Reversible merger of membranes at the early stage of influenza hemagglutinin-mediated fusion

Fusion mediated by influenza hemagglutinin (HA), a prototype fusion protein, is commonly detected as lipid and content mixing between fusing cells. Decreasing the surface density of fusion-competent HA inhibited these advanced fusion phenotypes and allowed us to identify an early stage of fusion at physiological temperature. Although lipid flow between membranes was restricted, the contacting membrane monolayers were apparently transiently connected, as detected by the transformation of this fusion intermediate into complete fusion after treatments known to destabilize hemifusion diaphragms. These reversible connections disappeared within 10-20 min after application of low pH, indicating that after the energy released by HA refolding dissipated, the final low pH conformation of HA did not support membrane merger. Although the dynamic character and the lack of lipid. mixing at 37 degrees C distinguish the newly identified fusion intermediate from the intermediate arrested at 4 degrees C described previously, both intermediates apparently belong to the same family of restricted hemifusion (RH) structures. Because the formation of transient RH structures at physiological temperatures was as fast as fusion pore opening and required less HA, we hypothesize that fusion starts with the formation of multiple RH sites, only a few of which then evolve to become expanding fusion pores.