Nuclear import of microinjected influenza virus ribonucleoproteins

Influenza virus ribonucleoproteins (vRNPs) devoid of the matrix protein (M1) were isolated and introduced into the cytoplasm of CHO and MDCK cells by microinjection. The injected vRNPs were found to be imported into the nucleus, and the RNA was transcribed. Their uptake into the nucleus was ATP-dependent, inhibited by antibodies to the nuclear pore complex, unaffected by the prior acidification of the vRNPs, and not inhibited by amantadine. The results showed that for productive infection, all the early stages of the viral entry pathway (receptor interaction, endocytosis, acid exposure, and membrane fusion) can be bypassed. Once the vRNPs are stripped of M1 and separated from each other, they are competent for import into the nucleus by constitutive cellular processes. Second, the results showed that while the amantadine block for incoming virus is manifested at the level of nuclear entry of the vRNPs, the actual import event per se is not affected. The results are consistent with a recent hypothesis that amantadine inhibits a step needed to prime the core for uncoating, which takes place before the virus has reached the cytosol.