Identification of the Moving Junction Complex of Toxoplasma gondii: A Collaboration between Distinct Secretory Organelles

Abstract

Apicomplexan parasites, including Toxoplasma gondii and Plasmodium sp., are obligate intracellular protozoa. They enter into a host cell by attaching to and then creating an invagination in the host cell plasma membrane. Contact between parasite and host plasma membranes occurs in the form of a ring-shaped moving junction that begins at the anterior end of the parasite and then migrates posteriorly. The resulting invagination of host plasma membrane creates a parasitophorous vacuole that completely envelops the now intracellular parasite. At the start of this process, apical membrane antigen 1 (AMA1) is released onto the parasite surface from specialized secretory organelles called micronemes. The T. gondii version of this protein, TgAMA1, has been shown to be essential for invasion but its exact role has not previously been determined. We identify here a trio of proteins that associate with TgAMA1, at least one of which associates with TgAMA1 at the moving junction. Surprisingly, these new proteins derive not from micronemes, but from the anterior secretory organelles known as rhoptries and specifically, for at least two, from the neck portion of these club-shaped structures. Homologues for these AMA1-associated proteins are found throughout the Apicomplexa strongly suggesting that this moving junction apparatus is a conserved feature of this important class of parasites. Differences between the contributing proteins in different species may, in part, be the result of selective pressure from the different niches occupied by these parasites.

Kinetic modeling of Toxoplasma gondii invasion

“…Thus, parasites that managed to invade remained attached about three times longer than parasites that eventually detached from the host cell. This may be due to a general defect in the detaching parasites but recent findings (Alexander et al, 2005 suggest attachment to be a multi-step process, of increasing strength rather than a binary process. Thus, the success of early attachment events may lead to longer and higher affinity interactions resulting in a longer attachment period.…”

“…The parasite traverses the host cell plasma membrane via distal translocation of the moving junction. Simultaneously rhoptry proteins (ROPs) are secreted and the parasitophorous vacuole forms around the intracellular portion of the parasite (Alexander et al, 2005). The penetrating stage is characterized by parasites staining green and red at their apical and distal ends, respectively.…”

“…Initial attachment through one or more of the parasite's abundant surface proteins (Dzierszinski et al, 2000, Jacquet et al, 2001) leads to tighter apical attachment upon release of proteins from secretory granules called micronemes. At least one of these microneme proteins associates with other proteins released from the neck of a second set of secretory organelles, the rhoptries, to the form of a structure called the moving junction (Alexander et al, 2005. During penetration this specialized ring of tight adhesion is translocated distally from its site of formation at the parasite's apex, thereby propelling the parasite into the host cell through the ring (Carruthers & Boothroyd, 2007).…”

Host cell manipulation by the human pathogen Toxoplasma gondii

“…Rhoptries, which are discharged immediately following micronemes, are comprised of two different substructures named the rhoptry neck and rhoptry bulb. MIC and rhoptry neck proteins (RON) assemble on the parasite surface to form the moving junction (MJ) [10], a tight apposition of the parasite and host plasma membranes visible during cell invasion. The moving junction ensures the formation of the parasitophorous vacuole membrane (PVM) from the host cell membrane while largely excluding host membrane proteins from the forming PVM [11].…”

Expansion of host range as a driving force in the evolution of Toxoplasma

“…Getting in -Following attachment, parasites actively enter into the host cell using a group of proteins that originate from the rhoptry necks and are referred to as the RON proteins (Alexander et al 2005, Lebrun et al 2005. These collaborate with a micronemal protein, AMA1, to form the ring-like moving junction where the invading parasite forms a ring of contact with the host cell's plasma membrane.…”