The influenza virus NS1 protein forms multimers in vitro and in vivo.

The NS1 protein of the influenza A virus inhibits both the nuclear export of mRNA and pre-mRNA splicing. Two functional domains, an RNA-binding domain and an effector domain, have been identified in this protein. Here we demonstrate that the NS1 protein exists as a dimer in vitro both in the absence of its RNA target and when it is bound to a specific RNA target, U6 snRNA. This indicates that it is most likely the dimer that binds to the RNA target. Mutational analysis indicated that the RNA-binding and dimerization domains are coincident. Multimerization also occurs in vivo, as assayed using the yeast two-hybrid system. In contrast to the situation in vitro, multimerization in vivo was mediated by not only the RNA-binding domain but also the effector domain. This suggests that multimerization in vivo involves a cellular protein cofactor that bridges more than one NS1 protein molecule together via their effector domains.