Apicoplast-derived isoprenoids are essential for biosynthesis of GPI protein anchors, and consequently for egress and invasion in Plasmodium falciparum

Abstract

Glycophosphatidylinositol (GPI) anchors are the predominant glycoconjugate in Plasmodium parasites, enabling modified proteins to associate with biological membranes. GPI biosynthesis commences with donation of a mannose residue held by dolichol-phosphate at the endoplasmic reticulum membrane. In Plasmodium dolichols are derived from isoprenoid precursors synthesised in the Plasmodium apicoplast, a relict plastid organelle of prokaryotic origin. We found that treatment of Plasmodium parasites with apicoplast inhibitors decreases the abundance of isoprenoid and GPI intermediates resulting in GPI-anchored proteins becoming untethered from their normal membrane association. Even when other isoprenoids were chemically rescued, GPI depletion led to an arrest in schizont stage parasites, which had defects segmentation and egress. In those daughter parasites (merozoites) that did form, proteins that would normally be GPI-anchored were mislocalised, and when these merozoites were artificially released they were able to attach to but not invade new red blood cells. Our data provides further evidence for the importance of GPI biosynthesis during the asexual cycle of P. falciparum, and indicates that GPI biosynthesis, and by extension egress and invasion, is dependent on isoprenoids synthesised in the apicoplast. Author summaryThe plastid apicoplast organelle of the malaria parasite Plasmodium falciparum has long been recognised as a drug target, however the downstream metabolic pathways have not been fully elucidated. In this study we inhibited apicoplast function in blood-stage P. falciparum and following the depletion of essential apicoplast-derived isoprenoids, we observed that these parasites exhaust their supplies of the polyisoprenoid alcohol dolichol. Dolichols form important components of biological membranes and are also required for the synthesis of the major parasite glycoconjugate, glycophosphatidylinositol (GPI) anchors. Concurrent with a reduction in dolichol levels, proteins normally conjugated to GPIs became mislocalised. Severe parasite impairments followed with incomplete membrane segmentation of their daughter merozoites, which could subsequently neither egress nor reinvade host red blood cells. Our data implicates dolichol as an essential parasite metabolite, dependent on normal apicoplast function, and reveals novel roles for GPI anchored proteins. The widespread phenotype following disrupted dolichol synthesis supports aspects of GPI biosynthesis as potential future drug targets.