title: Complement Evasion Mediated by Enhancement of Captured Factor H: Implications for Protection of Self-Surfaces from Complement.
abstract: In an attempt to evade annihilation by the vertebrate complement system, many microbes capture factor H (FH), the key soluble complement-regulating protein in human plasma. However, FH is normally an active complement suppressor exclusively on self-surfaces and this selective action of FH is pivotal to self versus non-self discrimination by the complement system. We investigated whether the bacterially captured FH becomes functionally enhanced and, if so, how this is achieved at a structural level. We found, using site-directed and truncation mutagenesis, surface plasmon resonance, nuclear magnetic resonance spectroscopy, and cross-linking and mass spectrometry, that the N-terminal domain of Streptococcus pneumoniae protein PspC (PspCN) not only binds FH extraordinarily tightly but also holds it in a previously uncharacterized conformation. Functional enhancement arises from exposure of a C-terminal cryptic second binding site in FH for C3b, the activation-specific fragment of the pivotal complement component, C3. This conformational change of FH doubles its affinity for C3b and increases 5-fold its ability to accelerate decay of the binary enzyme (C3bBb) responsible for converting C3 to C3b in an amplification loop. Despite not sharing critical FH-binding residues, PspCNs from D39 and Tigr4 S. pneumoniae exhibit similar FH-anchoring and enhancing properties. We propose that these bacterial proteins mimic molecular markers of self-surfaces, providing a compelling hypothesis for how FH prevents complement-mediated injury to host tissue while lacking efficacy on virtually all other surfaces. In hemolysis assays with 2-aminoethylisothiouronium bromide-treated erythrocytes that recapitulate paroxysmal nocturnal hemoglobinuria, PspCN enhanced protection of cells by FH, suggesting a new paradigm for therapeutic complement suppression.
authors.affiliation: School of Chemistry, University of Edinburgh, Edinburgh EH9 3FJ, United Kingdom;
authors.forename: Andrew P
authors.initials: AP
authors.lastname: Herbert
authors.affiliation: School of Chemistry, University of Edinburgh, Edinburgh EH9 3FJ, United Kingdom;
authors.forename: Elisavet
authors.initials: E
authors.lastname: Makou
authors.affiliation: School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3BF, United Kingdom; and.
authors.forename: Zhuo A
authors.initials: ZA
authors.lastname: Chen
authors.affiliation: School of Chemistry, University of Edinburgh, Edinburgh EH9 3FJ, United Kingdom; Queen's Medical Research Institute, Edinburgh EH16 4TJ, United Kingdom.
authors.forename: Heather
authors.initials: H
authors.lastname: Kerr
authors.affiliation: Queen's Medical Research Institute, Edinburgh EH16 4TJ, United Kingdom.
authors.forename: Anna
authors.initials: A
authors.lastname: Richards
authors.affiliation: School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3BF, United Kingdom; and.
authors.forename: Juri
authors.initials: J
authors.lastname: Rappsilber
authors.affiliation: School of Chemistry, University of Edinburgh, Edinburgh EH9 3FJ, United Kingdom; Paul.Barlow@ed.ac.uk A.Herbert@ed.ac.uk.
authors.forename: Paul N
authors.initials: PN
authors.lastname: Barlow
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000276:immunology; D003179:Complement C3b / Q000737:chemistry* / Q000276:immunology; D017242:Complement Factor H / Q000737:chemistry* / Q000276:immunology; D006457:Hemoglobinuria, Paroxysmal / Q000276:immunology; D006801:Humans; D017434:Protein Structure, Tertiary; D013296:Streptococcus pneumoniae / Q000737:chemistry* / Q000276:immunology
chemical_list: D001426:Bacterial Proteins; C110344:SpsA protein, Streptococcus pneumoniae; D003179:Complement C3b; D017242:Complement Factor H