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PubMed1984

Potassium, sodium, and chloride content of isolated intact chloroplasts in relation to ionic compartmentation in leaves.

Intact chloroplasts were isolated from spinach, sugarbeet, and pea leaves by a rapid procedure which included purification on a two-step Percoll gradient. The content of K+, Na+, and Cl- was determined together with the chloroplast volume from which the apparent concentration of these ions in the chloroplast was calculated. It was demonstrated by successive washing that K+ leaked out of the chloroplasts during isolation whereas Na+ and Cl- did not. For spinach and sugarbeet chloroplasts K+ was the predominant monovalent cation (160-200 mM) with Na+ present in lesser amounts (40-70 mM). Pea chloroplasts contained equal amounts of K+ and Na+ (40 mM). The Cl- concentration in the chloroplast was similar for the three species and was calculated to be near 100 mM. The Cl- concentration in the chloroplast was well above that of the leaf tissue implying accumulation of Cl- in the chloroplast. Qualitatively similar results were obtained for chloroplasts isolated by rapidly fractionating protoplasts of wheat and barley although the concentration of ions was lower than for the above three species. Spinach grown in 200 mM NaCl accumulated high levels of Na+ and Cl- in the leaves but K+ was decreased. The replacement of K+ by Na+ was also observed in the chloroplasts but the level of Cl- in the chloroplast was not greatly increased even at leaf Cl- in excess of 200 mM. Replacement of K+ by Na+ in the nutrient solution also resulted in replacement of K+ by Na+ both in the leaf and in the chloroplast. Spinach grown under Cl- deficiency showed a 70% decrease in growth and a similar decrease in leaf Cl- but the concentration of Cl- in the chloroplast was not significantly decreased. It is concluded that in most instances K+ is the dominant monovalent cation in chloroplasts, although replacement by Na+ can occur to some extent. Chloride is a major anion, being accumulated within the chloroplast at low leaf Cl- and excluded from the chloroplast at high leaf Cl-. Ionic compartmentation within the leaf cell appears to be regulated to maintain a relatively constant level of Cl- in the chloroplast.

Anions↗
PubMed1998

Protein import into cyanelles and complex chloroplasts.

Higher-plant, green and red algal chloroplasts are surrounded by a double membrane envelope. The glaucocystophyte plastid (cyanelle) has retained 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 surrounded 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 membrane chloroplast envelope. Analysis of presequence structure and in vitro import experiments indicate that proteins are imported directly from the cytoplasm across the two envelope membranes and peptidoglycan layer into cyanelles. Protein import into complex chloroplasts is however fundamentally different. Analysis of presequence structure and in vitro 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 and immunoelectronmicroscopy 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 presence of ribosomes on the outermost of the four envelope membranes suggests protein import into 4 membrane-bounded complex chloroplasts is directly from the ER like outermost membrane into the chloroplast. The fundamental difference in import mechanisms, posttranslational direct chloroplast import or co-translational translocation into the ER prior to chloroplast import, appears to reflect the evolutionary origin of the different chloroplast types. Chloroplasts with a two-membrane envelope are thought to have evolved through the primary endosymbiotic association 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 phototrophic eukaryotic host and a photosynthetic eukaryote.

Amino Acid Sequence↗