1.C.39 The Membrane Attack Complex/Perforin (MACPF) Family
Complement is part of the mammalian immune defense system against pathogenic microorganisms. More than 20 complement proteins control microbial invasion. One mechanism involves direct killing of the microbe, dependent on the C5b-8-9 or membrane attack complex (MAC) (Wang et al., 2000). This complex forms on the outer surface of a Gram-negative bacterial cell. The C5b-8 complex probably allows entry of C9 into the periplasm where the inactive protoxin is converted to the active toxin. This conversion process probably involves sulfhydryl oxidation to a disulfide linked protein. It has been shown that (C5b-8)1 (C9)1 which can lyse erythrocytes can not kill bacteria, but a (C5b-8)1 (C9)4 complex is bactericidal and also protozoan pathogens (Wang et al., 2000). Only the C-terminal portion of C9, residues 145-538 (the C9b fragment) is required to dissipate the membrane potential across respiring inner membrane vesicles, and only C9 is required if the protein is first targeted to the periplasm by any mechanism. While C9 therefore appears to form a pore in the inner membrane, C5b-8 appears to allow access of C9 to the periplasm. C6, C7, C8α, C8β and C9 all contain the pore forming MACPF domain (Rosado et al., 2008).
Native complement protein C9 is a soluble glycoprotein devoid of cellular toxicity, but its bactericidal activity is unmasked in the periplasm. This requires disulfide bond formation, a presumed conformational change, and insertion of a 'processed' toxin into the inner bacterial membrane, probably as an oligomer. Four disulfide bonds can potentially form in C9, three in the epidermal growth factor domain (residues 487-518) and one in the membrane attack/perforin region (Cys359 and Cys384). C9 has a 3-dimensional fold for its membrane interaction and pore forming domain as many other toxins of animal and bacterial origin (Anderluh and Lakey, 2008; Lukoyanova and Saibil, 2008). Kurschus et al., 2008 provided evidence that perforins and cholesterol-dependent cytolysins can deliver deadly proteins such as granzymes into the cytoplasm of target cells.
Proteins of the complement membrane attack complex (MAC) and the protein perforin (PF) share a common MACPF domain that is responsible for membrane insertion and pore formation. Hadders et al. 2007 determined the crystal structure of the MACPF domain of complement component C8α at 2.5 angstrom resolution and showed that it is structurally homologous to the bacterial, pore-forming, cholesterol-dependent cytolysins (TC #1.C.12). The structure displays two regions that (in the bacterial cytolysins) refold into transmembrane ß hairpins, forming the lining of a barrel pore. Local hydrophobicity explains why C8α is the first complement protein to insert into the membrane. The size of the MACPF domain is consistent with known C9 pore sizes. Thus, these mammalian and bacterial cytolytic proteins share a common mechanism of membrane insertion (Hadders et al., 2007).
Proteins containing membrane attack complex/perforin (MACPF) domains play important roles in vertebrate immunity, embryonic development, and neural-cell migration among others (Anderluh and Lakey, 2008). In vertebrates, the ninth component of complement and perforin form oligomeric pores that lyse bacteria and kill virus-infected cells, respectively. Rosada et al. 2007 determined the crystal structure of a bacterial MACPF protein, Plu-MACPF from Photorhabdus luminescens, to 2.0 angstrom resolution. The MACPF domain revealed structural similarity with pore-forming cholesterol-dependent cytolysins (CDCs; 1.C.12) from Gram-positive bacteria. This suggests that lytic MACPF proteins may use a CDC-like mechanism to form pores and disrupt cell membranes. Sequence similarity between bacterial and vertebrate MACPF domains suggests that the fold of the CDCs, a family of proteins important for bacterial pathogenesis, is probably used by vertebrates for defense against infection.
The secretory granule-mediated cell death pathway is the key mechanism for elimination of virus-infected and transformed target cells by cytotoxic lymphocytes (Voskoboinik et al. 2010). 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. Perforin's structure and function as well as its role in immune-mediated diseases have been reviewed (Voskoboinik et al. 2010). pH influsences perforin's permeabilizing activity but not its binding to membranes (Praper et al. 2010).
Law et al. (2010) elucidated the mechanism of perforin pore formation by determining the X-ray crystal structure of monomeric murine perforin, together with a cryo-electron microscopy reconstruction of the entire perforin pore. Perforin is a thin 'key-shaped' molecule, comprising an amino-terminal membrane attack complex perforin-like (MACPF)/cholesterol dependent cytolysin (CDC) domain followed by an epidermal growth factor (EGF) domain that, together with the extreme carboxy-terminal sequence, forms a central shelf-like structure. A C-terminal C2 domain mediates initial, Ca2+-dependent membrane binding. Electron microscopy revealed that the orientation of the perforin MACPF domain in the pore is inside-out relative to the subunit arrangement in CDCs. These data reveal remarkable flexibility in the mechanism of action of the conserved MACPF/CDC fold (Law et al., 2010). The 2.5Å structure of human C8 protein provided mechanistic insight into membrane pore formation by complement. C8-C9 and C9-C9 interactions facilitate refolding and insertion of putative MACPF transmembrane β-hairpins to form a circular pore (Lovelace et al., 2011).
Perforin (PFN) is produced by cytotoxic lymphocytes and aids in the clearance of tumor or virus-infected cells by a pore forming mechanism. Praper et al. (2011) showed that perforin forms heterogeneous pores with a broad range of conductances, from 0.15 to 21 nanosiemens. In comparison with large pores that possessed low noise and remained stably open, small pores exhibited high noise and were unstable. Furthermore, the opening step and the pore size were dependent on the lipid composition of the membrane. The heterogeneity in pore sizes was confirmed with cryo-electron microscopy and showed a range of sizes matching that observed in the conductance measurements. Two different membrane-bound PFN conformations were interpreted as pre-pore and pore states of the protein. PFN probably forms heterogeneous pores through a multistep mechanism.
The complement Membrane Attack Complex (MAC) forms transmembrane pores in pathogen membranes. The first step in MAC assembly is cleavage of C5 to generate metastable C5b, which forms a stable complex with C6, termed C5b-6. C5b-6 initiates pore formation via the sequential recruitment of homologous proteins: C7, C8, and 12-18 copies of C9, each of which comprise a central MACPF domain flanked by auxiliary domains. Aleshin et al. (2012) proposed a model of pore assembly, in which the auxiliary domains play key roles, both in stabilizing the closed conformation of the protomers, and in driving the sequential opening of the MACPF β-sheet of each new recruit to the growing pore. They also described an atomic model of C5b-6 at 4.2 Å resolution and showed that C5b provides 4 interfaces for the auxiliary domains of C6. The largest interface is created by the insertion of an interdomain linker from C6 into a hydrophobic groove created by a major reorganization of the α-helical domain of C5b. In combination with a rigid-body docking of N-terminal elements of both proteins, C5b becomes locked into a stable conformation. Both C6 auxiliary domains flanking the linker pack tightly against C5b. The net effect is to induce a clockwise rigid-body rotation of 4 auxiliary domains as well as an opening/twisting of the central β-sheet of C6, in the directions predicted to activate or prime C6 for the subsequent steps in MAC assembly. The complex also suggests novel small-molecule strategies for modulating pathological MAC assembly (Aleshin et al., 2012).
Cytotoxic lymphocytes use perforin to eliminate dangerous cells, while remaining refractory to lysis. At least two mechanisms jointly preserve the killer cell: the C-terminal residues of perforin dictate its rapid export from the ER, whose milieu otherwise favours pore formation. Perforin is then stored in secretory granules whose acidity prevent its oligomerisation. Following exocytosis, perforin delivers the proapoptotic protease, granzyme B, into the target cell by disrupting its plasma membrane. The defined crystal structure of the perforin monomer and cryo-electron microscopy (EM) of the entire pore suggest that passive transmembrane granzyme diffusion is the dominant proapoptotic mechanism (Lopez et al., 2012).
The generalized transport reaction catalyzed by MACPF family members is:
small and large molecules (in) ⇌ small and large molecules (out).