Glycosylphosphatidylinositol biosynthetic enzymes are localized to a stable tubular subcompartment of the endoplasmic reticulum in Leishmania mexicana

Glycosylphosphatidylinositols (GPI) are essential components in the plasma membrane of the protozoan parasite Leishmania mexicana, both as membrane an chors for the major surface macromolecules and as the sole class of free gl ycolipids. We provide evidence that L.mexicann dolichol-phosphate-mannose s ynthase (DPMS), a key enzyme in GPI biosynthesis, is localized to a distinc t tubular subdomain of the endoplasmic reticulum (ER), based on the localiz ation of a green fluorescent protein (GFP)-DPMS chimera and subcellular fra ctionation experiments. This tubular membrane (termed the DPMS tubule) is a lso enriched in other enzymes involved in GPI biosynthesis, can be specific ally stained with the fluorescent lipid, BODIPY-C-5-ceramide, and appears t o be connected to specific subpellicular microtubules that underlie the pla sma membrane. Perturbation of microtubules and DPMS tubule structure in viv o results in the selective accumulation of GPI anchor precursors, but not f ree GPIs. The DPMS tubule is closely associated morphologically with the si ngle Golgi apparatus in non-dividing and dividing cells, appears to exclude luminal ER resident proteins and is labeled, together with the Golgi appar atus, with another GFP chimera containing the heterologous human Golgi mark er pl,beta 1,2-N-acetylglucosaminyltransferase-I. The possibility that the DPMS-tubule is a stable transitional ER is discussed.