title: Sushi domain-containing protein 4 (SUSD4) inhibits complement by disrupting the formation of the classical C3 convertase.
abstract: Recently discovered Sushi domain-containing protein 4 (SUSD4) contains several Sushi or complement control protein domains; therefore, we hypothesized that it may act as complement inhibitor. Two isoforms of human SUSD4, fused to the Fc part of human IgG, were recombinantly expressed in Chinese hamster ovary (CHO) cells. The secreted soluble isoform of SUSD4 (SUSD4b) inhibited the classical and lectin complement pathways by 50% at a concentration of 0.5 μM. This effect was due to the fact that 1 μM SUSD4b inhibited the formation of the classical C3 convertase by 90%. The membrane-bound isoform (SUSD4a) inhibited the classical and alternative complement pathways when expressed on the surface of CHO cells but not when expressed as a soluble, truncated protein. In all functional studies, we used known complement inhibitors as positive controls, while Coxsackie adenovirus receptor, which has no effect on complement, expressed with Fc tag, was a negative control. We also studied the mRNA expression of both isoforms of SUSD4 in a panel of human tissues using quantitative PCR and primarily found SUSD4a in esophagus and brain, while SUSD4b was highly expressed in esophagus, ovary, and heart. Overall, our results show that SUSD4 is a novel complement inhibitor with restricted expression.
authors.affiliation: Department of Laboratory Medicine, Division of Medical Protein Chemistry, Lund University, Malmö, Sweden.
authors.forename: Emelie
authors.initials: E
authors.lastname: Holmquist
authors.forename: Marcin
authors.initials: M
authors.lastname: Okroj
authors.forename: Björn
authors.initials: B
authors.lastname: Nodin
authors.forename: Karin
authors.initials: K
authors.lastname: Jirström
authors.forename: Anna M
authors.initials: AM
authors.lastname: Blom
mesh_terms: D000818:Animals; D016466:CHO Cells; D003176:Complement C3 / Q000378:metabolism; D051574:Complement C3 Convertase, Classical Pathway / Q000037:antagonists & inhibitors* / Q000096:biosynthesis*; D003169:Complement Inactivator Proteins / Q000235:genetics / Q000502:physiology*; D003171:Complement Pathway, Classical; D006224:Cricetinae; D003412:Cricetulus; D005260:Female; D006801:Humans; D007113:Immunity, Innate / Q000235:genetics; D008565:Membrane Proteins / Q000235:genetics / Q000502:physiology*; D009369:Neoplasms / Q000276:immunology / Q000378:metabolism; D020033:Protein Isoforms / Q000235:genetics / Q000502:physiology; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000502:physiology; D014018:Tissue Distribution
chemical_list: D003176:Complement C3; D003169:Complement Inactivator Proteins; D008565:Membrane Proteins; D020033:Protein Isoforms; D011993:Recombinant Fusion Proteins; C582624:SUSD4 protein, human; D051574:Complement C3 Convertase, Classical Pathway
keywords: CCP domain; innate immunity