Evolutionary analysis of two complement C4 genes: Ancient duplication and conservation during jawed vertebrate...

Area of Science:

• Immunology
• Evolutionary Biology
• Biochemistry

Background:

• The complement C4 protein is essential for the classical pathway of the complement system.
• C4 contains a thioester bond, typically cleaved by a histidine residue for covalent binding to target cells.
• Some species have an alternative C4 isotype with an aspartic acid residue, which binds to proteins.

Purpose of the Study:

• To investigate the evolutionary history and distribution of C4 isotypes in jawed vertebrates.
• To understand the phylogenetic relationships and gene duplication events of C4.
• To explore the functional implications of different C4 isotypes.

Main Methods:

• Phylogenetic analysis of C4 genes across various vertebrate species.
• Comparative genomics to identify C4 isotypes and their genomic locations.
• Analysis of gene duplication events and evolutionary conservation.

Main Results:

• Two C4 isotypes (histidine- and aspartic acid-dependent) are present in diverse jawed vertebrates, including sharks, birds, and reptiles.
• Phylogenetic analysis indicates C4 gene duplication occurred early in jawed vertebrate evolution.
• A distinct D-lineage of C4 genes in bony fish (excluding mammals) evolved conservatively outside the MHC region; mammals lost this lineage but regenerated D-type C4 through recent duplications.

Conclusions:

• The presence of dual C4 molecules with distinct substrate specificities likely contributed to the development of the antibody-dependent classical complement pathway.
• Evolutionary dynamics of C4 genes, including duplication and loss, have shaped the vertebrate immune system.
• Understanding C4 evolution provides insights into the diversification of immune mechanisms across vertebrates.