Regulated degradation of an endoplasmic reticulum membrane protein in a tubular lysosome in Leishmania mexicana

The cell surface of the human parasite Leishmania mexicana is coated with g lycosylphosphatidylinositol (GPI)-anchored macromolecules and free GPI glyc olipids. We have investigated the intracellular trafficking of green fluore scent protein- and hemagglutinin-tagged forms of dolichol-phosphate-mannose synthase (DPMS), a key enzyme in GPI biosynthesis in L. mexicana promastig otes. These functionally active chimeras are found in the same subcompartme nt of the endoplasmic reticulum (ER) as endogenous DPMS but are degraded as logarithmically growing promastigotes reach stationary phase, coincident w ith the down-regulation of endogenous DPMS activity and GPI biosynthesis in these cells. We provide evidence that these chimeras are constitutively tr ansported to and degraded in a novel multivesicular tubule (MVT) lysosome. This organelle is a terminal lysosome, which is labeled with the endocytic marker FM 4-64, contains lysosomal cysteine and serine proteases and is dis rupted by lysomorphotropic agents. Electron microscopy and subcellular frac tionation studies suggest that the DPMS chimeras are transported from the E R to the lumen of the MVT via the Golgi apparatus and a population of 200-n m multivesicular bodies. In contrast, soluble ER proteins are not delectabl y transported to the MVT lysosome in either log or stationary phase promast igotes. Finally, the increased degradation of the DPMS chimeras in stationa ry phase promastigotes coincides with an increase in the lytic capacity of the MVT lysosome and changes in the morphology of this organelle. We conclu de that lysosomal degradation of DPMS may be important in regulating the ce llular levels of this enzyme and the stage-dependent biosynthesis of the ma jor surface glycolipids of these parasites.