Distribution of GPI-anchored proteins in the protozoan parasite Leishmania, based on an improved ultrastructural description using high-pressure frozen cells

The cellular distribution of two glycosyl-phosphatidylinositol (GPI)-anchored proteins and a trans-membrane protein and the compartments involved in their trafficking were investigated in the insect stage of Leishmania mexicana, which belongs to the phylogenetically old protozoan family Trypanosomatidae. Electron microscopy of sections from high-pressure frozen and freeze-substituted cells allowed a detailed description of exo- and endocytic structures located in the vesicle-rich, densely packed anterior part of the spindle-shaped cell. A complex of tubular clusters/translucent vesicles is the prominent structure between the trans-side of the single Golgi apparatus and the flagellar pocket, the only site of endo- and exocytosis. A tubulovesicular compartment lined by one or two distinct microtubules and extending along the length of the cell is proposed to be a post-Golgi and probably late endosomal/lysosomal compartment. Using biotinylation experiments, FACS analysis and quantitative immunoelectron microscopy it was found that, at comparable expression levels, 73-75% of the two GPI-anchored proteins but only 13% of the trans-membrane protein are located on the cell surface. The tubulovesicular compartment contains 46%, the ER 5%, the Golgi complex 1.9% and the tubular cluster/translucent vesicle complex 3.6% of the intracellular fraction of the GPI-anchored protease, GP63. The density of GP63 was found to be 23-fold higher on the plasma/flagellar pocket membrane than on the ER and about tenfold higher than on membranes of the Golgi complex or of endo- or exocytic vesicles. These results indicate that there is a considerable concentration gradient of GPI-anchored proteins between the plasma/flagellar pocket membrane and the ER as well as structures involved in exo- or endocytosis. Possible mechanisms how this concentration gradient is established are discussed.