Conservation of plasma regulatory proteins of the complement system in evolution: humans and fish.

The complement system is an important defense system of innate immunity. The recent identification of structurally and functionally related complement regulatory proteins in the teleost, barred sand bass (Paralabrax nebulifer), and humans, two species which are separated in evolution by 100 million years, indicates a high level of conservation and the early presence of this defense system in evolution. The complement regulatory protein of barred sand bass, SBP1, is related to both the human alternative pathway regulator factor H, and to the classical pathway regulator C4bp, and displays regulatory activities in both human pathways. In addition, molecules with homology to the recently identified human factor-H-related proteins are also present in the sand bass genome. Here, we summarize the structural and functional aspects of these homologies and discuss the consequences for the evolution of the complement system.