Heterogeneous MAC Initiator and Pore Structures in a Lipid Bilayer by Phase-Plate Cryo-electron Tomography

• MAC pores are cone shaped, twisted, and poorly sealed between C9 end and C5b-8 start
• Complement activation yields single and joined MAC pores in lipid membranes
• Oligomerization of three to four C5b-8 complexes causes ∼3.5-nm pores in membranes
• Phase plates markedly increase interpretability of cryo-electron tomograms

SummaryPore formation in membranes is important for mammalian immune defense against invading bacteria. Induced by complement activation, the membrane attack complex (MAC) forms through sequential binding and membrane insertion of C5b6, C7, C8, and C9. Using cryo-electron tomography with a Volta phase plate and subtomogram averaging, we imaged C5b-7, C5b-8, and C5b-9 complexes and determined the C5b-9 pore structure in lipid bilayers. The in situ C5b-9 pore structure at 2.3-nm resolution reveals a 10- to 11.5-nm cone-shaped pore starting with C5b678 and multiple copies of C9 that is poorly closed, yielding a seam between C9 and C6 substituting for the shorter β strands in C6 and C7. However, large variations of composite pore complexes are apparent in subtomograms. Oligomerized initiator complexes C5b-7 and C5b-8 show stages of membrane binding, deformation, and perforation that yield ∼3.5-nm-wide pores. These data indicate a dynamic process of pore formation that likely adapts to biological membranes under attack.

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