PROTEIN IMPORT INTO CYANELLES AND COMPLEX CHLOROPLASTS

Higher-plant, green and red algal chloroplasts are surrounded by a dou ble membrane envelope. The glaucocystophyte plastid (cyanelle) has ret ained a prokaryotic cell wall between the two envelope membranes. The complex chloroplasts of Euglena and dinoflagellates are surrounded by three membranes while the complex chloroplasts of chlorarachniophytes, cryptomonads, brown algae, diatoms and other chromophytes, are surrou nded by 4 membranes. The peptidoglycan layer of the cyanelle envelope and the additional membranes of complex chloroplasts provide barriers to chloroplast protein import not present in the simpler double membra ne chloroplast envelope. Analysis of presequence structure and in vitr o import experiments indicate that proteins are imported directly from the cytoplasm across the two envelope membranes and peptidoglycan lay er into cyanelles. Protein import into complex chloroplasts is however fundamentally different. Analysis of presequence structure and in vit ro import into microsomal membranes has shown that translocation into the ER is the first step for protein import into complex chloroplasts enclosed by three or four membranes. In vivo pulse chase experiments a nd immune electronmicroscopy have shown that in Euglena, proteins are transported from the ER to the Golgi apparatus prior to import across the three chloroplast membranes. Ultrastructural studies and the prese nce of ribosomes on the outermost of the four envelope membranes sugge sts protein import into 4 membrane-bounded complex chloroplasts is dir ectly from the ER like outermost membrane into the chloroplast. The fu ndamental difference in import mechanisms, posttranslational direct ch loroplast import or co-translational translocation into the ER prior t o chloroplast import, appears to reflect the evolutionary origin of th e different chloroplast types. Chloroplasts with a two-membrane envelo pe are thought to have evolved through the primary endosymbiotic assoc iation between a eukaryotic host and a photosynthetic prokaryote while complex chloroplasts are believed to have evolved through a secondary endosymbiotic association between a heterotrophic or possibly phototr ophic eukaryotic host and a photosynthetic eukaryote.