Structural insights on complement activation

Abstract

The proteolytic cleavage of C3 to generate C3b is the central and most important step in the activation of complement, a major component of innate immunity. The comparison of the crystal structures of C3 and C3b illustrates large conformational changes during the transition from C3 to C3b. Exposure of a reactive thio-ester group allows C3b to bind covalently to surfaces such as pathogens or apoptotic cellular debris. The displacement of the thio-ester-containing domain (TED) exposes hidden surfaces that mediate the interaction with complement factor B to assemble the C3-convertase of the alternative pathway (AP). In addition, the displacement of the TED and its interaction with the macroglobulin 1 (MG1) domain generates an extended surface in C3b where the complement regulators factor H (FH), decay accelerating factor (DAF), membrane cofactor protein (MCP) and complement receptor 1 (CR1) can bind, mediating accelerated decay of the AP C3-convertase and proteolytic inactivation of C3b. In the last few years, evidence has accumulated revealing that the structure of C3b in solution is significantly more flexible than anticipated. We review our current knowledge on C3b structural flexibility to propose a general model where the TED can display a collection of conformations around the MG ring, as well as a few specialized positions where the TED is held in one of several fixed locations. Importantly, this conformational heterogeneity in C3b impacts complement regulation by affecting the interaction with regulators. Role of TED relocation in complement activation and regulationComplement is a major component of innate immunity with crucial roles in microbial killing, apoptotic cell clearance, immune complex handling and modulation of adaptive immune responses. During complement activation, three activation pathways converge in the cleavage of the protein C3 to generate the activated fragment C3b. The transition from C3 to C3b involves large conformational changes [1][2][3]. These changes affect several regions in the C3 molecule, but the repositioning of the thio-ester-containing domain (TED) provides a key requirement for complement activation. In C3, the TED blocks most of the surfaces potentially capable of binding to other complement components, thus producing a rather inactive molecule. In C3b, the Abbreviations AP, alternative pathway; C3bB, C3

Structural Implications for the Formation and Function of the Complement Effector Protein iC3b

“…This arrangement is corroborated by EM studies indicating that the intact MG1/TED interface is the most frequently occupied conformation (16). Conversely, other EM studies suggest that C3b occupies various conformations in solution, with the TED domain in different positions around the molecule (17, 38); small-angle X-ray scattering data feature TED in two different orientations, both adjacent to the MG core (30, 39), and X-ray and neutron scattering data even propose a uniform population with TED extending away from MG1 (40). Similarly, there are discrepancies in the EM-based data concerning the arrangement of iC3b; some studies suggest a highly flexible positioning after conversion of CUB (16), whereas one indicates that TED moves into a compact position at the MG core and the CTC domain, with CUB adjacent (17).…”

Mapping the Complement Factor H-Related Protein 1 (CFHR1):C3b/C3d Interactions

“…When mapped onto the surface of the crystal-derived structure of unbound C3b, this second CFH binding surface is partially occluded by a neighboring macroglobulin domain (MG1) meaning that this site would only be accessible for CFH-engagement in the context of C3d, but not the larger C3b molecule [29]. It should be noted, however, that there is accumulating evidence that under solution conditions C3b is a flexible entity in which the TED can adopt a range of conformations with relation to the rest of C3b (reviewed in [30]). Kajander and colleagues have posited that this second C3d-binding site is physiologically important, with the ability of the C-terminus of CFH to simultaneously engage C3b and C3d resulting in a similar phenotype to that which occurs when CFH forms a ternary complex with C3b and anionic markers such as GAGs and sialic acid moieties.…”

Complement factor H in host defense and immune evasion

“…2). This is now understood at a molecular level based on the crystal structure of C3b in complex with CCPs 1–4 of FH [41], where this and other structural studies have provided valuable insights into the functional biology of FH [34–40, 42, 43]. Importantly, these CCPs of FH bind to C3b in an extended configuration, with a remarkably long contact interface that covers the whole flank of the C3b protein [41].…”