A rhoptry is a specialized Secretion organelle. They are club-shaped connected by thin necks to the extreme apical pole of the parasite. These , like , are characteristic of the motile stages of Apicomplexa . They can vary in number and shape and contain numerous that are released during the process of host penetration. The proteins they contain are important in the interaction between the host and the parasite, including the formation of the parasitophorous vacuole (PV).
Characteristics
Rhoptries are one of the three characteristic secretory
present in all
Apicomplexa along with
and
.
Rhoptries and
are localized at the
apical complex of the
organism, which suggests common ancestry of the members of the phylum and the evolution process they have experienced.
The name rhoptry indicates its shape as it comes from the Greek word for "club-shaped."
These large membrane-bound
Organelle are electron-dense and highly acidic
and have similar high density across those in
Apicomplexa species.
There is a variation in the number of rhoptries present in different species and during different developmental stages. For example, the tachyzoite stage of Toxoplasma gondii, which is found during the acute phase of toxoplasmosis, has 10 to 12 rhoptries, while the bradyzoite stage observed during the chronic phase of the infection has one to three rhoptries.
Structure and content
Rhoptry mainly comprises two regions: the rhoptry neck and the rhoptry bulb.
Those two regions physically divide the parasites and have different features and materials. The rhoptry neck is an electron-dense duct
that narrowly extends at the anterior tip
and contains the rhoptry neck proteins (RONs),
which are named after where they localize in the parasite.
On the other hand, the rhoptry bulb is a larger, bulbous base that is electron-lucent and contains the rhoptry bulb proteins (ROPs) and membranous materials.
So far, eight rhoptry bulb proteins, ROP1 through ROP8, have been identified in
T. gondii.
Those two classes of proteins, RONs and ROPs, follow the typical secretory pathway from the endoplasmic reticulum to the
Golgi apparatus, then finally, where they are normally stored, the rhoptry.
They have critical functions in the host invasion and replication within the host of
parasites.
During the host invasion process, the proteins are secreted at different times at which they each function. RONs are
Exocytosis first because they contribute during the invasion.
ROPs follow afterward and perform a post-invasion role.
Synthesis
De novo assembly of rhoptries occurs during cell replication.
They are first synthesized as pre-rhoptries, which are spherical-shaped,
-derived vesicles.
Yet, how these immature rhoptries are formed is still unknown.
Pre-rhoptries elongate and mature into the functional rhoptries just before
cytokinesis, which then move to the apexes of the parasites to localize to their normal position—the
Apical complex.
Functions
The three unique secretory
Organelle of
Apicomplexa—
, rhoptries, and
Dense granule—release their contents by
exocytosis at different stages of the host invasion as the process is regulated in time and space.
contents are secreted first to the apical end of the parasite when the parasite attaches to the host cell, followed by rhoptry as invasion proceeds, and then
Dense granule near post-invasion.
The micronemal proteins secreted to the parasite's surface direct the rhoptry proteins to the host cell by forming complexes together.
The rhoptry proteins then localize to different locations within the host cell, including the
plasma membrane, the
cytosol, the
Cell nucleus, the parasitophorous vacuole membrane (PVM), and the PV lumen.
The primary functions of rhoptries are to assist host invasion and to exploit host cellular functions for enhanced
parasitism.
Still, the specific roles differ depending on where they localize within the host upon direct injection into the host
cytoplasm and on the host
species.
During the initial stage of host invasion, rhoptry contents help the parasite attach to the host,
and the rhoptry membranous material forms the PVM around the parasite entering the host cell to establish its protective
intracellular protective compartment for successful development
by inducing
invagination of its
plasma membrane.
In
Plasmodium, some rhoptry proteins localize to the PVM and promote the formation of the vacuole.
parasites also utilize rhoptries to divert the host cell's
immune response. The host can come to favor the parasitic invasion if the rhoptry proteins manipulate the host's actin cytoskeleton.
Furthermore, rhoptry proteins in
Toxoplasma gondii can mistraffic the host's
Immune factor for its
virulence.
Another function of rhoptry proteins is nutrient import during the
lytic cycle of
Apicomplexa.
