Peculiar monomeric interferon gammas, IFNγrel 1 and IFNγrel 2, in ginbuna crucian carp

The existence of fish-specific isoforms of interferon (IFN)γ, known as IFNγ-related (IFNγrel), has been reported in several fish species. However, comparisons with deduced amino acid sequences of known IFNγrels among several fish species have indicated significant differences at the C-terminus basic amino acid continuous sequences, which indicate the existence of multiple IFNγrel isoforms. Two distinct cDNAs, encoding two IFNγrels, ifngrel 1 and ifngrel 2, were cloned from ginbuna crucian carp (Carassius auratus langsdorfii). Recombinant IFNγrel 1 and IFNγrel 2 have shown high antiviral activities against the lethal crucian carp hematopoietic necrosis virus. Both ligands exhibit biological activity as monomers despite the fact that the functional conformation of IFNγ is a homodimer. Both interferons have a high degree of sequence similarity, but differ in the C-terminus region. In this region, IFNγrel 1 contains a functional nuclear localization sequence which induces the translocation of green fluorescent protein from the cytoplasm to the nucleus. IFNγrel 2 lacks this sequence. These results indicate that IFNγrel 1 and IFNγrel 2 are functional antiviral cytokines. These structurally related ligands play distinct antiviral roles through different intracellular translocation mechanisms. Thus, IFNγrels form a novel, distinct subtype included in type II IFNs. The cyprinid fish IFNγ subtype currently consists of four members, including two IFNγ isoforms and two distinct additional IFNγrel isoforms specific to the fish.