The architectural transition of human complement component C9 to poly(C9).

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Biology

Background:

• C9 protein's role in biological processes is not fully understood.
• Understanding C9 polymerization is crucial for elucidating its function.
• Previous studies have identified key domains within the C9 polypeptide chain.

Purpose of the Study:

• To investigate the structural changes and domain exposure of C9 during polymerization.
• To identify the specific regions of C9 involved in the polymerization process.
• To elucidate the overall structure of polymerized C9 (poly(C9)).

Main Methods:

• Bacterial expression of C9 protein fragments and domains.
• Antibody production against specific C9 segments.
• Enzyme-linked immunosorbent assays (ELISAs) to probe epitope exposure.
• Immuno-electron microscopy to visualize poly(C9) structure.
• Computer-assisted image enhancement of electron micrographs.

Main Results:

• Thrombospondin (TS) and low-density lipoprotein receptor (LDL) modules are exposed in both monomeric and polymerized C9.
• The middle region of the C9 polypeptide chain is buried in monomers but exposed in polymers.
• Fab fragments targeting the middle region and EDGF module inhibited C9 polymerization.
• Immuno-electron microscopy revealed a 'U' shape for poly(C9), with TS and LDL modules on the upper rim.
• Subunits of poly(C9) exhibit a volute shape, with the midsection forming the tubule barrel.

Conclusions:

• TS and LDL modules do not directly participate in C9 polymerization but shield hydrophobic regions in monomers.
• Polymerization exposes the midsection of the C9 polypeptide chain, facilitating tubule formation.
• The 'U' shape and volute structure of poly(C9) subunits are key to its assembly and function.