Binds and inhibits the conjugation of the ubiquitin-like G1P2/ISG15 protein to its target proteins. Since G1P2/ISG15 is an early antiviral protein, NS1 may inhibit the host antiviral response. Prevents EIF2AK2/PKR activation, either by binding double strand RNA or by interacting directly with EIF2AK2/PKR. Also binds poly(A) and U6 snRNA. Homodimer. Interacts with and inhibits human G1P2 conjugation by UBE1L. Both N-terminus and C-terminus can inhibit IFN-beta promoter activation and IRF-3 nuclear translocation. Belongs to the influenza B viruses NS1 family.