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Complement component 9 (C9) is a MACPFprotein involved in the complement system, which is part of the innate immune system.[5] Once activated, about 12-18 molecules of C9 polymerize to form pores in target cell membranes, causing lysis and cell death.[6] C9 is one member of the complement membrane attack complex (MAC), which also includes complement components C5b, C6, C7 and C8.[7][8][9] The formation of the MAC occurs through three distinct pathways: the classical, alternative, and lectin pathways.[7] Pore formation by C9 is an important way that bacterial cells are killed during an infection, and the target cell is often covered in multiple MACs. The clinical impact of a deficiency in C9 is an infection with the gram-negative bacterium Neisseria meningitidis.[10]
Structure
C9 genes include 11 exons and 10 introns when found in fish.[11] In fish, the liver is the site where the majority of complement components are produced and expressed, but C9 can also be found in other tissues.[11] It is a single-chain glycoprotein with a four domain structure arranged in a globular bundle.[10][11]
Pore Formation in the Membrane Attack Complex
MAC formation starts with the assembly of a tetrameric complex with the complement components C6,C7, C8, and C5b.[12] The final step of MAC on target cell surfaces involves the polymerization of C9 molecules bound to C5b8 forming C5b-9.[8][10][13] C9 molecules allow cylindrical, asymmetrical transmembrane pores to form. The overall complex belongs to MAC/perforin-like (MACPF)/CDC superfamily.[14] Pore formation involves binding the C9 molecules to the target membrane, membrane molecules forming a pre-pore shape, and conformational change in the TMH1, the first transmembrane region, and TMH2, the second transmembrane region.[8][15] The formations of pores leads to the killing of foreign pathogens and infected host cells.
