Abstract

Toxoplasma gondii is an obligate intracellular protozoan parasite of the phylum Apicomplexa, and toxoplasmosis is an important disease in immuno-compromised individuals - especially AIDS patients, organ transplant recipients and patients receiving anti-cancer chemotherapy. Toxoplasmosis also has economic importance as it results in spontaneous abortion in economically important animals. Due to emerging drug resistance and the possible side effects of the existing therapeutics there is a necessity to explore new drug targets. Sphingolipids are essential components of eukaryotic cell membranes and signal transduction pathways. Mammals produce sphingomyelin (SM) via a SM synthase, whereas yeast, plants and some protozoa utilise an inositol phosphorylceramide (IPC) synthase to produce IPC. IPC synthases have no mammalian equivalent and have been proposed as targets for anti-fungals and anti-protozoals. In this study sphingolipid scavenging from the host was shown to be non-essential for Toxoplasma proliferation indicating de novo synthesis is key. To investigate this pathway in the parasite an orthologue of the SM synthase from the related apicomplexan Plasmodium falciparum was identified in the genome database as being encoded by a single copy gene. Subsequently it was characterized as a functional orthologue of the yeast AUR1p (IPC synthase), with mass spectrometry identifying this lipid species in parasite extracts for the first time. Like AUR1p and the human SM synthase, the Toxoplasma sphingolipid synthase (Tg SLS) is Golgi localized. However, unlike AUR1p Tg SLS is resistant to the inhibitor aureobasidin A and also facilitates production of a minor complex sphingolipid suggested to be SM. Further study characterized the sphingolipid synthases from the Trypanosoma species (protozoan Kinetoplastida) and the plant Arabidopsis. Bioinformatic and phylogenetic analyses of these and previously characterized SM and IPC synthases, indicate them to be a family of evolutionarily related, but functionally diverse, enzymes. Many of these may prove to be targets for pharmaceutical or herbicidal intervention.