Identification of ISG15 in golden pompano, Trachinotus ovatus, and its role in virus and bacteria infections

Interferon (IFN)-stimulated gene product 15 (ISG15) is a ubiquitin-like protein critical for the control of mi-crobial infections. Golden pompano, Trachinotus ovatus is one of the precious marine economic fish in the southern coast of China, always suffering from viruses, bacteria, and parasite infections. To date, the roles of golden pompano genes involved in viral and bacterial infections, especially IFN-related genes remained largely unknown. To identify the interferon system genes of golden pompano and explore their function, in this study, the ISG15 homolog (ToISG15) was cloned from golden pompano, and its role in response to grouper iridovirus (SGIV), nervous necrosis virus (NNV), and Aeromonas hydrophila infection was investigated. The whole ORF of ToISG15 was composed of 465 bp and encoded a polypeptide of 154 amino acids with different identity with the known ISG15 homologs from other fish species. Two conserved ubiquitin-like (UBL) domains and an Ub-conjugation domain (LRGG) were found in ToISG15 sequence. Expression analysis showed that ToISG15 was located mainly in the cytoplasm of golden pompano cells, and dramatically induced following SGIV, Aeromonas hydrophila, or poly I:C treatment, but little change was observed when NNV infection. Overexpression of ToISG15 in vitro significantly inhibited the replication of SGIV and NNV. Interestingly, ToISG15 possessed the ability to restrain the growth of Aeromonas hydrophila. Furthermore, To-ISG15 overexpression enhanced the expression of IFNc, IFNh, IRF3, IRF7, and viperin genes as well as, to a lesser extent, the IL-6 gene. Taken together, our results demonstrated the antiviral and antibacterial effect of To-ISG15, shedding light on the evolutionary conservation of ISG15 in the immune response to microbial infection.