Identification and characterization of the subpellicular network, a filamentous component of the membrane skeleton of the protozoan parasite Toxoplasma gondii

The protozoan Toxoplasma gondii is an obligate, intracellular parasite that is able to infect and thrive in most nucleated cells of warm-blooded animals. During its normal life cycle, Toxoplasma needs to withstand a variety of physically and chemically stressful environments. In order to maintain structural integrity under these conditions, Toxoplasma needs a source of mechanical strength.; To investigate the main structural elements that are responsible for the mechanical stability of Toxoplasma, we have developed a procedure for the isolation of stable cytoskeletal elements of T. gondii. Electron microscopic examination of these preparations revealed the presence of a novel cytoskeletal structure in T. gondii: the subpellicular network. This structure, which lies underneath the parasite pellicle, is composed of interwoven 8- to 10-nm filaments and surrounds the microtubule-based cytoskeleton. We believe that the subpellicular network forms a membrane skeleton that may provide the parasite with the mechanical strength necessary for survival. Two components of this network, TgIMC1 and TgIMC2, have been identified. A homologue of TgIMC1 has also been identified in the related apicomplexan Plasmodium falciparum, the causative agent of malaria, suggesting that structurally similar membrane skeletons may be a common feature of all apicomplexan parasites.; Investigation of the dynamics of the subpellicular network during the life cycle of the parasite has led to the discovery of a C-terminal proteolytic processing event of TgiMC1 that accompanies the maturation of the structure during parasite replication. Biochemical analysis suggests that this processing is involved in the conversion of the subpellicular network from a form that allows for the addition of new subunits as the daughter cells grow to a rigid conformation that is important for the mechanical strength of the mature parasite. Additionally, a loss of network-associated TgIMC2 is observed during parasite replication. This association was re-established upon parasite egress, suggesting that TgIMC2 may act to stabilize of the subpellicular network during the extracellular life of the parasite. A second pool of TgIMC2 has also been identified. This pool associates with the dense granules and may play a role in the targeting and/or secretion of these organelles.