Structures of complement component C3 provide insights into the function and evolution of immunity

Abstract

The mammalian complement system is a phylogenetically ancient cascade system that has a major role in innate and adaptive immunity. Activation of component C3 (1,641 residues) is central to the three complement pathways and results in inflammation and elimination of self and non-self targets. Here we present crystal structures of native C3 and its final major proteolytic fragment C3c. The structures reveal thirteen domains, nine of which were unpredicted, and suggest that the proteins of the alpha2-macroglobulin family evolved from a core of eight homologous domains. A double mechanism prevents hydrolysis of the thioester group, essential for covalent attachment of activated C3 to target surfaces. Marked conformational changes in the alpha-chain, including movement of a critical interaction site through a ring formed by the domains of the beta-chain, indicate an unprecedented, conformation-dependent mechanism of activation, regulation and biological function of C3.

Deciphering complement mechanisms: The contributions of structural biology

“…Thus, only a few of the deposited structures are of complete proteins, and most of them correspond to functional fragments or domains. A remarkable exception to this rule is the structure of the large protein C3 (1641 residues, 13 domains), which was recently solved by X-ray crystallography (Janssen et al, 2005;Fredslund et al, 2006).…”

Conformational Complexity of Complement Component C3

“…C3o and C3c differ by the presence of residues 933-942 in C3o. Thus, the binding data suggest that this residue stretch (forming strand β5 and its flanking loops 5 in the CUB domain in C3) is involved in factor B binding. One possible binding site for factor B located on the β-chain of C3 has been suggested based on sequence homology.…”

“…The thioester is intact in the crystals, as indicated by the electron density at the thioester linkage, and is occluded in the structure of C3 as expected for the native conformation of C3. The structure of C3 is characterized by an intricate arrangement of 13 domains (see Figure 1; domain names and secondary structure labeling as defined in [5]). The core of the structure is formed by eight homologous domains from both the β-and α-chains with an additional five domains appearing as inserts and a C-terminal extension.…”

“…Properdin is predominantly positively charged, especially the domains TSR-3, 5, and 6 72 (TSR-4, 5, and 6 have been shown to be involved in C3b binding 72,77 ). C3 and C3c are mainly negatively charged 5 , and factor Bb has dispersed positive-and negative-charged patches 19 . Possibly some of the positive patches of Bb are involved in electrostatic interaction with C3b, resulting in a mainly negatively charged C3bBb complex in good agreement with ionic strengthdependent binding of the positively charged properdin.…”

“…Nonetheless, in the last decade significant advances have been made in the structure determination of complement proteins and protein domains (see Table 1). Most recently, we published the structures of native C3 and its major proteolytic fragment C3c 5 . These structures provide insight into the domain organization, structure, and dynamics of the central C3 protein of the complement system.…”