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CryoEM reveals how the complement membrane attack complex ruptures lipid bilayers

Anaïs Menny1, Marina Serna1,2, Courtney M Boyd1

  • 1Department of Life Sciences, Sir Ernst Chain Building, Imperial College London, London, SW7 2AZ, UK.

Nature Communications
|December 16, 2018
PubMed

Insights

The membrane attack complex (MAC) uses distinct mechanisms to disrupt cell membranes. It bends membranes initially and later increases rigidity, leading to pore formation for pathogen lysis and cell regulation.

Area of Science:

  • Immunology
  • Structural Biology
  • Biophysics

Background:

  • The membrane attack complex (MAC) is a crucial component of the complement system, involved in innate immunity.
  • MAC forms pores in target cell membranes, leading to lysis and impacting tissue homeostasis.
  • The precise mechanism by which MAC disrupts lipid bilayers remains incompletely understood.

Purpose of the Study:

  • To elucidate the structural mechanisms by which the membrane attack complex (MAC) interacts with and disrupts lipid bilayers.
  • To provide a detailed structural understanding of MAC pore formation and its functional implications.

Main Methods:

  • Electron cryo-microscopy (CryoEM) was employed to determine high-resolution structures of MAC intermediates.
  • Flicker spectroscopy was utilized to probe the biophysical interactions of MAC components with lipid bilayers.
  • Structural reconstructions revealed the dynamic assembly and architecture of the MAC pore.

Main Results:

  • MAC exhibits dual modes of interaction with lipid bilayers: initial association with the outer leaflet (C6/C7) reducing bending energy, followed by bilayer traversal (C8/C9) increasing membrane rigidity.
  • CryoEM reconstructions unveiled the plasticity of the MAC pore and the role of C5b6 as a scaffold for β-barrel assembly.
  • A glycan scaffold was identified as supporting the structure of the assembled MAC pore.

Conclusions:

  • The study provides a structural framework for understanding how β-pore forming proteins, like MAC, breach membranes.
  • MAC employs a multi-step mechanism involving distinct protein-lipid interactions to achieve membrane disruption.
  • This research sheds light on the molecular mechanisms underlying MAC-mediated pathogen killing and regulation of cellular functions.

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