The ORF, regulated synthesis, and persistence-specific variation of influenza C viral NS1 protein

The open reading frame (ORF) and the regulated synthesis of the influenza C viral NS1 protein were analyzed in view of viruses possessing different bi ological activities. We provide evidence for a 246-amino-acid NS1-ORF, enco ded by five viral strains and variants. Prokaryotic expression of the proto type NS1-ORF resulted in a product of 27 kDa, confirming the predicted mole cular weight. Using an antiserum raised against recombinant NS1 protein, no nstructural proteins of wild-type virus were detected in infected cells for a limited course of time, whereas a persistent virus variant was character ized by a long-term nonstructural gene expression. As examined by infection experiments, the intracellular distribution of nonstructural protein was n uclear and cytoplasmic, whereas in NS1 gene-transfected cells, the cytoplas mic localization occurred in a fine-grained structure, suggesting an analog y to influenza A viral NS1 protein. Concerning persistent infection, NS1 pr otein species differing in sizes and posttranslational modifications were o bserved for a persistent virus variant, as particularly illustrated by a hi gh degree of NS1 phosphorylation. Virus reassortant analyses proved the imp ortance of the NS-coding genomic segment: the minimal viral properties requ ired for the establishment of persistence were transferred with this segmen t to a monoreassortant virus. Thus the influenza C viral NS1 protein is a 2 46-amino-acid nuclear-cytoplasmic phosphoprotein that can be subject to spe cific variations being functionally linked to a persistent virus phenotype. (C) 1999 Academic Press.