In the clinical phase of malaria infection, the parasite cells invade their host erythrocytes and create an intracellular vacuole, inside which they replicate. When the daughter parasites are mature, after about 48 h, they need to break through both vacuole and erythrocyte membranes in order to invade new erythrocytes. The process by which they escape (“egress”) is a highly organised sequence of secretion, activation and proteolytic events, culminating in the explosive release of the new parasites (Thomas et al, 2018). Plasmodium species have several perforin-like proteins, two of which are expressed at the blood stage of their life cycle, and it has been hypothesised that these are used in the membrane breakage steps. In the related apicomplexan parasite Toxoplasma gondii, the perforin like protein TgPLP1 is essential for vacuole breakage and parasite egress.
We have used video microscopy, electron and X-ray tomography along with mutants and pharmacological blockers of different steps in egress, to study the membrane disruption and breakage during the process of egress. We found an early step of vacuole membrane permeabilization preceding the secretion of protein kinase G and activation of the protease cascade in P. falciparum (Hale et al, 2017), but the effector of this permeabilization is still unknown. In contrast, T.gondii secretes PLP1 which clearly disrupts the vacuole membrane and most likely also the host fibroblast plasma membrane.
Subsequent to vacuole permeabilization by P. falciparum, the vacuole membrane is completely disrupted, followed by erythrocyte membrane permeabilisation and finally disruption to allow parasite escape. The major merozoite surface protein of P. falciparum, MSP1, is proteolytically processed during egress, and an MSP1 mutant shows a substantial defect in egress (Das et al, 2015). We are currently comparing the membrane structures of wild type and mutant parasites at different stages of egress.