Abstract
Rhoptries are secretory organelles involved in the virulence of the human pathogen Toxoplasma gondii. We have used high performance liquid chromatography and capillary gas-liquid chromatography to isolate and quantify lipids from whole Toxoplasma cells and their purified rhoptries. This comparative lipidomic analysis revealed an enrichment of cholesterol, sphingomyelin and, most of all, saturated fatty acids in the rhoptries. These lipids are known, when present in membranes, to be contributing to their rigidity and, interestingly, fluorescence anisotropy measurements confirmed that rhoptry-derived membranes have a lower fluidity than membranes from whole T. gondii cells. Moreover, while rhoptries were initially thought to be highly enriched in cholesterol, we demonstrated it is present in lower proportions and provided additional evidence towards a lack of involvement of rhoptry cholesterol in the process of host cell invasion by the parasite. Indeed, depleting the cholesterol content of the parasites did not prevent the secretion of protein-containing rhoptry-derived vesicles and the parasites could still establish a structure called the moving junction, which is necessary for invasion. Instead, the crucial role for host cholesterol for invasion, which has already been demonstrated (Coppens, I. and Joiner, K. A. (2003), Mol.Biol.Cell 14, 3804-3820), might be explained by the need of a cholesterol-rich region of the host cell we could visualise at the point of contact with the attached parasite, in conditions where parasite motility was blocked.