ROP2 localizes to both mitochondria (yellow) and ER (green).

The intracellular parasite Toxoplasma gondii hides out in a specialized compartment surrounded by the parasitophorous vacuole membrane (PVM). The PVM associates with host cell organelles, possibly as a way of scavenging lipids from the host cell. Sinai and Joiner (page 95), describe a protein that mediates this association. The new work illuminates a poorly understood aspect of parasite biology, and may be relevant for understanding organelle interactions in eukaryotic cells.

Sinai and Joiner focused on the T. gondii ROP2 protein, as this protein appears on the PVM around the time that the PVM associates with mitochondria, and it contains a domain with characteristics of a mitochondrial targeting signal. In vitro, they find that the N-terminal domain of ROP2 partially translocates into the mitochondrial outer membrane, and appears to stably integrate there. The same domain also contains a motif that promotes a high-affinity interaction with the endoplasmic reticulum (ER). In vivo, an expressed N-terminal domain of ROP2 targets both organelle systems.

The results suggest that one organelle might use a protein such as ROP2 to “capture” other organelles. Because PVM–organelle interactions are morphologically similar to normal interactions between organelles, such as ER–mitochondria interactions, the authors propose that this type of tethering mechanism may be a general phenomenon. ▪