ELONGATION OF THE CYTOPLASMIC TAIL INTERFERES WITH THE FUSION ACTIVITY OF INFLUENZA-VIRUS HEMAGGLUTININ

The hemagglutinin (HA) of fowl plague virus was lengthened and shorten ed by site-specific mutagenesis at the cytoplasmic tail, and the effec ts of these modifications on HA functions were analyzed after expressi on from a simian virus 40 vector. Elongation of the tail by the additi on of one to six histidine (His) residues did not interfere with intra cellular transport, glycosylation, proteolytic cleavage, acylation, ce ll surface expression, and hemadsorption, However, the ability to indu ce syncytia at a low pH decreased dramatically depending on the number of His residues added, Partial fusion (hemifusion), assayed by fluore scence transfer from octadecylrhodamine-labeled erythrocyte membranes, was also reduced, but even with the mutant carrying six His residues, significant transfer was observed. However, when the formation of fus ion pores was examined with hydrophilic fluorescent calcein, transfer from erythrocytes to HA-expressing cells was not observed with the mut ant carrying six histidine residues. The addition of different amino a cids to the cytoplasmic tail of HA caused an inhibitory effect similar to that caused by the addition of His. On the other hand, a mutant la cking the cytoplasmic tail was still able to fuse at a reduced level. These results demonstrate that elongation of the cytoplasmic tail inte rferes with the formation and enlargement of fusion pores. Thus, the l ength of the cytoplasmic tail plays a critical role in the fusion proc ess.