Poster Presentation Lorne Infection and Immunity 2023

The Plasmodium apicoplast is essential for the construction of glycophosphatidylinositol anchors needed for egress and invasion (#142)

Long K Huynh 1 , Michaela S Bulloch 1 , Kit Kennedy 1 , Julie Ralton 1 , Malcolm McConville 1 , Stuart Ralph 1
  1. Department of Biochemistry and Pharmacology, University of Melbourne, Melbourne, VIC, Australia

The apicoplast is a relict plastid of cyanobacterial origin found in most apicomplexan parasites, including P. falciparum, which can be selectively targeted by antibiotic-like drugs. Treatment of P. falciparum with apicoplast inhibitors causes a peculiar ‘delayed death’ phenotype, where treated parasites only arrest in the subsequent intraerythrocytic development cycle. Despite the widespread use of such drugs as malaria prophylactics (e.g. doxycycline), the molecular basis of delayed death has not been thoroughly described. Understanding these mechanisms will assist in informing appropriate clinical usage of apicoplast inhibitors. The apicoplast is responsible for the biosynthesis of isoprenoid precursors, which have multiple downstream fates in the parasite, including protein prenylation, ubiquinone and dolichols. The effect of protein prenylation and ubiquinone loss in the parasite has been previously described, however, the effect of dolichol loss has not been well-characterised. Dolichols are required for the biosynthesis of glycosylphosphatidylinositol (GPI) anchors, the predominant type of protein glycosylation present in P. falciparum. Many GPI-anchored proteins are thought to be essential for both parasite egress and reinvasion. We performed immunofluorescence assays on P. falciparum with inhibited apicoplasts but with exogenous rescue of their prenylation depletion. In these parasites, GPI‑anchored proteins become mislocalised from their normal membrane association. These GPI‑anchorless parasites also exhibited an egress defect and are unable to undergo proper segmentation and rupture the parasitophorous vacuole. Through flow cytometry‑based invasion assays, we found that these GPI-anchorless parasites are unable to reinvade red blood cells. Our data indicates that apicoplast inhibitors cause a defect in GPI anchor biosynthesis that prevents egress and reinvasion of asexual-stage P. falciparum.