Signification structurale et fonctionnelle des enveloppes chloroplastiques d'Euglena: Etude immunocytologique et en cryofracture

Topology of Euglena Chloroplast Protein Precursors within Endoplasmic Reticulum to Golgi to Chloroplast Transport Vesicles

“…Transmission electron microscopy has shown that the three Euglena chloroplast membranes are closely appressed in some regions reminiscent of higher plant envelope contact sites, while in other regions they are well separated (10,11). Regions of adherence between the Euglena intermediate and inner membrane similar to the contact sites between the plant outer and inner envelope membrane have been identified in freeze etch replicas (11). These ultrastructural studies support the assumption that protein translocation through the Euglena chloroplast intermediate and inner membranes is mechanistically similar to post-translational protein import into the plastids of cyanelles, plants, and green and red algae.…”

“…Euglena is one of a group of organisms having complex chloroplasts, chloroplasts with a three-or four-membrane envelope rather than a two-membrane envelope as found for the chloroplasts of plants, red and green algae and the cyanelles of glaucocystophytes (10,11,(31)(32)(33)(34)(35)(36). Immunoelectron microscopy (37,38) and pulse-chase intracellular localization studies (13,14) have clearly demonstrated that protein import into Euglena chloroplasts is fundamentally different from import into chloroplasts with a double envelope membrane.…”

“…The ancestral Euglenoid is believed to have been a phagotrophic trypanosome-like organism that engulfed a eukaryotic algae (10,31,33,36). The third envelope membrane is thought to have evolved from the host's phagocytic vacuole membrane (11,33). Vacuolar protein precursors contain signal peptides and are transported from the ER to the Golgi apparatus, where they are sorted into transport vesicles for delivery to the vacuole (1, 3).…”

“…Just as the ancestral prokaryotic preprotein translocation system was retained for thylakoid protein import during the evolution of a photosynthetic prokaryotic endosymbiont into a chloroplast (4), the ancestral chloroplast protein import system was retained as the chloroplast envelope membranes of the photosynthetic eukaryotic endosymbiont evolved into the intermediate and inner membranes of the Euglena chloroplast (32,33). The bipartite Euglena presequence arose by the addition of a signal peptide to the existing stromal targeting presequence (32,33), enabling the precursor to be transported first to the phagocytic vacuole-derived Euglena outer chloroplast membrane (11,33), separating the host cytoplasm from the endosymbiont, and subsequently through the intermediate and inner chloroplast membranes derived from the endosymbiont's chloroplast envelope (32,33).…”

“…Euglena is a protist having complex chloroplasts with a three-membrane envelope (10,11). Euglena chloroplast precursors are synthesized on membrane-bound polysomes (12) rather than free polysomes as found for higher plant, green and red algal chloroplast precursors.…”