Structural basis of soluble membrane attack complex packaging for clearance

Abstract

Unregulated complement activation causes inflammatory and immunological pathologies with consequences for human disease. To prevent bystander damage during an immune response, extracellular chaperones (clusterin and vitronectin) capture and clear soluble precursors to the membrane attack complex (sMAC). However, how these chaperones block further polymerization of MAC and prevent the complex from binding target membranes remains unclear. Here, we address that question by combining cryo electron microscopy (cryoEM) and cross-linking mass spectrometry (XL-MS) to solve the structure of sMAC. Together our data reveal how clusterin recognizes and inhibits polymerizing complement proteins by binding a negatively charged surface of sMAC. Furthermore, we show that the pore-forming C9 protein is trapped in an intermediate conformation whereby only one of its two transmembrane β-hairpins has unfurled. This structure provides molecular details for immune pore formation and helps explain a complement control mechanism that has potential implications for how cell clearance pathways mediate immune homeostasis.

Soluble MAC is primarily released from MAC-resistant bacteria that potently convert complement component C5

“…sMAC is believed to consist of C5b-7, C8, multiple copies of C9, and several copies of Vn and Clu ( Barnum et al, 2020 ; Preissner et al, 1989 ). This was recently supported by Menny et al, 2021 , who revealed that sMAC generated by zymosan particles in serum contains at least one to three copies of C9. Our data suggest that sMAC generated by bacteria is present in similar stoichiometries, but on average, contains less C9 molecules, especially for sMAC generated by MAC-resistant E. coli ( Figures 6 and 7 ).…”

“… Menny et al, 2021 recently reported that the commercially available sMAC used in our study consists of C5b-8, one to three copies of C9 and several copies of Vn and Clu, using proteomics, cross-linking MS, and cryo-electron microscopy. Here, we compared the average composition of sMAC generated by MAC-resistant E. coli with commercial sMAC by profiling sMAC components with liquid chromatography-tandem mass spectrometry (LC-MS/MS).…”

The chaperone Clusterin in neurodegeneration−friend or foe?

“…[38] In addition to its chaperone capacity, Clusterin functions in sperm maturation, [39] cell differentiation, [40] regulation of cell death and survival mechanisms, [41] and as an anti-inflammatory inhibitor of the complement system. [42,43] Moreover, it is often overlooked that Clusterin is an apolipoprotein (ApoJ) that has been identified in plasma high-density lipoprotein particles, suggesting a role in lipid and cholesterol metabolism. [44] Indeed, together with ApoE, Clusterin is one of the major apolipoproteins in the brain parenchyma, but its role in lipid metabolism in the central nervous system (CNS) is not well understood.…”

“…[137] Clusterin exerts anti-inflammatory effects mainly by suppressing complement activation. [42,43] Because synapse pruning by astrocytes and microglia during development involves the complement system, [138] upregulation of complement proteins in AD has been associated with synapse loss and cognitive decline. [139] Accordingly, inhibition of the complement system by Clusterin may be neuroprotective.…”

The neoepitope of the complement C5b-9 Membrane Attack Complex is formed by proximity of adjacent ancillary regions of C9

“…Recent structural studies of the MAC and its components have provided insight into the final assembled architecture and the structural rearrangements that enable its assembly, such as the canonical unravelling of the transmembrane β-hairpins within the MACPF/CDC domain and global shifts of its ancillary domains (20). These studies and others indicate that large scale conformational rearrangements, such as movement of TMH1 in C9, are characteristic of MAC assembly (21)(22)(23).…”