The human c1q globular domain: structure and recognition of non-immune self ligands

Area of Science:

• Immunology
• Structural Biology
• Biochemistry

Background:

• C1q is the complement system's C1 complex ligand-binding unit.
• It acts as a pattern recognition molecule, detecting various self-structures, including apoptotic cells.
• C1q's recognition function is mediated by its C-terminal globular domain.

Purpose of the Study:

• To provide an overview of recent advances in understanding C1q recognition mechanisms.
• To elucidate the biological implications of C1q's ligand-binding properties.
• To detail the structural insights into C1q's interaction with self-ligands.

Main Methods:

• X-ray crystallography was used to determine the three-dimensional structure of the C1q globular domain.
• This technique, combined with other approaches, deciphered C1q's interaction mechanisms with non-immune self-ligands.
• Analysis focused on identifying common binding areas for ligands like phosphatidylserine and DNA.

Main Results:

• The C1q globular domain structure reveals a heterotrimeric assembly with distinct subunit surface patterns.
• A common binding area for ligands such as phosphatidylserine and DNA was identified in subunit C.
• Ligand recognition via this area was shown to down-regulate C1 activation.

Conclusions:

• C1q exhibits versatile binding properties due to its globular domain structure.
• The identified binding site for self-ligands like phosphatidylserine and DNA offers insights into C1q's recognition mechanisms.
• C1q's ligand recognition plays a role in controlling inflammatory reactions by down-regulating C1 activation.