In a tour de force effort by Piet Gros and colleagues, the long-awaited structure of the central complement component C3 has now been resolved — possibly the largest multidomain protein structure that has been determined so far. With it, some of the mysteries that surround the origin of the complement system have been unlocked, indicating that its key defence mechanisms have changed little over hundreds of millions of years.

The crucial defensive functions of C3 are regulated by several proteolytic-cleavage steps that yield, first, the anaphylatoxin C3a and the active intermediate fragment C3b and, second, the inactive fragment C3c, following the release of C3f and C3dg. To understand how its activity is regulated, the authors resolved and compared the crystal structures of both C3 and C3c.

The core of C3 (and C3c) is composed of eight structurally homologous domains, which the authors term macroglobulin domains, six of which form a ring structure (see figure). This pattern of repeated domains indicates that these domains probably arose through duplication of a primordial gene, but in the intervening millennia, the sequences of the eight domains have diverged. Resting on top of the core ring structure are three domains with specialized functions: the anaphylatoxin domain, the CUB fold and the thioester-containing domain (TED). These domains seem to have been added to the core through gene-insertion events, possibly to provide necessary regulation of these active elements.

Based on the structure by Janssen et al., this figure illustrates the conformational changes that occur on cleavage and activation of complement component C3 to yield the active fragments C3a and C3b. Release of C3a from C3 allows macroglobulin domain 8 to rotate away from the thioester-containing domain (TED). This exposes the reactive thioester and enables it to interact with a histidine residue to form a reactive intermediate. In this activated state, C3b can bind its target cells and other complement components, resulting in the formation of the membrane-attack complex. Modified, with permission, from Nature © (2005), Macmillan Magazines Ltd.

One of the key and most primitive elements of the complement defence mechanism is the presence of an activated thioester in the TED. In C3b, the thioester is exposed, allowing it to bind covalently to its target cells (such as bacteria and apoptotic cells). Importantly, in full-length C3, the thioester is buried between the TED and macroglobulin domain 8 and is therefore protected from hydrolysis (and thereby inactivation). In addition, the orientation of the TED of C3, which is in part maintained by the anaphylatoxin domain, prevents a histidine residue in the TED from interacting with the thioester and forming a reactive intermediate. Cleavage of the anaphylatoxin domain of C3 to produce C3b induces a conformational change that disrupts both of these protective mechanisms and exposes the reactive thioester.

These structures should provide further clues about the function and origin of the complement system and create new avenues for treating diseases that are associated with defects in complement.