TOXOPLASMA INVASION - THE PARASITOPHOROUS VACUOLE IS FORMED FROM HOST-CELL PLASMA-MEMBRANE AND PINCHES OFF VIA A FISSION PORE

Most intracellular pathogens avoid lysing their host cells during inva sion by wrapping themselves in a vacuolar membrane, This parasitophoro us vacuole membrane (PVM) is often retained, serving as a critical tra nsport interface between the parasite and the host cell cytoplasm, To test whether the PVM formed by the parasite Toxoplasma gondii is deriv ed from host cell membrane or from lipids secreted by the parasite, we used time-resolved capacitance measurements and video microscopy to a ssay host cell surface area during invasion. We observed no significan t change in host cell surface area during PVM formation, demonstrating that the PVM consists primarily of invaginated host cell membrane, Pi nching off of the PVM from the host cell membrane occurred after an un expected delay (34-305 sec) and was seen as a 0.219 +/- 0.006 pF drop in capacitance, which corresponds well to the predicted surface area o f the entire PVM (30-33 mu m(2)). The formation and closure of a fissi on pore connecting the extracellular medium and the vacuolar space was detected as the PVM pinched off, This final stage of parasite entry w as accomplished without any breach in cell membrane integrity.