Basis for selective incorporation of glycoproteins into the influenza virus envelope

The ability of mutant or chimeric A/Japan hemagglutinins (HAs) to compete for space in the envelope of A/WSN influenza viruses was investigated with monkey kidney fibroblasts that were infected with recombinant simian virus 40 vectors expressing the Japan proteins and superinfected with A/WSN influenza virus. Wild-type Japan HA assembled into virions as well as WSN HA did. Japan HA lacking its cytoplasmic sequences, HA^tail-, was incorporated into influenza virions at half the efficiency of wild-type Japan HA. Chimeric HAs containing the 11 cytoplasmic amino acids of the herpes simplex virus type 1 gC glycoprotein or the 29 cytoplasmic amino acids of the vesicular stomatitis virus G protein were incorporated into virions at less than 1% the efficiency of HA^tail. Thus, the cytoplasmic domain of HA was not required for the selection process; however, foreign cytoplasmic sequences, even short ones, were excluded. A chimeric HA having the gC transmembrane domain and the HA cytoplasmic domain (HgCH) was incorporated at 4% the efficiency of HA^tail-. When expressed from simian virus 40 recombinants in this system, vesicular stomatitis virus G protein with or without (G^tail-) its cytoplasmic domain was essentially excluded from influenza virions. Taken together, these data indicate that the HA transmembrane domain is required for incorporation of HA into influenza virions. The slightly more efficient incorporation of HgCH than G or G^tail- could indicate that the region important for assembling HA into virions extends into part of the cytoplasmic domain.