Assembly and Activation of the C1 Complex of Complement

Complement is an essential component of the innate immune system. The classical pathway of complement activation is initiated by the 774 kDa C1 complex. The C1 complex comprises two C1r and two C1s serine proteases, synthesised as zymogens and arranged as a C1s-C1r-C1r-C1s, Ca2+ dependent, heterotetramer bound to the centre of one C1q molecule. C1q is a hexameric protein formed from six copies of three different polypeptide chains, designated A, B and C, with a bouquet-like architecture. The C1 complex is activated upon interaction with an activating surface such as antigen-antibody complexes, leading to auto-activation of C1r followed by activation of C1s. Two contrasting models have been proposed to explain how the C1 complex assembles and activates; the intramolecular model in which activation is driven by separation and subsequent activation of C1r polypeptides when C1 binds to an activating surface, and the intermolecular model whereby C1r polypeptides separate upon binding to C1q, and neighbouring C1 complexes activate each other. 

In this project, I investigated how C1 assembles, and initiates complement activation at the molecular level. I created stable C1r homodimers by incorporating a disulphide bond at the C1r-C1r interface that prevents the separation of the polypeptides. I showed that these modified C1r molecules could be incorporated into the C1r2C1s2 tetramer and the C1 complex, determining that C1r polypeptides do not need to separate upon binding to C1q and hence providing evidence to support the intramolecular activation mechanism of C1. 

Alongside these findings, I have solved several crystal structures of a fragment of C1s in complex with collagen-like peptides derived from the A, B and C polypeptide chains of C1q. The chains of the collagen triple helix of C1q are staggered, creating a leading, middle, and lagging strand, but the order of these chains was unknown until now. From these structures, I propose that C1qB forms the leading strand, C1qA forms the middle strand, and C1qC forms the lagging strand of C1q.