Abstract
The complement membrane attack complex (MAC) forms transmembrane pores in pathogen membranes. The first step in MAC assembly is cleavage of C5 to generate metastable C5b, which forms a stable complex with C6, termed C5b-6. C5b-6 initiates pore formation via the sequential recruitment of homologous proteins: C7, C8, and 12-18 copies of C9, each of which comprises a central MAC-perforin domain flanked by auxiliary domains. We recently proposed a model of pore assembly, in which the auxiliary domains play key roles, both in stabilizing the closed conformation of the protomers and in driving the sequential opening of the MAC-perforin β-sheet of each new recruit to the growing pore. Here, we describe an atomic model of C5b-6 at 4.2 Å resolution. We show that C5b provides four interfaces for the auxiliary domains of C6. The largest interface is created by the insertion of an interdomain linker from C6 into a hydrophobic groove created by a major reorganization of the α-helical domain of C5b. In combination with the rigid body docking of N-terminal elements of both proteins, C5b becomes locked into a stable conformation. Both C6 auxiliary domains flanking the linker pack tightly against C5b. The net effect is to induce the clockwise rigid body rotation of four auxiliary domains, as well as the opening/twisting of the central β-sheet of C6, in the directions predicted by our model to activate or prime C6 for the subsequent steps in MAC assembly. The complex also suggests novel small molecule strategies for modulating pathological MAC assembly.
Highlights
The C5b-6 complex triggers assembly of the Membrane Attack Complex
We recently proposed a model of pore assembly, in which the auxiliary domains play key roles, both in stabilizing the closed conformation of the protomers and in driving the sequential opening of the membrane attack complex (MAC)-perforin -sheet of each new recruit to the growing pore
Conformational Changes in C6—In addition to the gross conformational changes observed in the C-terminal CCPs and FIMs, more subtle changes can be discerned that we propose to be linked to the activation or “priming” of C6 to initiate MAC assembly
Summary
The C5b-6 complex triggers assembly of the Membrane Attack Complex. Results: The structure of C5b-6 at 4.2 Å resolution allowed an atomic model to be built. We recently proposed a model of pore assembly, in which the auxiliary domains play key roles, both in stabilizing the closed conformation of the protomers and in driving the sequential opening of the MAC-perforin -sheet of each new recruit to the growing pore. The net effect is to induce the clockwise rigid body rotation of four auxiliary domains, as well as the opening/twisting of the central -sheet of C6, in the directions predicted by our model to activate or prime C6 for the subsequent steps in MAC assembly. We suggested how the rotation of auxiliary modules at the leading edge of the nascent MAC could mediate optimal packing interactions with the new recruit and trigger the opening of its -sheet, leading to the release of clusters of helices (CH), which would form the membrane attachment/spanning elements [14]. Crystal Structure of Complement C5b-6 particular, it shows how C5b engages the auxiliary domains of C6 in an intimate embrace that “primes” C6 for initiating MAC assembly
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
