Interferon-stimulated gene 15 (ISG15) is an
ubiquitin homolog that is significantly induced by
type I interferons or
viral infections. Groupers, Epinephelus spp. being maricultured in China and Southeast Asian countries, always suffer from
virus infection, including iridovirus and nodavirus. To date, the roles of grouper genes, especially
interferon related genes in
virus infection remained largely unknown. Here, the ISG15 homolog (EcISG15) was cloned from grouper Epinephelus coioides and its immune response to Singapore grouper iridovirus (SGIV) and grouper nervous
necrosis virus (GNNV) was investigated. The full-length EcISG15
cDNA was composed of 948 bp and encoded a
polypeptide of 155
amino acids with 37-68% identity with the known ISG15 homologs from other fish species.
Amino acid alignment analysis indicated that EcISG15 contained two
ubiquitin-like (UBL) domains and an Ub-conjugation domain (LRGG). Expressional analysis showed that EcISG15 was dramatically induced by GNNV
infection,
poly I:C or
poly dA-dT treatment, but no obvious changes were observed during SGIV
infection. Immunofluorescence assay showed that EcISG15 localized mainly in the cytoplasm of grouper cells in response to
poly I:C stimulation or GNNV
infection, but not in mock or SGIV infected cells. Western blot analysis indicated that the ISGylation was absent in SGIV-infected cells, but significantly enhanced in GNNV-infected or
poly I:C transfected cells, suggesting that EcISG15 might play different roles in SGIV and GNNV
infection. Furthermore, overexpression of EcISG15 in vitro inhibited the transcription of GNNV genes significantly. Taken together, the results indicated that fish ISG15 might exert important roles against
RNA virus infection.