FIG 3.

TTSP cleavage generates fusion-competent IBV HA. (A) Experiment setup. HeLa cells expressing IBV (B/Yam) HA plus a TTSP or KLK protease were exposed to an acidic buffer 0.1 units below the fusion pH. (B) Representative photographs showing polykaryon formation in cells expressing IBV (B/Yam) HA. Polykaryon formation was similar in cells expressing B/Vic HA (data not shown). The trypsin control (third photograph) received an empty (i.e., protease-lacking) plasmid and underwent HA activation by a 15- min exposure to trypsin just before the acidic pulse.