Cloning and structural analysis of complement component 3d in wild birds provides insight into its functional...

Area of Science:

• Immunology
• Evolutionary Biology
• Biochemistry

Background:

• Complement component 3d (C3d) is vital for linking innate and adaptive immunity, significantly boosting B-cell antigen sensitivity.
• Human C3d possesses distinct surfaces: a convex surface for antigen binding and a concave surface with an acidic pocket for CR2 interaction.

Purpose of the Study:

• To investigate the evolutionary divergence of C3d in wild birds and its functional implications for immunity.
• To analyze the phylogenetic relationships and structural characteristics of C3d across avian and mammalian species.

Main Methods:

• Cloning and sequencing of C3d cDNA fragments from 15 wild bird species.
• Phylogenetic analysis of C3d sequences from birds, chickens, and mammals.
• Structural modeling to analyze C3d protein sequences and predict CR2 binding capabilities.

Main Results:

• Phylogenetic analysis revealed two main branches (mammals and birds), with five wild bird species clustering closer to mammals.
• Specific wild bird C3d proteins exhibited amino acid residues (Glu163 or Asp163) forming a typical mammalian acidic pocket for CR2 interaction.
• Structure modeling confirmed that these five bird C3d variants possess the characteristic acidic pocket structure for CR2 binding via electrostatic forces.

Conclusions:

• Certain wild bird C3d proteins may possess the ability to bind CR2 through electrostatic forces, similar to mammals.
• These birds, with more typical C3d acidic pockets, might have enhanced immunity against pathogens.
• The study provides insights into the evolutionary divergence of avian and mammalian immune factor functions.