Post-translational Modification of Influenza Virus-induced Proteins During Productive and Abortive Infections

Abstract

The replication of influenza virus is abortive in HeLa cells and the virus yield from these cells is less than 1% of that from permissive cell lines such as MDBK cells. We have previously shown that there is a host cell defect in the final stages of virus maturation which results in a block in the release of influenza virus from HeLa cells and the accumulation of elongated budding virus particles on the plasma membrane (2). In the present study we examined the synthesis and post-translational modification of virus-specific proteins in HeLa cells and MDBK cells infected with influenza virus A/WSN (H1N1) at a multiplicity of 50 PFU/cell. In both cell lines the NP (nucleocapsid protein) and NS1 (non-structural) polypeptides were detected by 4 hr after infection. In the presence of 32PO4, both NP1 and NS1 were phosphorylated in MDBK cells; however, only NS1 was phosphorylated in the abortive infection of HeLa cells. NP was not phosphorylated in infected HeLa cells at any time. The phosphorylation of NS1 in HeLa cells occurred later and at a reduced level. Virus released from MDBK cells contained phosphorylated NP and the small quantity of virus released from HeLa cells also contained phosphorylated NP. These results suggest that the phosphorylation of NP is important for efficient assembly of influenza virus. The lack of phosphorylation of the NP protein in HeLa cells may alter the charge of the viral ribonucleoprotein (RNP) complexes and modify the interaction of the RNPs with the viral matrix protein (M), resulting in particles of aberrant morphology which fail to bud from the surface of HeLa cells.