Secretory and endocytic pathways converge in a dynamic endosomal system ina primitive protozoan

Leishmania are a group of primitive eukaryotic trypanosomatid protozoa that are apically polarized with a flagellum at their anterior end. Surrounding the base of the flagellum is the flagellar reservoir that constitutes the site for endocytosis and exocytosis in these organisms. In the present stud y, we define a novel multivesicular tubular compartment involved in the int racellular trafficking of macromolecules in Leishmania. This dynamic struct ure appears to subtend the flagellar reservoir and extends towards the post erior end of the cell. Functional domains of several surface-expressed prot eins, such as the gp63 glycosyl phosphatidyl inositol anchor and the 3' nuc leotidase/nuclease transmembrane domain were fused to green fluorescent pro tein. These chimeric proteins were found to traffic through the secretory p athway and, while reaching their intended destinations, also accumulated wi thin the intracellular tubular compartment. Using various compounds that ar e efficient fluid-phase markers used to track endocytosis in higher eukaryo tes, we showed that this tubular compartment constitutes an important stati on in the endocytic pathway of these cells. Based on our functional observa tions of its role in the trafficking of expressed proteins and endocytosed markers, this compartment appears to have properties similar to endosomes o f higher eukaryotes.