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Crystal Structure of Human Complement Component C8
Structural highlights
DiseaseCO8A_HUMAN Defects in C8A are a cause of complement component 8 deficiency type 1 (C8D1) [MIM:613790. A rare defect of the complement classical pathway associated with susceptibility to severe recurrent infections, predominantly by Neisseria gonorrhoeae or Neisseria meningitidis. FunctionCO8A_HUMAN Constituent of the membrane attack complex (MAC) that plays a key role in the innate and adaptive immune response by forming pores in the plasma membrane of target cells. C8A inserts into the target membrane, but does not form pores by itself.[1] [2] Publication Abstract from PubMedC8 is one of five complement proteins that assemble on bacterial membranes to form the lethal pore-like "membrane attack complex" (MAC) of complement. The MAC consists of one C5b, C6, C7, and C8 and 12-18 molecules of C9. C8 is composed of three genetically distinct subunits, C8alpha, C8beta, and C8gamma. The C6, C7, C8alpha, C8beta, and C9 proteins are homologous and together comprise the MAC family of proteins. All contain N- and C-terminal modules and a central 40-kDa membrane attack complex perforin (MACPF) domain that has a key role in forming the MAC pore. Here, we report the 2.5 A resolution crystal structure of human C8 purified from blood. This is the first structure of a MAC family member and of a human MACPF-containing protein. The structure shows the modules in C8alpha and C8beta are located on the periphery of C8 and not likely to interact with the target membrane. The C8gamma subunit, a member of the lipocalin family of proteins that bind and transport small lipophilic molecules, shows no occupancy of its putative ligand-binding site. C8alpha and C8beta are related by a rotation of approximately 22 degrees with only a small translational component along the rotation axis. Evolutionary arguments suggest the geometry of binding between these two subunits is similar to the arrangement of C9 molecules within the MAC pore. This leads to a model of the MAC that explains how C8-C9 and C9-C9 interactions could facilitate refolding and insertion of putative MACPF transmembrane beta-hairpins to form a circular pore. Structure of human C8 protein provides mechanistic insight into membrane pore formation by complement.,Lovelace LL, Cooper CL, Sodetz JM, Lebioda L J Biol Chem. 2011 May 20;286(20):17585-92. Epub 2011 Mar 25. PMID:21454577[3] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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