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Structure of the poly-C9 component of the complement membrane attack complex.

Natalya V Dudkina1, Bradley A Spicer2,3, Cyril F Reboul2,3

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The membrane attack complex (MAC) component 9 (C9) forms pores by assembling from solution, not requiring a membrane. Its structure reveals how thrombospondin-1 domains facilitate this unique self-assembly process.

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Area of Science:

  • Immunology
  • Structural Biology
  • Complement System

Background:

  • The membrane attack complex (MAC) is crucial for pathogen lysis.
  • Complement component 9 (C9) is the primary building block of the MAC.
  • Unlike related proteins, C9 assembles from solution, a mechanism not fully understood.

Purpose of the Study:

  • To elucidate the molecular mechanism of MAC assembly.
  • To determine the structure of soluble poly-C9.

Main Methods:

  • Cryo-electron microscopy (cryo-EM) at 8 Å resolution.
  • Structural analysis of soluble poly-C9.

Main Results:

  • Revealed a 22-fold symmetrical arrangement of C9 molecules.
  • Identified an 88-strand pore-forming β-barrel structure.
  • Demonstrated the significant role of the N-terminal thrombospondin-1 (TSP1) domain in C9 oligomerization and solution-based assembly.

Conclusions:

  • The structure of soluble poly-C9 provides insights into MAC assembly.
  • TSP1 domains are key to C9's ability to assemble in solution.
  • This mechanism facilitates the recruitment of C9 to the MAC on target membranes.