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...