University of Leicester, United Kingdom
Posters & Accepted Abstracts: J Clin Cell Immunol
The classical pathway of complement activation triggers lysis and opsonization of invading pathogens and stimulates inflammatory and adaptive immune responses resulting in auto-immune diseases and graft-rejection in organ transplantation. It is initiated via large multicomponent assembly, known as C1 (790kDa), that binds to immune complexes, protein modulators (e.g., C-reactive protein), and polyanionic structures on pathogens and apoptotic cells. It is composed of a large recognition subcomponent, C1q (460kDa), with a bouquet-like architecture consisting of six collagenous stems, each linked to a globular head, and four serine protease subcomponents, two C1r polypeptides (90kDa) and two C1s polypeptides (80kDa) that in the absence of C1q form a Ca2+-dependent heterotetramer. Binding to pathogens induces auto-activation in a stepwise fashion: C1r auto- activates and then activates C1s. C1s subsequently cleaves substrates C4 and C4b-bound C2 to form the C3 convertase (C4b2a), the next enzyme in the pathway. Here we describe the structure of the C1rÔ??C1s interaction in the form of a complex between the CUB1-EGF-CUB2 fragments of each protease highlighting the conformational changes during activation. The fragments form Ca2+-dependent heterodimers both in solution and in the crystals. The interface is extensive and spans all three domains of each protease. Supporting the traditional arrangement in which C1r-C1s heterodimers are linked via interactions between the catalytic domains of C1r. In association with C1q, the C1r-C1r contacts would prevent auto-activation of C1r as the proteases fold up with the C1r-C1s dimers at the center. Disruption of the C1r contacts when C1 binds to an activating surface very likely triggers auto-activation of C1r and subsequent activation of C1s. Activation is likely facilitated through hyper flexibility at the C1s EGFCUB2 junction, enabling considerable movement of the catalytic domains.
Jamal Almitairi is a 3rd year PhD student at the University of Leicester working in structural biology and complement related diseases. Part of his work was published in the following journal: PNAS