Structural insights into the central complement component C3

C3 is a central protein of the complement system, which is important to immune defense and provides a link between innate and adaptive immunity. Three pathways of complement activation converge at the activation of C3 yielding a diverse set of biological responses. This versatile and flexible molecule interacts with various proteins to fulfill its functions. Here we review recent insights gained from the crystal structure determinations of human, native C3 and its physiological down-regulation product C3c. The data provided, for the first time, a complete and detailed view of the composition and arrangement of the domains in C3. Comparison of C3 with C3c indicates marked flexibility of the molecule, particularly in the α-chain. We discuss the observed domain rearrangements, conformational changes and the location of various protein binding sites. These detailed, and structural, insights are important for developing models of the molecular mechanisms underlying the diverse biological activities of this large and complex molecule.