The first milliseconds of the pore formed by a fusogenic viral envelope protein during membrane fusion.

Fibroblasts expressing the influenza virus hemagglutinin on their plasma membrane were patch clamped while they fused to erythrocytes. An increase in the fibroblast's membrane capacitance indicated the opening of the "fusion pore," the first aqueous connection between the fusing cells. We show here that the capacitance increase is preceded by a brief current transient, generated as the erythrocyte discharges its membrane potential through the nascent fusion pore. This signal allows one to calculate the pore conductance during the first milliseconds of its existence. The pore conductance jumps from 0 to approximately 150 pS and then grows more gradually over the subsequent tens of milliseconds until growth is arrested. The initial conductance is similar to that of a large ion channel and suggests that the pore is initially only 1-2 nm wide. Hence, we are probably observing events caused by only a small number of hemagglutinin molecules.