Interferons type II and their receptors R1 and R2 in fish species: Evolution, structure, and function

Interferon gamma (IFN-gamma) is one of the key players in the immune system of vertebrates. The evolution and properties of IFN-gamma and its receptors in fish species are of special interest as they point to the origin of innate immunity in vertebrates. We studied the phylogeny, biophysical and structural properties of IFN-gamma and its receptors. Our phylogeny analysis suggests the existence of two groups of IFN-gamma related proteins, one specific for Acanthomorpha, the other for Cypriniformes, Characiformes and Silurifonnes. The analysis further shows an ancient duplication of the gene for IFN-gamma receptor 1 (IFN-gamma R1) and the parallel existence of the duplicated genes in all current teleost fish species. In contrast, only one gene can be found for receptor 2, IFN-gamma R2. The specificity of the interaction between IFN-gamma and both types of IFN-gamma R1 was determined by microscale thermophoresis measurements of the equilibrium dissociation constants for the proteins from three fish species. The measured preference of IFN-gamma for one of the two forms of receptor 1agrees with the bioinformatic analysis of the coevolution between IFN-gamma and receptor 1. To elucidate structural relationships between IFN-gamma of fish and other vertebrate species, we determined the crystal structure of IFN-gamma from olive flounder (Paralichthys olivaceus, PoliIFN-gamma) at crystallographic resolution of 2.3 angstrom and the low-resolution structures of Takifugu rubripes, Oreochromis niloticus, and Larimichthys crocea IFN-gamma by small angle X-ray diffraction. The overall PoliIFN-gamma fold is the same as the fold of the other known IFN-gamma structures but there are some significant structural differences, namely the additional C-terminal helix G and a different angle between helices C and D in PoliIFN-gamma.