Complement component 9 (C9) is aMACPFprotein involved in thecomplement system, which is part of theinnate immune system.[5] Once activated, about 12-18 molecules of C9 polymerize to form pores in targetcell membranes, causinglysis and cell death.[6] C9 is one member of thecomplement membrane attack complex (MAC), which also includes complement componentsC5b,C6,C7 andC8.[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 bacteriumNeisseria meningitidis.[10]
C9 genes include 11exons and 10introns 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]
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][11] C9 molecules allow cylindrical, asymmetrical transmembrane pores to form. The overall complex belongs to MAC/perforin-like (MACPF)/CDC superfamily.[6] 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] The formations of pores leads to the killing of foreign pathogens and infected host cells.
^Wickramaarachchi WD, Wan Q, Lee Y, Lim BS, De Zoysa M, Oh MJ, et al. (October 2012). "Genomic characterization and expression analysis of complement component 9 in rock bream (Oplegnathus fasciatus)".Fish & Shellfish Immunology.33 (4):707–17.doi:10.1016/j.fsi.2012.06.019.PMID22796422.
^abcdLi L, Chang MX, Nie P (August 2007). "Molecular cloning, promoter analysis and induced expression of the complement component C9 gene in the grass carp Ctenopharyngodon idella".Veterinary Immunology and Immunopathology.118 (3–4):270–82.doi:10.1016/j.vetimm.2007.05.005.PMID17604124.
^Fu YW, Zhu CK, Zhang QZ (May 2019). "Molecular characterization and expression analysis of complement components C3 and C9 in largemouth bronze gudgeon (Coreius guichenoti) in response to Ichthyophthirius multifiliis infection".Aquaculture.506:270–279.Bibcode:2019Aquac.506..270F.doi:10.1016/j.aquaculture.2019.03.046.S2CID133378035.
