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ABSTRACTErythrocytes infected with malaria parasites have increased permeability to ions and nutrients, as mediated by the plasmodial surface anion channel (PSAC) and recently linked to parasite clag3genes. Although the encoded protein is integral to the host membrane, its precise contribution to solute transport remains unclear because it lacks conventional transmembrane domains and does not have homology to ion channel proteins in other organisms. Here, we identified a probable CLAG3 transmembrane domain adjacent to a variant extracellular motif. Helical-wheel analysis revealed strict segregation of polar and hydrophobic residues to opposite faces of a predicted α-helical transmembrane domain, suggesting that the domain lines a water-filled pore. A single CLAG3 mutation (A1210T) in a leupeptin-resistant PSAC mutant falls within this transmembrane domain and may affect pore structure. Allelic-exchange transfection and site-directed mutagenesis revealed that this mutation alters solute selectivity in the channel. The A1210T mutation also reduces the blocking affinity of PSAC inhibitors that bind on opposite channel faces, consistent with global changes in channel structure. Transfected parasites carrying this mutation survived a leupeptin challenge significantly better than a transfection control did. Thus, the A1210T mutation contributes directly to both altered PSAC activity and leupeptin resistance. These findings reveal the molecular basis of a novel antimalarial drug resistance mechanism, provide a framework for determining the channel's composition and structure, and should guide the development of therapies targeting the PSAC.

Severe malarial anemia (SMA) is a leading cause of morbidity and mortality in children residing in regions where Plasmodium falciparum transmission is holoendemic. Although largely unexplored in children with SMA, interleukin-18 (IL-18) is important for regulating innate and acquired immunity in inflammatory and infectious diseases. As such, we selected two functional single-nucleotide polymorphisms (SNPs) in the IL-18 promoter (–137G→C [rs187238] and –607C→A [rs1946518]) whose haplotypes encompass significant genetic variation due to the presence of strong linkage disequilibrium among these variants. The relationship between the genotypes/haplotypes, SMA (hemoglobin [Hb], <5.0 g/dl], and longitudinal clinical outcomes were then investigated in Kenyan children (n = 719). Multivariate logistic regression analyses controlling for age, gender, sickle cell trait, glucose-6-phosphate dehydrogenase (G6PD) deficiency, HIV-1, and bacteremia revealed that carriage of the –607AA genotype was associated with protection against SMA (odds ratio [OR] = 0.440 [95% confidence interval {CI} = 0.21 to 0.90], P = 0.031) in children with acute infection. In contrast, carriers of the –137G/–607C (GC) haplotype had increased susceptibility to SMA (OR = 2.050 [95% CI = 1.04 to 4.05], P = 0.039). Measurement of IL-18 gene expression in peripheral blood leukocytes demonstrated that elevated IL-18 transcripts were associated with reduced hemoglobin concentrations (ρ = –0.293, P = 0.010) and that carriers of the "susceptible" GC haplotype had elevated IL-18 transcripts (P = 0.026). Longitudinal investigation of clinical outcomes over a 3-year follow-up period revealed that carriers of the rare CC haplotype (∼1% frequency) had 5.76 times higher mortality than noncarriers (P = 0.001). Results presented here demonstrate that IL-18 promoter haplotypes that condition elevated IL-18 gene products during acute infection are associated with increased risk of SMA. Furthermore, carriage of the rare CC haplotype significantly increases the risk of childhood mortality.