Mutations in the putative lipid-interaction domain of complement C9 result in defective secretion of the functional protein

The mystery behind membrane insertion: a review of the complement membrane attack complex

The membrane attack complex (MAC) is an important innate immune effector of the complement terminal pathway that forms cytotoxic pores on the surface of microbes. Despite many years of research, MAC structure and mechanism of action have remained elusive, relying heavily on modelling and inference from biochemical experiments. Recent advances in structural biology, specifically cryo-electron microscopy, have provided new insights into the molecular mechanism of MAC assembly. Its unique 'split-washer' shape, coupled with an irregular giant β-barrel architecture, enable an atypical mechanism of hole punching and represent a novel system for which to study pore formation. This review will introduce the complement terminal pathway that leads to formation of the MAC. Moreover, it will discuss how structures of the pore and component proteins underpin a mechanism for MAC function, modulation and inhibition.This article is part of the themed issue 'Membrane pores: from structure and assembly, to medicine and technology'.

Perforin: structure, function, and role in human immunopathology

The secretory granule-mediated cell death pathway is the key mechanism for elimination of virus-infected and transformed target cells by cytotoxic lymphocytes. The formation of the immunological synapse between an effector and a target cell leads to exocytic trafficking of the secretory granules and the release of their contents, which include pro-apoptotic serine proteases, granzymes, and pore-forming perforin into the synapse. There, perforin polymerizes and forms a transmembrane pore that allows the delivery of granzymes into the cytosol, where they initiate various apoptotic death pathways. Unlike relatively redundant individual granzymes, functional perforin is absolutely essential for cytotoxic lymphocyte function and immune regulation in the host. Nevertheless, perforin is still the least studied and understood cytotoxic molecule in the immune system. In this review, we discuss the current state of affairs in the perforin field: the protein's structure and function as well as its role in immune-mediated diseases.

Topology of the membrane-bound form of complement protein C9 probed by glycosylation mapping, anti-peptide antibody binding, and disulfide modification

The two N-linked oligosaccharides in native human C9 were deleted by site-specific mutagenesis. This aglycosyl-C9 did not differ from its native form in hemolytic and bactericidal activity. A new N-glycosylation site (K311N/E313T) was introduced into the turn of a helix-turn-helix [HTH] fold that had been postulated to form a transmembrane hairpin in membrane-bound C9. This glycosylated form of human C9 was as active as the native protein suggesting that the glycan chain remains on the external side of the membrane and that translocation of this hairpin is not required for membrane anchoring. Furthermore, flow cytometry provided evidence for the recognition of membrane-bound C9 on complement-lysed ghosts by an antibody specific for the HTH fold. A new N-glycosylation site (P26N) was also introduced close to the N-terminus of C9 to test whether this region was involved in C9 polymerization, which is thought to be required for cytolytic activity of C9. Again, this glycosylated C9 was as active as native C9 and could be induced to polymerize by heating or incubation with metal ions. The two C-terminal cystines within the MACPF domain could be eliminated partially or completely without affecting the hemolytic activity. Free sulfhydryl groups of unpaired cysteines in such C9 mutants are blocked since they could not be modified with SH-specific reagents. These results are discussed with respect to a recently proposed model that, on the basis of the MACPF structure in C8alpha, envisions membrane insertion of C9 to resemble the mechanism by which cholesterol-dependent cytolysins enter a membrane.

An abnormal but functionally active complement component C9 protein found in an Irish family with subtotal C9 deficiency

Two independently segregating C9 genetic defects have previously been reported in two siblings in an Irish family with subtotal C9 deficiency. One defect would lead to an abnormal C9 protein, with replacement of a cysteine by a glycine (C98G). The second defect is a premature stop codon at amino acid 406 which would lead to a truncated C9. However, at least one of two abnormal proteins was present in the circulation of the proband at 0.2% of normal C9 concentration. In this study, the abnormal protein was shown to have a molecular weight approximately equal to that of normal C9, and to carry the binding site for monoclonal antibody (mAb) Mc42 which is known to react with an epitope at amino acid positions 412-426, distal to 406. Therefore, the subtotal C9 protein carries the C98G defect. The protein was incorporated into the terminal complement complex, and was active in haemolytic, bactericidal and lipopolysaccharide release assays. A quantitative haemolytic assay indicated even slightly greater haemolytic efficiency than normal C9. Epitope mapping with six antihuman C9 mAbs showed the abnormal protein to react to these antibodies in the same way as normal C9. However, none of these mAbs have epitopes within the lipoprotein receptor A module, where the C98G defect is located. The role of this region in C9 functionality is still unclear. In conclusion, we have shown that the lack of a cysteine led to the production of a protein present in the circulation at very much reduced levels, but which was fully functionally active.

Role of a disulfide-bonded peptide loop within human complement C9 in the species-selectivity of complement inhibitor CD59

CD59 antigen is a membrane glycoprotein that inhibits the activity of the C9 component of the C5b-9 membrane attack complex (MAC), thereby protecting human cells from lysis by human complement. The complement-inhibitory activity of CD59 is species-selective, and is most effective toward C9 derived from human or other primate plasma. The species-selective activity of CD59 was recently used to map the segment of human C9 that is recognized by this MAC inhibitor, using recombinant rabbit/human C9 chimeras that retain lytic function within the MAC [Husler, T., Lockert, D. H., Kaufman, K. M., Sodetz, J. M., & Sims, P. J. (1995) J. Biol. Chem. 270,3483-3486]. These experiments suggested that the CD59 recognition domain was contained between residues 334 and 415 in human C9. By analyzing the species-selective lytic activity of recombinant C9 with chimeric substitutions internal to this segment, we now demonstrate that the site in human C9 uniquely recognized by CD59 is centered on those residues contained between C9 Cys359/Cys384, with an additional contribution by residues C-terminal to this segment. Consistent with its role as a CD59 recognition domain, CD59 specifically bound a human C9-derived peptide corresponding to residues 359-384, and antibody (Fab) raised against this C9-derived peptide inhibited the lytic activity of human MAC. Mutant human C9 in which Ala was substituted for Cys359/384 was found to express normal lytic activity and to be fully inhibited by CD59. This suggests that the intrachain Cys359/Cys384 disulfide bond within C9 is not required to maintain the conformation of this segment of C9 for interaction with CD59.

Chimeras of human complement C9 reveal the site recognized by complement regulatory protein CD59

CD59 antigen is a membrane glycoprotein that inhibits the activity of the C9 component of the C5b-9 membrane attack complex, thereby protecting human cells from lysis by human complement. The complement-inhibitory activity of CD59 is species-selective and is most effective toward C9 derived from human or other primate plasma. By contrast, rabbit C9, which can substitute for human C9 in the membrane attack complex, mediates unrestricted lysis of human cells. To identify the peptide segment of human C9 that is recognized by CD59, rabbit C9 cDNA clones were isolated, characterized, and used to construct hybrid cDNAs for expression of full-length human/rabbit C9 chimeras in COS-7 cells. All resulting chimeras were hemolytically active, when tested against chicken erythrocytes bearing C5b-8 complexes. Assays performed in the presence or absence of CD59 revealed that this inhibitor reduced the hemolytic activity of those chimeras containing human C9 sequence between residues 334-415, irrespective of whether the remainder of the protein contained human or rabbit sequence. By contrast, when this segment of C9 contained rabbit sequence, lytic activity was unaffected by CD59. These data establish that human C9 residues 334-415 contain the site recognized by CD59, and they suggest that sequence variability within this segment of C9 is responsible for the observed species-selective inhibitory activity of CD59.

A null mutation in the perforin gene impairs cytolytic T lymphocyte- and natural killer cell-mediated cytotoxicity

Lymphocyte-mediated cytotoxicity has been proposed to consist of the polarized secretion of granule-stored perforin leading to target-cell lysis. Nevertheless, perforin-independent pathways were postulated to explain the cytolytic activity of apparently perforin-free lymphocytes and the DNA degradation found in dying target cells. To evaluate the role of perforin, we used gene targeting in embryonic stem cells to produce mice lacking perforin. Mice homozygous for the disrupted gene have no perforin mRNA. The mice are healthy. Activation and granzyme A secretion of perforin-free cytolytic T cells are unaltered. The killing activity of cytolytic T cells as well as natural killer (NK) cells, however, is impaired but not abolished. Approximately one-third of the killing activity remains when lysis of 3T3 fibroblast targets and the apoptotic cell death of YAC-1 NK targets are analyzed. We conclude that perforin is a crucial effector molecule in T cell- and NK cell-mediated cytolysis. However, alternative perforin-independent lytic mechanisms also exist.

A method for in vitro synthesis of unglycosylated recombinant complement component C9

A method for in vitro synthesis of human complement component C9 has been established in order to generate unglycosylated normal and mutant proteins without the need to sub-clone. One or two step polymerase chain reaction (PCR) was used to add the T7 RNA polymerase promoter and introduce multiple mutations within the cDNA. The cDNA was then transcribed by T7 RNA polymerase and the mRNA translated in a rabbit reticulocyte lysate or wheat germ system. Successful synthesis was confirmed by: the correct size of PCR product DNA on agarose gel electrophoresis, incorporation of [alpha-32P]UTP into mRNA, and formation of [35S]methionine-labelled protein of the correct molecular mass for full length C9. The wheat germ extract generated up to 1.5 micrograms of recombinant C9. This unglycosylated C9 had at least 10% of the haemolytic activity of native C9. Unglycosylated C9 polymerised more readily than the native protein. This spontaneous polymerisation was increased by removal of the first 23 amino acids or mutating two cysteines at positions 33 and 36. This therefore provides a rapid method for screening the effect of multiple mutations on the biological activity and polymerisation of pore forming proteins.