The lipid-anchored ectodomain of influenza virus hemagglutinin (GPI-HA) iscapable of inducing nonenlarging fusion pores

GPI-linked hemagglutinin (GPI-HA) of influenza virus was thought to induce hemifusion without pore formation. Cells expressing either HA or GPI-HA wer e bound to red blood cells, and their fusion was compared by patch-clamp ca pacitance measurements and fluorescence microscopy. It is now shown that un der more optimal fusion conditions than have been used previously, GPI-HA i s also able to induce fusion pore formation before Lipid dye spread, althou gh with fewer pores formed than those induced by HA. The GPI-HA pores did n ot enlarge substantially, as determined by the inability of a small aqueous dye to pass through them. The presence of 1,1'-dioctadecyl-3,3,3',3'-tetra methylindocarbocyanine perchlorate or octadecylrhodamine B in red blood cel ls significantly increased the probability of pore formation by GPI-HA; the dyes affected Fore formation to a much lesser degree for HA. This greater sensitivity of pore formation to Lipid composition suggests that Lipids are a more abundant component of a GPI-HA fusion pore than of an HA pore. The finding that GPI-HA can induce pores indicates that the ectodomain of HA is responsible for all steps up to the initial membrane merger and that the t ransmembrane domain, although not absolutely required, ensures reliable por e formation and is essential for pore growth. GPI-HA is the minimal unit id entified to date that supports fusion to the point of pore formation.