An analysis of the role of the target membrane on the Gp64-induced fusion pore

Influenza hemagglutinin (HA) and GP64 of the baculovirus Autographa califor nica multicapsid nuclear polyhedrosis virus induce strikingly different ini tial fusion pores when mediating fusion between host cells that express the se fusion proteins and target cells (Plonsky and Zimmerberg, 1996; Spruce e t al., 1989, 1991; Zimmerberg el al., 1994). However, in these experiments, variations in host and target membranes confounded the analysis of the rol e that major components of the fusion reaction play in determining initial pore characteristics. To determine the contribution of the target cell plas ma membrane to the fusion pore phenotype, we studied GP64-induced fusion of stably transfected cells (Sf9(Op1D)) to either red blood cells (RBCs) or S f9 cells. Initial fusion pores in Sf9(Op1D)/RBC and Sf9(Op1D)/Sf9 cell pair s exhibited the same conductance, and pore flickering was not observed in e ither combination of cells. This indicates that the target cell determines neither the size nor the reversibility of the initial pore. However, the ta rget cell does influence the kinetics of pore formation. The waiting time b etween triggering and pore appearance was shorter for Sf9(Op1D)/RBC fusion than for Sf9(Op1D)/Sf9 pairs. No correlation between pore waiting lime and conductance was found. This argues against a molecular model that assumes a ggregation of the pore wall from a nonfixed number of components as the rat e-limiting step in GP64 fusion pore formation. (C) 1999 Academic Press.