1.A.91 The Plasmodial Surface Anion Channel (PSAC) Family
Erythrocytes infected with malaria parasites have increased permeability to ions and nutrients as mediated by the plasmodial surface anion channel (PSAC), recently linked to parasite Cytoadherence-linked asexual protein 3.2 (CLAG3.2) gene. Although the CLAG3.2-encoded protein is integral to the host membrane, its contribution to solute transport was unclear because it lacks conventional transmembrane domains and does not have homology to ion channel proteins in other organisms. Sharma et al. 2015 identified a probable CLAG3.2 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 segment, suggesting that the domain lines a water-filled pore. A single CLAG3.2 mutation (A1210T) in a leupeptin-resistant PSAC mutant fell 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 blocking affinity of PSAC inhibitors that bind at opposite channel faces, consistent with global changes in channel structure. Transfected parasites carrying this mutation survived leupeptin challenge significantly better than a transfection control. 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 development of therapies targeting PSAC. Plasmodium transporters have been reviewed (Staines et al. 2017).
Malaria parasites use the RhopH complex for erythrocyte invasion and channel-mediated nutrient uptake. Member proteins are unique to Plasmodium spp. Schureck et al. 2021 showed that RhopH is synthesized as a soluble complex of CLAG3, RhopH2, and RhopH3 with 1:1:1 stoichiometry. After transfer to a new host cell, the complex crosses a vacuolar membrane surrounding the intracellular parasite and becomes integral to the erythrocyte membrane through a PTEX translocon-dependent process. A 2.9 Å single-particle cryo-EM structure of the trafficking complex, revealed that CLAG3 interacts with the other subunits over large surface areas. This soluble complex is tightly assembled with extensive disulfide bonding and predicted transmembrane helices shielded. Schureck et al. 2021 proposed that the large protein complex is stabilized for trafficking but poised for host membrane insertiond through large-scale rearrangements, paralleling smaller two-state pore-forming proteins in other organisms.
The generalized reaction catalyzed by the PSAC is:
Solutes (out) ↔ Solutes (in).