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Structure of human complement C5 complexed with tick inhibitors OmCI, RaCI1 and CirpT1
Structural highlights
FunctionC5I_ORNMO Bifunctional protein derived from blood-feeding ticks that specifically prevents complement-mediated C5 activation and acts as a scavenger for inflammatory modulators such as leukotriene B4 (LTB4). C5 and LTB4 binding activities are independent (PubMed:23625922). Inhibits classical and alternative pathways of complement activation by preventing the cleavage of complement C5 by both classical and alternative C5 convertases (Probable) (PubMed:15699138). Inhibits complement in all species tested (rabbit, rat, guinea pig, mouse, pig, and human) (PubMed:27018802). Also binds fatty acids such as palmitoleic acid and ricinoleic acid, as well as inflammatory modulators LTB4 (and presumably arachidonic acid (AA)) that may be sequestered to interfere with the host inflammatory response (PubMed:23625922). Does not bind to leukotriene C4 and thromboxane B2 (PubMed:23625922). In vivo, when tested on the mouse model of immune complex-induced acute lung injury (IC-ALI), shows a potent inhibitory activity of the pathology, equally dependent on both C5 inhibition and LTB4 binding for full activity (PubMed:23625922).[1] [2] [3] [4] Publication Abstract from PubMedThe complement system is a crucial part of innate immune defenses against invading pathogens. The blood-meal of the tick Rhipicephalus pulchellus lasts for days, and the tick must therefore rely on inhibitors to counter complement activation. We have identified a class of inhibitors from tick saliva, the CirpT family, and generated detailed structural data revealing their mechanism of action. We show direct binding of a CirpT to complement C5 and have determined the structure of the C5-CirpT complex by cryoelectron microscopy. This reveals an interaction with the peripheral macro globulin domain 4 (C5_MG4) of C5. To achieve higher resolution detail, the structure of the C5_MG4-CirpT complex was solved by X-ray crystallography (at 2.7 A). We thus present the fold of the CirpT protein family, and provide detailed mechanistic insights into its inhibitory function. Analysis of the binding interface reveals a mechanism of C5 inhibition, and provides information to expand our biological understanding of the activation of C5, and thus the terminal complement pathway. An inhibitor of complement C5 provides structural insights into activation.,Reichhardt MP, Johnson S, Tang T, Morgan T, Tebeka N, Popitsch N, Deme JC, Jore MM, Lea SM Proc Natl Acad Sci U S A. 2019 Dec 23. pii: 1909973116. doi:, 10.1073/pnas.1909973116. PMID:31871188[5] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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