Structure of human C8 protein provides mechanistic insight into membrane pore formation by complement

Area of Science:

• Immunology
• Structural Biology
• Biochemistry

Background:

• The complement system is crucial for innate immunity, involving protein complexes that target bacterial membranes.
• The membrane attack complex (MAC) is a key effector of complement, forming pores in target cells.
• Complement protein C8 is a vital component of the MAC, but its structure and function remain incompletely understood.

Purpose of the Study:

• To determine the high-resolution crystal structure of human complement protein C8.
• To elucidate the structural basis for C8's role in MAC formation and pore assembly.
• To provide insights into the molecular mechanisms of complement-mediated bacterial lysis.

Main Methods:

• Purification of human C8 from blood.
• X-ray crystallography to determine the 2.5 Å resolution structure of C8.
• Structural analysis and comparison with homologous proteins.

Main Results:

• The crystal structure of human C8 was determined at 2.5 Å resolution, representing the first structure of a MAC family member and a human MACPF-containing protein.
• The structure revealed that the N- and C-terminal modules of C8α and C8β subunits are peripherally located, suggesting limited direct interaction with target membranes.
• The C8γ subunit, a lipocalin family member, showed no ligand occupancy in its putative binding site, and the arrangement of C8α and C8β subunits suggested a conserved binding geometry with C9.

Conclusions:

• The determined structure of human C8 provides a molecular basis for understanding its role in MAC formation.
• The findings suggest a model where C8-C9 and C9-C9 interactions facilitate the refolding and insertion of MACPF transmembrane β-hairpins to form the MAC pore.
• This study advances our understanding of complement-mediated membrane attack and bacterial killing mechanisms.