Extracellular truncated influenza virus nucleoprotein

In the culture medium of MDCK cells infected with influenza A/Duck/Ukraine/1/63(H3N8) virus two kinds of virus nucleoprotein (NP) are detected: full-length 56 kDa NP and truncated 53 kDa NP. However, in infected cells 53 kDa NP may be detected only at short pulse and after 10 min chase it becomes nondetectable. The extracellular truncated 53 kDa NP is detected in free RNP, and not in the virions. Both extracellular free 53 and 56 kDa NP in the virions are completely oligomerized. Several data argue against the possibility of extracellular 53 kDa NP formation being a result of extracellular 56 kDa NP proteolytic degradation. Thus, the accumulation of extracellular 53 kDa NP takes place only in the course of infection, and the amount of 53 kDa NP is not increased during prolonged storage of cell-free culture medium at +37°C. Moreover, all extracellular 56 kDa NP of A/Duck/Ukraine/1/63 influenza virus is present in the oligomeric form, and the latter, in contrast to the mononeric form, is highly resistant to proteases. The possibility is discussed that in the course of A/Duck/Ukraine/1/63 (H3N8) influenza virus infection a fraction of the synthesized 56 kDa monomeric NP undergoes the proteolytic cleavage in the infected cells before oligomerization and forms the 53 kDa NP. This 53 kDa NP is then oligomerized, enters the RNP and is quickly secreted from the cells.