Interaction cloning of NS1-I, a human protein that binds to the nonstructural NS1 proteins of influenza A and B viruses

The yeast interaction trap system was used to identify NS1-I (for NS1 interactor), which is a human protein that binds to the nonstructural NS1 protein of the influenza A virus, NS1-I is a human homolog of the porcine 17 beta-estradiol dehydrogenase precursor protein, to which it is 84% identical, We detected only one NS1-I mRNA species, of about 3.0 kb, in HeLa cells, and the NS1-I cDNA was found to have a coding capacity for a 79.6-kDa protein, However, immunoblot analysis detected predominantly a 55-kDa protein in human cells, suggesting that NS1-I, like the porcine 17 beta-estradiol dehydrogenase, is posttranslationally processed, Using an in vitro coprecipitation assay, we showed that NS1-I interacts with NS1 proteins from extracts of cells infected with five different influenza A virus strains as well as with the NS1 of an influenza B virus. The fact that influenza A and influenza B virus NS1 proteins bind to NS1-I suggests that this cellular protein plays a role in the influenza virus life cycle.