Rhoptry

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Rhoptries are a type of secretory organelle in certain parasites called Apicomplexa, which include malaria parasites and Toxoplasma. They are club-shaped, with a narrow neck and a larger bulb, located at the parasite’s front end. Rhoptries are one of three secretory organelles at the parasite’s apex, along with micronemes and dense granules.

The number of rhoptries varies by species and life stage. For example, Toxoplasma gondii tachyzoites have about 10–12 rhoptries, while bradyzoites have 1–3. Plasmodium merozoites have 2, sporozoites have 2–4, and ookinetes have none; Cryptosporidium sporozoites have 1.

Rhoptries have two main regions: the rhoptry neck and the rhoptry bulb. The neck contains rhoptry neck proteins (RONs), and the bulb contains rhoptry bulb proteins (ROPs). These proteins are made in the usual secretory path (ER to Golgi) and then sent to the rhoptries where they are stored.

RONs are released first during invasion, followed by ROPs after invasion. The contents of rhoptries help the parasite attach to the host and form a protective compartment called the parasitophorous vacuole (PV) inside the host cell. Some rhoptry proteins become part of the PV membrane or move into the host cell’s cytosol or nucleus, where they can modify the host.

Rhoptry proteins also help the parasite manipulate the host’s immune response and can aid nutrient uptake during the parasite’s growth. Their actions work together with proteins from micronemes (which help with attachment) and dense granules (which modify the host after invasion) to enable successful infection. Rhoptries are formed anew during cell replication from precursor vesicles that mature and move to the parasite’s apex before division.