title: Identification of a catalytic exosite for complement component C4 on the serine protease domain of C1s.
abstract: The classical pathway of complement is crucial to the immune system, but it also contributes to inflammatory diseases when dysregulated. Binding of the C1 complex to ligands activates the pathway by inducing autoactivation of associated C1r, after which C1r activates C1s. C1s cleaves complement component C4 and then C2 to cause full activation of the system. The interaction between C1s and C4 involves active site and exosite-mediated events, but the molecular details are unknown. In this study, we identified four positively charged amino acids on the serine protease domain that appear to form a catalytic exosite that is required for efficient cleavage of C4. These residues are coincidentally involved in coordinating a sulfate ion in the crystal structure of the protease. Together with other evidence, this pointed to the involvement of sulfate ions in the interaction with the C4 substrate, and we showed that the protease interacts with a peptide from C4 containing three sulfotyrosine residues. We present a molecular model for the interaction between C1s and C4 that provides support for the above data and poses questions for future research into this aspect of complement activation.
authors.affiliation: Department of Biochemistry and Molecular Biology, Monash University, Clayton, Victoria 3800, Australia.
authors.forename: Renee C
authors.initials: RC
authors.lastname: Duncan
authors.forename: Frida
authors.initials: F
authors.lastname: Mohlin
authors.forename: Deni
authors.initials: D
authors.lastname: Taleski
authors.forename: Theresa H
authors.initials: TH
authors.lastname: Coetzer
authors.forename: James A
authors.initials: JA
authors.lastname: Huntington
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Payne
authors.forename: Anna M
authors.initials: AM
authors.lastname: Blom
authors.forename: Robert N
authors.initials: RN
authors.lastname: Pike
authors.forename: Lakshmi C
authors.initials: LC
authors.lastname: Wijeyewickrema
mesh_terms: D000595:Amino Acid Sequence; D001666:Binding Sites, Antibody / Q000276:immunology; D020134:Catalytic Domain / Q000276:immunology*; D003167:Complement Activation / Q000276:immunology*; D003173:Complement C1s / Q000378:metabolism*; D003181:Complement C4 / Q000276:immunology / Q000378:metabolism*; D003171:Complement Pathway, Classical / Q000276:immunology*; D006801:Humans; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000378:metabolism; D057057:Serine Proteases / Q000378:metabolism*
chemical_list: D003181:Complement C4; D010446:Peptide Fragments; D057057:Serine Proteases; D003173:Complement C1s