Studies of the association of the eighth and ninth components of human complement within the membrane-bound cytolytic complex

Expression of the human complement C8 subunits is independently regulated by interleukin 1 beta, interleukin 6, and interferon gamma

The eighth component of human complement (C8) is composed of two subunits which are products from three separate genes. The alpha-gamma- and the beta-subunit of C8 are expressed independently, and are part of the membrane attack complex. C8 is primarily synthesized in the liver. It has been shown in previous studies that the human hepatoma cell line HepG2 constitutively expresses C8, and thus is a suitable model system for studying C8 biosynthesis in vitro. Expression is modulated by the cytokines IL-1 beta, IL-6 and IFN-gamma. The effect of the different cytokines on the expression of these subunits was examined using biosynthetical labelling and immunoprecipitation methods. C8 alpha-gamma is expressed first and secreted independently from C8 beta. After 5 h labelling, the expression is strongly reduced, and the majority of C8 alpha-gamma is found in the supernatant. C8 beta expression exhibits a different pattern with a much slower rate of biosynthesis and secretion. Evidence was obtained for an independent secretion of the C8 beta chain. C8 alpha-gamma expression is strongly enhanced after stimulation with the cytokines IL-6, IFN-gamma and IL-1 beta. In contrast, only IFN-gamma but not IL-6 and IL-1 beta had an increasing effect on the expression of C8 beta. Thus the total amount of assembled functionally active C8 appears to be limited by the rate of C8 beta expression.

The membrane attack complex of complement. Assembly, structure and cytotoxic activity

The membrane attack complex of complement is formed by the molecular fusion of the five terminal complement proteins, C5, C6, C7, C8, and C9. While the assembly process on a target membrane and its modulation by restriction factors present on host cells is now quite well understood the molecular details of the architecture of the complex still need much further clarification. This is especially true for the interaction of the last acting protein C9, which provides the cytotoxic action of the complex, with the precursor C5b-8 complex. Because of this lack of structural details the molecular mechanisms that lead to complement-mediated cell death remain cryptic, however, it is hoped that recent advances in controlling the assembly process and in site-specific modification of the terminal complement proteins by recombinant DNA techniques should change this predicament quickly.

Sequence homology of complement C8 gamma chain with alpha 1-microglobulin and its implications for C8 structure and function

Complement C8 gamma is a disulphide bonded subunit of C8 with no known homology or function. We show here that it strongly resembles alpha 1-microglobulin and protein HC in both length and sequence suggesting a common genetic origin. By extrapolation from the alpha 1-microglobulin family we are able to suggest that the cysteine residue of C8 gamma is involved in disulphide bonding to C8 alpha, and a possible physiological role in the regulation of the inflammatory response. Comparison of C8 gamma with C8 alpha shows that one region of C8 alpha bears a structural homology with the region of C8 gamma containing the free cysteine residue. This homology includes a cysteine residue of C8 alpha, for which no equivalent is found in C8 beta or C9, and which is therefore a likely candidate for C8 gamma attachment.

A mechanism for the insertion of complement component C9 into target membranes

Complement component C9 is a globular serum protein which can insert and polymerise in a target membrane to form a large membrane channel. The ability to insert in the membrane is conferred by amphipathetic elements of secondary structure in the central part of the molecule. Towards each end high cysteine domains are found, one of which is homologous to the apoprotein binding domains of the LDL receptor. From the sequence and topological data for C9 we present a model for its structure and insertion into the membrane.

Ninth component of complement: self-aggregation and interaction with lipids

We have investigated environmental conditions that might be of importance for the polymerization of the ninth component (C9) of human complement. In disagreement with earlier reports summarized by Tschopp et al. [Tschopp, J., Müller-Eberhard, H. J., & Podack, E. R. (1982) Nature (London) 298, 534-538] we find no evidence for significant aggregation or loss of hemolytic activity of C9 when incubated at 37 degrees C even after 12 days of incubation. Higher temperatures cause denaturation of the protein and formation of stringlike aggregates. In contrast, short-term proteolysis with 1% (w/w) trypsin at room temperature causes rapid polymerization of part of the C9 into tubular structures (poly-C9), and the remainder of the monomeric C9 is digested. This polymerization reaction is inhibitable by trypsin inhibitor; alpha-thrombin and proteinase K are ineffective in creating polymers. A second discrepancy to the earlier reports is our finding that monomeric C9 immediately interacts with small unilamellar lipid vesicles (SUV) without a required heating step. As a result of this interaction about half of the C9 aggregates to form strings and tubules, and these aggregates cause agglutination of vesicles. The other half of the C9 associates with a second population of SUV without causing a change in Stokes' radius of these vesicles, and no proteinaceous structures are detectable on the vesicle surface by electron microscopy. When these two vesicle populations are tested for their membrane integrity, no release of an encapsulated fluorescent marker can be detected, nor is there leakage of potassium ions across the bilayer membrane since a membrane diffusion potential can be developed.(ABSTRACT TRUNCATED AT 250 WORDS)

Complement protein C9 labeled with fluorescein isothiocyanate can be used to monitor C9 polymerization and formation of the cytolytic membrane lesion

Human complement protein C9 was covalently labeled with the fluorescent chromophore fluorescein isothiocyanate (FITC) with only a small reduction in the cytolytic activity of the protein. Polymerization of the labeled protein--either by incubating with lipid vesicles treated with complement proteins C5b-8 (activating the C5b-9 membrane lesion) or by heating the protein [Tschopp, J., Muller-Eberhard, H.J., & Podack, E.R. (1982) Nature (London) 298, 534]--resulted in a 40-60% decrease in the fluorescence emission from FITC. The decrease in total fluorescence was accompanied by an increase in the steady-state anisotropy following activation and polymerization of FITC-C9 by C5b-8 membranes, while heat-induced aggregation of the protein resulted in a dramatic depolarization of fluorescence. Only small changes in either the absorbance spectrum or fluorescence lifetime of the chromophore were detected upon FITC-C9 polymerization. Evidence is presented that the measured changes in FITC fluorescence upon C9 activation are due to self energy transfer between closely apposed fluorescein chromophores which occur in the polymerized form of the protein. The significance of these observations to the molecular structure of the assembled C5b-9 complex is discussed, as are the potential applications of this fluorescent derivative of C9.