Location of influenza virus M, NP and NS1 proteins in microinjected cells

When microinjected as cloned DNA, the nucleoprotein (NP) of influenza virus A/NT/60/68 (H3N2) accumulated in the nuclei of Xenopus laevis oocytes, and cultured cells of rodent and primate origin. This accumulation appeared to be specific and a property of the NP itself (or conceivably NP in association with unknown cellular constituents) since no other influenza virus components were present in DNA-injected cells. In the oocyte nucleus, clonally derived NP achieved an eightfold concentration over that in the cytoplasm. Such NP was full-length as judged by its mobility during PAGE and had the native conformation of H3N2 virus NP according to its reaction with a panel of monoclonal antibodies. NP appeared to be in the soluble fraction of the nucleus as it did not sediment under conditions which removed particulate matter from nuclear extracts. Microinjection of extracts of chick embryo fibroblast cells infected with A/FPV/Rostock/34 (H7N1) showed that exogenous NP had an affinity for the nucleus similar to that synthesized intracellularly from cloned NP DNA. This conclusion was supported by an experiment in which cloned NP from the oocyte nucleus re-entered the nucleus after injection into the cytoplasm of fresh oocytes. Injection of mRNA, extracted from chick embryo fibroblast cells infected with A/FPV/Rostock/34, into oocytes directed the synthesis of the viral proteins M (Mr 28 000), and NS1 (Mr 27 000) as well as NP (Mr 56 000). While NP from this source concentrated in the nucleus as before, M merely associated with the nucleus without exceeding the cytoplasmic level. Even more remarkable was NS1; although in injected cells this protein is concentrated in nucleoli, in microinjected oocytes its nuclear concentration was threefold less than that in the cytoplasm, despite the very large number (greater than 1500) of nucleoli present in Xenopus oocytes. It seems likely that the karyophilic nature of M and NS1, unlike that of NP, is a property not of the proteins themselves, but of a complex which they form with some other product of the infected cell. These findings were repeated when extracts from infected chick embryo cells containing NP, M and NS1 proteins radiolabelled in vivo, were injected into the cytoplasm of oocytes.