Assembly and Regulation of the Membrane Attack Complex Based on Structures of C5b6 and sC5b9

Susan M. Lea,Doryen Bubeck,Svetlana Hakobyan,Federico Forneris,Pietro Roversi,Piet Gros,Michael K. Pangburn,Oscar Llorca,Michael A. Hadders,B. Paul Morgan

doi:10.1016/j.celrep.2012.02.003

Abstract

Activation of the complement system results in formation of membrane attack complexes (MACs), pores that disrupt lipid bilayers and lyse bacteria and other pathogens. Here, we present the crystal structure of the first assembly intermediate, C5b6, together with a cryo-electron microscopy reconstruction of a soluble, regulated form of the pore, sC5b9. Cleavage of C5 to C5b results in marked conformational changes, distinct from those observed in the homologous C3-to-C3b transition. C6 captures this conformation, which is preserved in the larger sC5b9 assembly. Together with antibody labeling, these structures reveal that complement components associate through sideways alignment of the central MAC-perforin (MACPF) domains, resulting in a C5b6-C7-C8β-C8α-C9 arc. Soluble regulatory proteins below the arc indicate a potential dual mechanism in protection from pore formation. These results provide a structural framework for understanding MAC pore formation and regulation, processes important for fighting infections and preventing complement-mediated tissue damage.

Highlights

Proteins of the terminal pathway of complement provide immune protection by forming lytic pores, membrane attack complexes (MACs), in membranes (Esser, 1994)
Crystals diffracted anisotropically to a resolution between 3.5 and 4.2 A, and the structure was solved by molecular replacement (Table S1)
The concerted movement of MG7 and CUB resembles that observed for the C3-C3b transition, the position of the connected TED differs dramatically (Figures 1D and 1E, Table S2)

Summary

Introduction

Proteins of the terminal pathway of complement provide immune protection by forming lytic pores, membrane attack complexes (MACs), in membranes (Esser, 1994). Inserted C5b8 functions as a receptor for C9 and catalyzes its oligomerization, leading to membrane perforation and target cell lysis (Podack et al, 1982; Tschopp, 1984; Tschopp et al, 1985). Off-target assembly of the MAC in solution leads to binding of clusterin and vitronectin, yielding a soluble complex called sC5b9 or sMAC. Dissociation of these chaperones by detergents reconstitutes membrane binding (Bhakdi et al, 1979; Podack and Muller-Eberhard, 1980). SC5b9 and the lytic MAC share a neo-epitope present in C9 that is associated with pore formation (Mollnes et al, 1985), suggesting similarities in how the soluble and membrane-associated complexes are assembled

Results

Discussion

Conclusion