The Toxoplasma gondii protein ROP2 mediates host organelle association with the parasitophorous vacuole membrane

Toxoplasma gondii replicates within a specialized vacuole surrounded by the parasitophorous vacuole membrane (PVM). The PVM forms intimate interaction s with host mitochondria and endoplasmic reticulum (ER) in a process termed PVM-organelle association. In this study we identify a likely mediator of this process, the parasite protein ROP2. ROP2, which is localized to the PV M, is secreted from anterior organelles termed rhoptries during parasite in vasion into host cells. The NH2-terminal domain of ROP2 (ROP2hc) within the PVM is exposed to the host cell cytosol, and has characteristics of a mito chondrial targeting signal. In in vitro assays, ROP2hc is partially translo cated into the mitochondrial outer membrane and behaves like an integral me mbrane protein. Although ROP2hc does not translocate across the ER membrane , it does exhibit carbonate-resistant binding to this organelle. In vivo, R OP2hc expressed as a soluble fragment in the cytosol of uninfected cells as sociates with both mitochondria and ER. The 30-amino acid (aa) NH2-terminal sequence of ROP2hc, when fused to green fluorescent protein (GFP), is suff icient for mitochondrial targeting. Deletion of the 30-aa NH2-terminal sign al from ROP2hc results in robust localization of the truncated protein to t he ER. These results demonstrate a new mechanism for tight association of d ifferent membrane-bound organelles within the cell cytoplasm.