Influenza B virus causes a respiratory disease in people with a compromised immune system. The NS1B protein of influenza B virus is essential for virus growth and plays a crucial role in inhibiting the anti-viral responses mounted by the infected host cell. The N terminal 104 amino acids of NS1B bind a cellular protein called ISG15. ISG15 is an interferon induced 'ubiquitin-like' protein, and upon interferon induction, is conjugated to hundreds of targets. It has been found that both ISG15 and its conjugation inhibit many viruses. The focus of the current study was to characterize the interaction between NS1B and ISG15. Study of a recombinant influenza B virus which encoded a mutant NS1B protein that is unable to bind ISG15 revealed that ISG15 is mis-localized in cells infected with wild type but not the mutant influenza B virus. Further, such a mutant virus is attenuated in growth as compared to wild type virus in human cell lines but is not attenuated in canine cell lines. This result led to the discovery of the species specific nature of the interaction between NS1B and ISG15. Specifically, NS1B was found to bind ISG15 homologs from human and old world monkeys like Rhesus macaques and African green monkeys but not those from mouse or canines. These findings were extended by identifying the hinge between the N and C terminal domains of ISG15 as one of the major determinants of species specificity. These results highlight the importance of using human or primate cell culture models to study the effect of ISG15 on influenza B virus, and raises new possibilities on differences in the function of the ISG15 system in different species. / text