Figure 9.

Transitional hemifusion as an intermediate of HA- and GPI-HA–mediated membrane fusion. GPI-HA pores (lipidic pores) can naturally result from the state of transitional hemifusion by reconfiguration of a few lipids within the initial hemifusion diaphragm. (The same would be the case for HA pores.) If hemifusion never occurred, and instead GPI-HA generated a “hemi-pore” as an intermediate of a “protein-lipid pore,” lipid dye would not be able to spread, nor would the rearrangement of lipids be likely to proceed in the GPI-HA–expressing membrane. All experimental evidence is consistent with HA inducing pores by means essentially the same as those used by GPI-HA. We thus propose that HA-mediated fusion proceeds through transitional hemifusion as well.