A soluble secretory protein of the intracellular parasite Toxoplasma gondii associates with the parasitophorous vacuole membrane through hydrophobic interactions

The obligate intracellular parasite Toxoplasma gondii resides within a specialized, nonfusogenic vacuole termed the parasitophorous vacuole. We are exploring the hypothesis that parasite proteins contained in dense core granules and secreted into the vacuole after cell invasion contribute to the modification of the parasitophorous vacuole membrane (PVM). The dense granule protein GRA3 associates preferentially with the PVM. Here we show that GRA3 liberated from dense granules is a soluble protein. Following secretion, one- third or more of GRA3 in the parasitophorous vacuole becomes stably associated with the vacuole membrane as judged by sucrose flotation gradients and Triton X-114 phase partitioning experiments. GRA3 cannot be removed from the membrane by extraction with sodium carbonate at pH 11.0, nor can it be labeled in infected cells with [1-¹⁴C]acetate. In sucrose velocity sedimentation gradients, GRA3 from vacuolar membranes solubilized in Triton X-100 sediments as an oligomer. Membrane-associated GRA3 cross-links to itself, migrating as a ladder of bands in SDS-polyacrylamide gel electrophoresis. No other parasite or cell proteins co-precipitate with cross-linked GRA3. We conclude that a portion of GRA3 secreted from the parasite as a soluble protein directly inserts into the vacuole membrane in an oligomeric form. This is the first report in Toxoplasma or related parasites of a protein which inserts into the vacuole membrane for some purpose other than to lyse that membrane.