9 complex of human complement

Assembly of the terminal C5b-C9 complement components into the cytolytic C5b-9 complex is accompanied by exposure of characteristic neoantigens on the macromolecule. We report the production and characterization of mouse monoclonal antibodies to C9-dependent neoantigens of human C5b-9. Binding-inhibition assays with EDTA-human plasma and micro-ELISA assays with purified C9 showed that the antibodies did not react with native complement components and thus confirmed the specificity of the antibodies for the neoantigens. The monoclonal antibodies did, however, cross-react with cytolytically inactive, fluid-phase C5b-9 complexes. Thus, expression of the neoantigenic determinants was not dependent on the formation of high molecular weight C9 polymers with the complex, since these are absent in fluid-phase C5b-9. Radioiodinated antibodies could be utilized in immunoradiometric assays for the detection and quantitation of C5b-9 on cell membranes. Cross-reactivities of the antibodies with C9-dependent neoantigens of several other animal species were examined and antibody clones cross-reacting with rabbit (clones 3B1, 3D8, and 2F3), sheep (clones 3D8 and 2F3) and guinea-pig (clone 3D8) neoantigens were identified. Three of four tested clones (3D8, 2F3, 1A12) precipitated C5b-9 complexes in double-diffusion assays, probably due to their interaction with multiple and repeating C9-epitopes on the terminal complexes. The monoclonal antibodies will be of value for definitive identification and quantitation of C5b-9 on cell membranes and in tissues, and for establishing immunoassays for detection and quantitation of terminal fluid-phase C5b-9 complexes in plasma.

Mechanism of complement cytolysis and the concept of channel-forming proteins

Complement damages membranes via the terminal reaction sequence that leads to the formation of membrane-bound, macromolecular C5b-9(m) protein complexes. These complexes represent C5b-8 monomers to which varying numbers of C9 molecules can be bound. Complexes carrying high numbers of C9 (ca. 6/8-12/16?) exhibit the morphology of hollow protein channels. Because they are embedded within the lipid bilayer, aqueous transmembrane pores are generated that represent the primary lesions caused by complement in the target cell membrane. Many other proteins damage membranes by forming channels in a manner analogous to the C5b-9(m) complex. Two prototypes of bacterial exotoxins, Staphylococcus aureus alpha-toxin and streptolysin-O, are discussed in this context, and attention is drawn to the numerous analogies existing among these protein systems. Common to all is the process of self-association of the native proteins to form supramolecular complexes. This event is in turn accompanied by a unique transition of the molecules from a hydrophilic to an amphiphilic state.

Is the membrane attack complex of complement an enzyme?

Recent studies on the functional activities of the membrane attack complex of complement, C5b-9, are reviewed. A new speculative hypothesis has been advanced to account for the ability of complement to mediate lysis of various targets. This hypothesis has three major elements: 1) that the membrane attack complex is an enzyme; 2) that the substrate for this putative enzyme is a membrane constituent; 3) that the substrate specificity of the putative enzyme is dependent on the species source of individual complement components within the C5b-9 complex.

Complement-mediated lysis of pigeon erythrocyte ghosts analysed by flow cytometry. Evidence for the involvement of a 'threshold' phenomenon

Flow-cytometric analysis of complement-mediated lysis of antibody-coated pigeon erythrocyte ghosts containing fluorescein was carried out to determine whether lysis involved a gradual release of fluorescein or a 'threshold' release from individual cells. Antibody-coated ghosts were comprised of three subpopulations identified by fluorescence and scatter (size). These were: (a) highly fluorescent, medium scatter, (b) medium fluorescence, high scatter, and (c) low (or zero) fluorescence, low scatter. Lysed ghosts and isolated nuclei were identified by fluorescence microscopy and scanning electron microscopy. Fluorescence distributions analysed by flow cytometry indicated that, after complement attack, those ghosts remaining intact retained all their fluorescent label. A time course of changes in ratios of the three subpopulations indicated that once lysis of an individual ghost was initiated, release of label was complete within 1 min; no stages of intermediary fluorescence appeared, and those ghosts remaining at the end of the experiment retained the same fluorescence intensity as control ghosts. The results supported the hypothesis that complement-mediated cell lysis is a 'threshold' phenomenon; a submaximal response by a cell population representing a complete response by only some of the cells rather than a partial response by all of the cells.

Freeze-fracture analysis of the membrane lesion of human complement

The structure and membrane insertion of the human C5b-9(m) complex, generated by lysis of antibody-coated sheep erythrocytes with whole human serum under conditions where high numbers of classical ring-shaped lesions form, were studied in single and complementary freeze-fracture replicas prepared by unidirectional and rotary shadowing. The intramembrane portion of the C5b-9(m) cylinder was seen on EF-faces as an elevated, circular structure. In nonetched fractures it appeared as a solid stub; in etched fractures a central pit confirmed the existence of a central, water-filled pore in the molecule. Complementary replicas showed that each EF-face ring corresponded to a hole in the lipid plateau of the PF-face. Etched fractures of proteolytically stripped membranes revealed the extramembrane annulus of the C5b-9(m) cylinder on ES-faces and putative internal openings on PS-faces. Allowing for the measured thickness of deposited Pt/C, the dimensions of EF-face rings and ES-face annuli conformed to anticipations derived from negatively stained preparations. Our results support the concept that the hollow cylindrical C5b-9(m) complex penetrates into the inner leaflet of the target erythrocyte membrane bilayer, forming a stable transmembrane protein channel.

Molecular composition of the terminal membrane and fluid-phase C5b-9 complexes of rabbit complement. Absence of disulphide-bonded C9 dimers in the membrane complex

The terminal membrane C5b-9(m) and fluid-phase SC5b-9 complexes of rabbit complement were isolated from target sheep erythrocyte membranes and from inulin-activated rabbit serum respectively. In the electron microscope, rabbit C5b-9(m) was observed as a hollow protein cylinder, a structure identical with that of human C5b-9(m). Monodispersed rabbit C5b-9(m) exhibited an apparent sedimentation coefficient of 29 S in deoxycholate-containing sucrose density gradients, corresponding to a composite protein-detergent molecular-weight of approx. 1.4 X 10(6). Protein subunits corresponding to human C5b-C9 were found on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. By densitometry, there were consistently six molecules of monomeric C9 present for each monomeric C5b-8 complex. Fluid-phase rabbit SC5b-9 was a hydrophilic 23 S ma macromolecule that differed in subunit composition from its membrane counterpart in that it contained S-protein and only two to three molecules of C9 per monomer complex. The data are in accord with the previous report on human C5b-9 that C5b-9(m) contains more C9 molecules than SC5b-9 [Ware & Kolb (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 6426-6430]. They corroborate the previous molecular-weight estimate of approx. 10(6) for C5b-9(m) and thus support the concept that the fully assembled, unit lesion of complement is a C5b-9 monomer [Bhakdi & Tranum-Jensen (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 1818-1822]. They also show that C9 dimer formation is not required for assembly of the rabbit C5b-9(m) protein cylinder, or for expression of its membrane-damaging function.

Terminal membrane C5b-9 complex of human complement: transition from an amphiphilic to a hydrophilic state through binding of the S protein from serum

The membrane-damaging C5b-9(m) complex of complement is a cylindrically structured, amphiphilic molecule that is generated on a target membrane during complement attack. Isolated C5b-9(m) complexes are shown here to possess the capacity of binding a protein, termed "S"-protein, that is present in human plasma. Binding of this protein apparently shields the apolar surfaces of C5b-9(m), since the resulting "SC5b-9(m)" complex is hydrophilic and no longer aggregates in detergentfree solution. Dispersed SC5b-9(m) complexes exhibit an apparent sedimentation coefficient of 29S in sucrose density gradients, corresponding to a molecular weight of approximately 1.4 million. SDS PAGE analyses indicate binding of 3-4 molecules of S-protein per C5b-9(m) complex. These data are consistent with a monomer nature and molecular weight of 1-1.1 million of the C5b-9(m) complex. Ultrastructural analysis of SC5b-9(m) shows preservation of the hollow cylindrical C5b-9(m) structure. Additional material, probably representing the S-protein itself, can be visualized attached to the originally membrane-embedded portion of the macromolecule. The topography of apolar surfaces on a molecule thus appears directly probed and visualized through the binding of a serum protein.

Transmembrane channel formation by complement: functional analysis of the number of C5b6, C7, C8, and C9 molecules required for a single channel

Earlier studies have shown that sequential treatment of resealed erythrocyte ghosts with C5b6, C7, C8, and C9 leads to insertion of hydrophobic peptides from these complement proteins into the membrane and assembly of transmembrane channels. The number of molecules of each of the proteins required for assembly of the membrane-associated channel structure was evaluated by measuring the quantitative relationship between the doses of the individual proteins and the release of two trapped markers, sucrose and inulin, from ghosts after channel formation. The incubation period was sufficient to attain equilibrium of marker distribution between the ghosts and the extracellular fluid. Two markers of different size (sucrose and inulin, 0.9 and 3 nm molecular diameter, respectively) were used in order to develop information on the molecular composition of small and large channels, respectively. We found that participation of C5b6, C7, and C8 in channel formation displayed one-hit characteristics, regardless of marker size. By contrast, the participation of C9 was one-hit with respect to the sucrose marker, whereas with respect to the inulin marker the C9 reaction was multi-hit. Our results are compatible with the view that these markers are released through a channel structure in the membrane that is a monomer of C5b--9 of the composition C5b61 C71C81C9n, in which n = 1 for channels permitting passage of sucrose and n = 2 for channels allowing transit of inulin.

Molecular organization of C9 within the membrane attack complex of complement. Induction of circular C9 polymerization by the C5b-8 assembly

Evidence has been presented suggesting that during assembly of the membrane attack complex (MAC) of complement, the C5b-8 complex induces polymerization of C9. The C9 polymer was detected by sodium dodecyl sulfate (SDS) gel electrophoresis of MAC isolated from complement-lysed erythrocytes. It resembled the previously described polymerized C9 (poly C9) produced from isolated monomeric C9 by prolonged incubation at 37 degrees C in that it was resistant to dissociation by SDS and reducing agents and had an apparent molecular weight of approximately 1.1 million. The presence of poly C9 in the MAC was further supported by the expression of identical neoantigens by the MAC and poly C9 and by the high C9 content of the MAC relative to its other constituents. Isolated C8 in solution was found to have a single C9-binding site. In mixture, the two proteins formed a reversible equimolar complex that had a sedimentation coefficient of 10.5S. In contrast, a single, cell-bound C5b-8 complex was found to bind up to 12-15 C9 molecules and clusters of C5b- 8 bound 6-8 C9 molecules per C8 molecule. In either case, typical ultrastructural membrane lesions were observed, suggesting that the membrane lesion is identical with the tubular poly C9 consisting of 12-16 C9 molecules, and that the MAC can have either the composition (C5b-8)polyC9 or (CSb-8)(2)polyC9. When C9 input was restricted so that the molar C9/C8 ratio was less than or equal to 3, C9-induced aggregates of C5b-8 were observed but virtually no circular membrane lesions were found. We suggest, therefore, that C9, at low dosage, causes cross-linking of multiple C5b-8 complexes within the target membrane and that, at high dosage, C9 is polymerized by C5b-8 to form a transmembrane channel within the MAC assembly. It is primarily the C9 polymer that evokes the ultrastructural image of the MAC or of membrane lesions caused by complement.

Polymerization of the ninth component of complement (C9): formation of poly(C9) with a tubular ultrastructure resembling the membrane attack complex of complement

The ninth component of complement (C9) has a marked propensity to polymerize. C9 polymers [poly(C9)] formed spontaneously in Veronal-buffered saline upon incubation of purified C9 for 64 hr at 37 degrees C or within 2 hr at 46--56 degrees C. Poly(C9) formed at 37 degrees C was visualized by electron microscopy as a tubular structure with an internal diameter of 110 A and a length of 160 A. Its ultrastructure suggested a dodecameric composition and resembled that of the membrane attack complex of complement. The wider end of the tubular structure was formed by an approximately 30-A-thick torus with inner and outer diameters of 110 A and 220 A, respectively. Because the dimensions of C9 within poly(C9) were 160 x 55 A (maximal) and 20 A (minimal) and because monomeric C9 has dimensions of approximately 80 x 55 A, it is proposed that monomeric C9 unfolds during polymerization into tubules. Polymerization also occurred upon treatment of C9 for 1 hr at 37 degrees C with 0.6 M guanidine . HCl, 0.1 M octyl glucoside, or 1.5% sodium deoxycholate. Guanidine . HCl-induced C9 polymers consisted of elongated highly curved strands 55--80 A wide, suggesting that these polymers were formed by globular C9 that had not unfolded.

Assembly of the functional membrane attack complex of human complement: formation of disulfide-linked C9 dimers

The 158,000 Mr protein, previously designated C5c, present in fully assembled complement (C) membrane attack complexes (MC5b-9) has been identified as a disulfide-bonded dimer of C9. This conclusion was based on the observations that: (i) a portion of the 125I-radiolabeled precursor C9 incorporated into MC5b-9 complexes comigrated with the 158,000 Mr protein band in NaDodSO4/polyacrylamide slab gels; (ii) monospecific antisera produced against native C9 and the 158,000 Mr protein immunologically crossreacted with monomeric native C9 by double immunodiffusion and with monomeric C9 and the 158,000 Mr protein on immunoreplication procedures; and (iii) two-dimensional NaDodSO4/polyacrylamide slab gel electrophoresis, in which the second dimension was conducted under reducing conditions, revealed that the 158,000 Mr protein contained two identical 71,000 Mr subunits which comigrated with monomeric C9. Molar ratio estimates indicated that 1 mol of C5b, C9 dimer, C6, C7, and C8 and 3-4 mol of C9 monomer were present per MC5b-9 complex. Each fully assembled membrane-bound MC5b-9 complex would therefore have a calculated Mr of 982,000. The presence of C9 dimers in the hemolytically active 29S dimeric form of the MC5b-9 complex and the absence of C9 dimers in the hemolytically inactive 23S monomeric form of the fluid phase SC5b-9 complex strongly suggest an important role for C9 dimer formation in MC5b-9 complex structure and function. The most probable function of C9 dimers would be the formation of intercomplex disulfide crosslinks which would provide a mechanism to stabilize the assembly of MC5b-9 into aggregates of increasing size on the target membrane surface which would thus be responsible for the observed pore size heterogeneity of functional C lesions.