Intracellular oligomerization of influenza virus nucleoprotein.

It has previously been shown that the purified influenza virus nucleoprotein (NP) forms the oligomers in vitro in NP preparations obtained from virions (Wiley et al., 1977, Virology, 79, 446-448; Ruigrok and Baudin, 1995, J. Gen. Virol., 76, 1009-1014) and infected cells (Becht and Weiss, 1991, Behring Inst Mitt., Justus-Liebig Universitat, Giessen, 89, 1-11). We have shown in this report that boiling-sensitive NP oligomers (di- and trimers) are formed in vivo in the course of intracellular influenza virus replication. They are detected by PAGE about 10 min after monomeric 56-kDa NP molecules are synthesized. NP oligomers are formed by different strains of influenza virus in different cell lines. Some influenza virus strains are characterized by complete conversion of NP monomers into oligomers and others by only partial conversion. In the Triton X-114 phase partitioning system NP oligomers show more hydrophobicity than NP monomers. NP oligomers are detected in the sedimentable and soluble fractions of both cell lysate and extracellular medium. The possibility is discussed that oligomeric NP is a native and functionally significant form of influenza virus NP.