PLASMODIUM-FALCIPARUM EXPORTS THE GOLGI MARKER SPHINGOMYELIN SYNTHASEINTO A TUBOVESICULAR NETWORK IN THE CYTOPLASM OF MATURE ERYTHROCYTES

This work describes two unusual features of membrane development in a eukaryotic cell. (a) The induction of an extensive network of tubovesi cular membranes by the malaria parasite Plasmodium falciparum in the c ytoplasm of the mature erythrocyte, and its visualization with two cer amide analogues C-5-DMB-ceramide and C-6-NBD-ceramide. ''Sectioning'' of the infected erythrocytes using laser confocal microscopy has allow ed the reconstruction of detailed three-dimensional images of this nov el membrane network. (b) The stage-specific export of sphingomyelin sy nthase, a biosynthetic activity concentrated in the Golgi of mammalian cells, to this tubovesicular network. Evidence is presented that in t he extracellular merozoite stage the parasite retains sphingomyelin sy nthase within its plasma membrane. However, intracellular ring- and tr ophozoite-stage parasites export a substantial fraction (similar to 26 %) of sphingomyelin synthase activity to membranes beyond their plasma membrane. Importantly we do not observe synthesis of new enzyme durin g these intracellular stages. Taken together these results strongly su ggest that the export of this classic Golgi enzyme is developmentally regulated in Plasmodium. We discuss the significance of this export an d the tubovesicular network with respect to membrane development and f unction in the erythrocyte cytosol.