Ultrastructures and interactions of complement factors H and I

Area of Science:

• Complement system biology
• Structural biology
• Protein-protein interactions

Background:

• The human complement system is crucial for innate immunity but requires tight regulation to prevent self-damage.
• Factor H and Factor I are key regulatory proteins that control complement activation by cleaving C3b.
• Understanding the structure and interaction of these proteins is vital for comprehending complement-mediated diseases.

Purpose of the Study:

• To determine the molecular structure of human factor H using high-resolution transmission electron microscopy.
• To characterize the structural and binding properties of factor I.
• To elucidate the cooperative mechanism of action between factors H and I in the regulation of C3b.

Main Methods:

• High-resolution transmission electron microscopy (TEM) to visualize factor H and factor I.
• Hydrodynamic analysis to confirm molecular weight and shape.
• Binding studies to determine stoichiometry and affinity constants for factor I-C3b interactions.
• Scatchard analysis to assess binding in the presence and absence of factor H.

Main Results:

• Factor H is an extended, flexible monomer (approx. 155,000 M(r)) with a contour length of 495 Å, typically folding in half.
• Factor I is a bilobal protein (130 Å length) with distinct domains for C3b binding and protease activity.
• Factor H enhances the affinity of factor I for C3b by at least 15-fold, revealing a cooperative regulatory mechanism.

Conclusions:

• Factor H and Factor I cooperate to efficiently regulate the complement system.
• The structural insights into factor H and factor I provide a molecular basis for their regulatory functions.
• This study proposes a model for the combined action of factors H and I in C3b processing.