Binding of C4b-Binding Protein to Porin

The group B streptococcus (GBS) is the most important cause of life-threatening bacterial infections in newborn infants. Protective immunity to GBS infection is elicited by several surface proteins, one of which, the ␤ protein, is known to bind human IgA-Fc. Here, we show that the ␤ protein also binds human factor H (FH), a negative regulator of complement activation. Absorption experiments with whole human plasma demonstrated binding of FH to a GBS strain expressing ␤ protein but not to an isogenic ␤-negative mutant. This binding was due to a direct interaction between ␤ and FH, as shown by experiments with purified proteins. Inhibition tests and studies with ␤ fragments demonstrated that FH and IgA-Fc bind to separate and nonoverlapping regions in ␤. Heparin, a known ligand for FH, specifically inhibited the binding between ␤ and FH, suggesting that FH has overlapping binding sites for ␤ and heparin. Bacteria-bound FH retained its complement regulatory activity, implying that ␤-expressing GBS may use bound FH to evade complement attack. The finding that ␤ protein binds FH adds to a growing list of interactions between human pathogens and complement regulatory proteins, supporting the notion that these interactions are of general importance in bacterial pathogenesis.

Section: Discussionmentioning

confidence: 99%

Streptococcal β Protein Has Separate Binding Sites for Human Factor H and IgA-Fc

Areschoug

Stålhammar‐Carlemalm

Karlsson

et al. 2002

“…Expression of M protein is important for the ability of S. pyogenes to cause the diseases associated with this pathogen: acute pharyngitis and impetigo (6). Many other pathogens share the ability to sequester C4BP (7)(8)(9)(10)(11)(12). Thus escape from complement attack by hijacking of C4BP is emerging as a widespread contributor to immune evasion strategies and is a therapeutic target.…”

mentioning

confidence: 99%

Human C4b-binding Protein, Structural Basis for Interaction with Streptococcal M Protein, a Major Bacterial Virulence Factor

Jenkins

Mark

Ball

et al. 2006

Self Cite

Human C4b-binding protein (C4BP) protects host tissue, and those pathogens able to hijack this plasma glycoprotein, from complement-mediated destruction. We now show that the first two complement control protein (CCP) modules of the C4BP ␣-chain, plus the four residues connecting them, are necessary and sufficient for binding a bacterial virulence factor, the Streptococcus pyogenes M4 (Arp4) protein. Structure determination by NMR reveals two tightly coupled CCP modules in an elongated arrangement within this region of C4BP. Chemical shift perturbation studies demonstrate that the N-terminal, hypervariable region of M4 binds to a site including strand 1 of CCP module 2. This interaction is accompanied by an intermodular reorientation within C4BP. We thus provide a detailed picture of an interaction whereby a pathogen evades complement.Bacteria that enter human blood or tissues will be opsonized by complement and destroyed by phagocytes unless they have a means of overcoming attack by the complement system. An extensively studied example of such a virulence mechanism is the ability of many Streptococcus pyogenes M proteins to bind the key human complement regulator C4b-binding protein (C4BP), 2 an interaction that endows the bacteria with resistance to phagocytosis (1). C4BP is a polymeric, soluble, glycoprotein (ϳ200 mg/liter in plasma) (2). The M proteins, classical bacterial virulence factors first recognized over 75 years ago (3), are fibrillar surface structures exhibiting antigenic variation within a 50 -100-amino acid residue hypervariable N terminus (4, 5). Expression of M protein is important for the ability of S. pyogenes to cause the diseases associated with this pathogen: acute pharyngitis and impetigo (6). Many other pathogens share the ability to sequester C4BP (7-12). Thus escape from complement attack by hijacking of C4BP is emerging as a widespread contributor to immune evasion strategies and is a therapeutic target.The complement system, a vital molecular component of innate immunity, consists of plasma and cell-surface proteins that conspire to rid the body of infectious particles (13,14). Because complement is potentially harmful to host tissue it is tightly regulated. Like other members of the regulators of complement activation (RCA) protein family, the plasma protein C4BP acts upon the bimolecular C3 convertases that are the main drivers of the complement cascade. The C3 convertases cleave and thereby activate the third component of complement, C3, leading to deposition of C3b on the surface of an unprotected particle such as an invading microorganism. This process, known as opsonization, marks the particle as a target for phagocytosis. C3b also nucleates assembly of further convertase complexes and progression of the cascade toward formation of the membrane attack complex. C4BP regulates the C3 convertase of the classical pathway, a complex of C4b and C2a, by acting as a cofactor for the proteolysis of C4b (15, 16) and accelerating the decay of C4b⅐C2a complexes deposited on host cells (17...

“…Most gonococci that cause bacteremic or disseminated infection are resistant to killing by nonimmune normal human serum (NHS), 2 a property termed "stable" serum resistance (4). Stable serum resistance may in part be mediated by the ability of gonococcal porin B (PorB) to bind plasma complement inhibitory proteins such as factor H (fH) (5) and/or C4b-binding protein (C4BP) (6). PorB molecules are 34 -37-kDa proteins, which exist as trimers in their native configurations and function as selective ion channels (7).…”

mentioning

confidence: 99%

“…Most gonococcal PorB.1A strains can bind fH and/or C4BP (hence they exhibit stable serum resistance), whereas PorB.1B strains bind fH weakly (5) unless their lipooligosaccharide (LOS) is sialylated (5,10). Some PorB.1B strains can bind C4BP independently of LOS sialylation (6).…”

mentioning

confidence: 99%

Molecular Characterization of the Interaction between Sialylated Neisseria gonorrhoeae and Factor H

Shaughnessy¹,

Ram²,

Bhattacharjee

et al. 2011

Self Cite

Human factor H (HufH), a key inhibitor of the alternative pathway of complement, binds to Neisseria gonorrhoeae and constitutes an important mechanism of human-specific complement evasion. The C-terminal domain 20 of HufH contains the binding site for sialylated gonococci. We exploited differences in amino acid sequences between human and non-binding chimpanzee fH domain 20 to create cross-species mutations to define amino acids important for binding to sialylated gonococci. We used fH/Fc fusion constructs that contained contiguous fH domains 18 -20 fused to Fc fragments of murine IgG2a. The Fc region was used both as a tag for detection of each fusion molecule on the bacterial surface and as an indicator for complement-dependent killing. Arg-1203 was critical for binding to both porin (Por) B.1A and PorB.1B strains. Modeling of the R1203N human-to-chimpanzee mutation using the crystal structure of HufH19 -20 as a template showed a loss of positive charge that protrudes at the C terminus of domain 20. We tested the functional importance of Arg-1203 by incubating sialylated gonococci with normal human serum, in the presence of wildtype HufH18 -20/Fc or its R1203A mutant. Gonococci bound and were killed by wild-type HufH18 -20/Fc but not by the R1203A mutant. A recombinant fH/Fc molecule that contained chimpanzee domain 20, humanized only at amino acid 1203 (N1203R) also bound to sialylated gonococci and restored killing. These findings provide further insights into the species specificity of gonococcal infections and proof-of-concept of a novel therapeutic approach against gonorrhea, a disease rapidly becoming resistant to conventional antibiotics.