SUMMARY Development of malaria parasites within vertebrate erythrocytes requires nutrient uptake at the host cell membrane. The plasmodial surface anion channel (PSAC) mediates this transport and is an antimalarial target, but despite its importance, its molecular basis has been unknown. We now report a parasite gene family responsible for PSAC activity. We performed high-throughput screening to find transport inhibitors specific for distinct lines of the human pathogen P. falciparum. One inhibitor, 800-fold more active against PSAC from the Dd2 line than from HB3 parasites, was used with a genetic cross to map a single parasite locus on chromosome 3. DNA transfection and in vitro selections indicate that PSAC-inhibitor interactions are determined by two clag genes previously assumed to function in cytoadherence. These genes are conserved in plasmodia, exhibit expression switching, and encode an integral protein on the host membrane, as predicted by functional studies. This protein establishes novel ion channel activity on the erythrocyte surface.

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 malaria1. 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 responses2,3. The mechanism by which 59 of the 60 var genes are silenced remains largely unknown4–7. 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.

The altered permeability characteristics of erythrocytes infected with malaria parasites have been a source of interest for over 30 years. Recent electrophysiological studies have provided strong evidence that these changes reflect transmembrane transport through ion channels in the host erythrocyte plasma membrane. However, conflicting results and differing interpretations of the data have led to confusion in this field. In an effort to unravel these issues, the groups involved recently came together for a week of discussion and experimentation. In this article, the various models for altered transport are reviewed, together with the areas of consensus in the field and those that require a better understanding. KeywordsPatch-clamp; Ion channels; New permeability pathways; PSAC; Plasmodium; Oxidation It has been known for several decades that Plasmodium falciparum-infected erythrocytes exhibit increased permeability to a wide range of structurally unrelated solutes as the internal parasite matures. These changes are thought to be important for the survival of the parasite. They may be involved in nutrient uptake, metabolite removal, volume regulation and/or