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ItemCharacterization of a Leishmania mexicana mutant defective in synthesis of free and protein-linked GPI glycolipidsNaderer, T ; McConville, MJ (ELSEVIER, 2002-11-01)The cell surface of the promastigote stage of the protozoan parasite, Leishmania mexicana is coated by a number of glycosylphosphatidylinositol (GPI)-anchored proteins, a GPI-anchored lipophosphoglycan (LPG) and an abundant class of free GPIs, termed glycoinositolphospholipids (GIPLs). We have developed a new screen for isolating L. mexicana mutants that are defective in GPI biosynthesis, involving concanavalin A selection of a parental strain with a modified surface coat. One mutant was isolated that lacked the major GIPL species and mature GPI-protein anchor precursors, but synthesized normal levels of LPG anchor precursors. Based on analysis of apolar GIPLs that accumulate in this mutant and in vivo and in vitro synthesized GPIs, this mutant was found to have a defect in the addition of an alpha1-6 linked mannose to the common precursor, Man(1)GlcN-PI. The apolar GIPLs were transported to the cell surface with the same kinetics as mature GIPLs. However, non-anchored isoforms of the major GPI-anchored protein, gp63, were either slowly secreted (with a t(1/2) of 2 h) or retained within the endoplasmic reticulum, respectively. These findings suggest that common enzymes are involved in the synthesis of GIPLs and protein anchors and have implications for understanding how the biosynthesis of the major surface components of these parasites is regulated.
ItemIntracellular trafficking of glycosylphosphatidylinositol (GPI)-anchored proteins and free GPIs in Leishmania mexicanaRalton, JE ; Mullin, KA ; McConville, MJ (PORTLAND PRESS, 2002-04-15)Free glycosylphosphatidylinositols (GPIs) are an important class of membrane lipids in many pathogenic protozoa. In this study, we have investigated the subcellular distribution and intracellular trafficking of an abundant class of free GPIs [termed glycosylinositolphospholipids (GIPLs)] in Leishmania mexicana promastigotes. The intracellular transport of the GIPLs and the major GPI-anchored glycoprotein gp63 was measured by following the incorporation of these molecules into sphingolipid-rich, detergent-resistant membranes (DRMs) in the plasma membrane. In metabolic-labelling experiments, mature GIPLs and gp63 were transported to DRMs in the plasma membrane with a t(1/2) of 70 and 40 min, respectively. Probably, GIPL transport to the DRMs involves a vesicular mechanism, as transport of both the GIPLs and gp63 was inhibited similarly at 10 degrees C. All GIPL intermediates were quantitatively recovered in Triton X-100-soluble membranes and were largely orientated on the cytoplasmic face of the endoplasmic reticulum, as shown by their sensitivity to exogenous phosphatidylinositol-specific phospho-lipase C. On the contrary, a significant proportion of the mature GIPLs ( approximately 50% of iM4) were accessible to membrane-impermeable probes on the surface of live promastigotes. These results suggest that the GIPLs are flipped across intracellular or plasma membranes during surface transport and that a significant fraction may populate the cytoplasmic leaflet of the plasma membrane. Finally, treatment of L. mexicana promastigotes with myriocin, an inhibitor of sphingolipid biosynthesis, demonstrated that ongoing sphingolipid biosynthesis is not required for the plasma-membrane transport of either gp63 or the GIPLs and that DRMs persist even when cellular levels of the major sphingolipid are depleted by 70%.