Analysis of Nuclear Accumulation of Influenza NP Antigen in von Magnus Virus‐Infected Cells

Abstract

When 1-50-4 cells were infected with von Magnus virus derived from influenza AJRIJ5+ virus by four successive undiluted passages in chick embryos, virus-specific proteins were synthesized but production of infectious virus was inhibited. In these cells the synthesis of viral RNA was suppressed and the nucleoprotein (NP) antigen was found predominantly in the nucleus in contrast to standard virus-infected cells in which the antigen was distributed throughout the whole cell. The intracellular location and migration of NP were determined by isotope labeling and sucrose gradient centrifugation of subcellular fractions. In standard virus-infected cells NP polypeptide was present predominantly in the cytoplasm in the form of viral ribonucleoprotein (RNP) and intranuclear RNP was detected in reduced amounts. In contrast, in von Magnus virus-infected cells NP polypeptide was present predominantly in the nucleus in a non assembled, soluble form and the amount of cytoplasmic RNP was considerably reduced. After short-pulse labeling NP was detected exclusively in the cytoplasm in a soluble form and after a chase a large proportion of such soluble NP was seen in the nucleus. It is suggested that a large proportion of the NP synthesized in von Magnus virus-infected cells is not assembled into cytoplasmic RNP because of the lack of available RNA and the NP migrated into the nucleus and remained there.Canavanine, an arginine analog, is a potent inhibitor of the growth of influenza virus (11). It inhibits the synthesis of viral RNA and the formation of viral RNP.Under these conditions, most of the NP is found in the nucleus in a nonassembled, soluble form, in contrast to untreated cells in which a large amount of NP is distributed in the cytoplasm in the form of RNP. Influenza RNP consists of multiple molecules ofNP and one segment of virion RNA (vRNA) or cRNA complementary to vRNA (2, 17). We have suggested the possibility that the NP synthesized in the presence of canavanine is not assembled into cytoplasmic RNP because of the lack of available RNA and that the NP readily migrates into the nucleus and ac-1 Present address: Gamagori-Fukashi Hospital, Gamagori, Aichi 433. 283

Analysis of Nuclear Accumulation of Influenza Nucleoprotein Antigen Using a Temperature‐Sensitive Mutant

“…NP itself may be able to accumulate in the nucleus but, once amplified vRNA synthesis begins, soluble NPs are assembled with the vRNA to form cytoplasmic progeny nucleocapsids that are about to be packaged into virions. The nuclear accumulation of NP was also observed with von Magnus virus infection (13,17) as well as host cell restriction of viral growth by L cells (3,4). In these cases, the suppression of genome replication is also predictable and therefore, the absence or shortage of vRNA may possibly be responsible for such nuclear accumulation.…”