A Novel Family of Toxoplasma IMC Proteins Displays a Hierarchical Organization and Functions in Coordinating Parasite Division

Apicomplexans employ a peripheral membrane system called the inner membrane complex (IMC) for critical processes such as host cell invasion and daughter cell formation. We have identified a family of proteins that define novel sub-compartments of the Toxoplasma gondii IMC. These IMC Sub-compartment Proteins, ISP1, 2 and 3, are conserved throughout the Apicomplexa, but do not appear to be present outside the phylum. ISP1 localizes to the apical cap portion of the IMC, while ISP2 localizes to a central IMC region and ISP3 localizes to a central plus basal region of the complex. Targeting of all three ISPs is dependent upon N-terminal residues predicted for coordinated myristoylation and palmitoylation. Surprisingly, we show that disruption of ISP1 results in a dramatic relocalization of ISP2 and ISP3 to the apical cap. Although the N-terminal region of ISP1 is necessary and sufficient for apical cap targeting, exclusion of other family members requires the remaining C-terminal region of the protein. This gate-keeping function of ISP1 reveals an unprecedented mechanism of interactive and hierarchical targeting of proteins to establish these unique sub-compartments in the Toxoplasma IMC. Finally, we show that loss of ISP2 results in severe defects in daughter cell formation during endodyogeny, indicating a role for the ISP proteins in coordinating this unique process of Toxoplasma replication

PfSETvs methylation of histone H3K36 represses virulence genes in Plasmodium falciparum

The variant antigen, Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1), expressed on the surface of P. falciparum infected Red Blood Cells (iRBCs) is a critical virulence factor for malaria(1). Each parasite encodes 60 antigenically distinct var genes encoding PfEMP1s, but during infection the clonal parasite population expresses only one gene at a time before switching to the expression of a new variant antigen as an immune evasion mechanism to avoid the host’s antibody responses(2,3). The mechanism by which 59 of the 60 var genes are silenced remains largely unknown(4–7). Here we show that knocking out the P. falciparum variant-silencing SET gene (PfSETvs), which encodes an ortholog of Drosophila melanogaster ASH1 and controls histone H3 lysine 36 trimethylation (H3K36me3) on var genes, results in the transcription of virtually all var genes in the single parasite nuclei and their expression as proteins on the surface of individual iRBCs. PfSETvs-dependent H3K36me3 is present along the entire gene body including the transcription start site (TSS) to silence var genes. With low occupancy of PfSETvs at both the TSS of var genes and the intronic promoter, expression of var genes coincides with transcription of their corresponding antisense long non-coding RNA (lncRNA). These results uncover a novel role of the PfSETvs-dependent H3K36me3 in silencing var genes in P. falciparum that might provide a general mechanism by which orthologs of PfSETvs repress gene expression in other eukaryotes. PfSETvs knockout parasites expressing all PfEMP1s may also be applied to the development of a malaria vaccine