Chloroplast envelope membranes

In many of the chloroplasts exhibiting incipient changes, clusters of ordered arrays of fibrils and crystalline bodies were observed in the stroma (Fig. 2, 3F). These fibrils and crystalloids were frequently associated with the chloroplast envelope and were often observed in conjunction with regions where small but noticeable alterations of the ultrastructure of the envelope were apparent. These alterations consisted primarily of an increase in the staining density of the envelope membranes (Fig. 2, arrows) and a frequent accumulation of electron-dense material between the two membranes of the envelope (Fig. 3, arrows). Larger deposits of electron-dense material also occurred in association with the membranes in these areas. Another unusual observation was the frequent presence of what appears to be a complex of several membranes including the chloroplast envelope membranes associated with the indentation of the chloroplast surface (Fig. 1, arrows) and the crystalloids in the stroma (Fig. 3F). 

Falah M, Gupta RS (1994) Cloning of the hsp70 (dnaK) genes from Rhizobium meliloti and Pseudomonas cepacia phylogenetic analyses of mitochondrial origin based on a highly conserved protein sequence. J Bacteriol 176 7748-7753 Ferro M et al. (2003) Proteomics of the chloroplast envelope membranes from Arabidopsis thaliana. Mol Cell Proteomics 2 325-345... 

Soil, J., and Tien, R. (1998). Protein translocation into and across the chloroplastic envelope membranes. Plant Mol. Biol. 38, 191-207. 

Reumann, S., Davila-Aponte, J., and Keegstra, K. (1999). The evolutionary origin of the protein-translocating channel of chloroplastic envelope membranes identification of a cyanobacterial homolog. Proc. Natl. Acad. Sci. USA 96, 784-789. 

The most abundant membranes in nature are the thylakoids inside chloroplasts of green plants. A surprising amount of lipid traffic is involved in the assembly of these membranes. Almost all the acyl chains that form the core of the photosynthetic membranes are first produced by fatty acid synthase in the chloroplast. In most plants these acyl chains are then exported to the ER where they become esterified to glycerol, desaturated while they are part of phosphatidylcholine and then are returned to the plastid. The exact mechanisms for the export and return of acyl chains are still uncertain although much has been learned (Chapter 17) [10]. The export from plastids across the chloroplast envelope membranes is known to involve a fatty acid intermediate, and probably is a channeled or facilitated process rather than free diffusion because only a tiny pool of free fatty acid is ever detected (A. Koo, 2004). An acyl-CoA synthetase on the envelope membrane is believed to quickly convert the exported fatty acid to a thioester form that is then a substrate for acyltransferases. Transfer of acyl groups to the ER may occur via diffusion of the acyl-CoAs however, recent evidence suggests this initial acyl transfer reaction involves acylation of lyso-phosphatidylcholine and may occur at the chloroplast envelope. 

BICK, J.A., LANGE, B.M., Metabolic cross talk between cytosolic and plastidial pathways of isoprenoid biosynthesis unidirectional transport of intermediates across the chloroplast envelope membrane. Arch. Biochem. Biophys., 2003, 415, 146-154. 

In closing, it is noteworthy that several of the mesophyll cell-specific steps of the C, cycle are light-modulated in C plants such as maize, including the transport of pyruvate across the chloroplast envelope-membrane (32) and the ensuing reaction sequence of pyruvate - PEP - oxaloacetate (OA) -> malate catalyzed by stromal... 

Jacobson and Stumpf, in 1972, reported a very rapid entry of free acetate into isolated chloroplasts. The half-time for equilibration across the envelope membrane was less than 5 s. However, acetyl-CoA was essentially nonpermeable across the chloroplast envelope membrane. That a coupled acetyl-CoA hydrolase-acetyl-CoA synthetase system may serve as an efficient transport unit is suggested by the observation that free acetate is rarely found in the plant cell. 

Malherbe, A., Block, M.A., Douce, R. and Joyard, J. (1995) Solubilisation and biochemical properties of phosphatidate phosphatase from spinach chloroplast envelope membranes. Plant Physiol. Biochem. 33, 149-161... 

Miquel M., Block M.A., Joyard J., Dome A.J., Dubacq J.P,, Kader J.C.and Douce R. (1987) - Protein mediated transfer of phosphatidyl -choline from liposomes to spinach chloroplast envelope membranes. Biochim. Biophys. Acta 937, 219-228. 

Chloroplast envelope membranes. The two envelope membranes that surround chloroplasts contain numerous enzyme systems some of which are involved in the formation and reduction of semiquinone radicals. These include quinol oxidase and NADPHiquinone/semiquinone reductases. These reactions are one step in more complex reactions including the biosynthesis of specific plastid membrane constituents and the formation of polyunsaturated fatty adds. Some of these enzymes are also involved in the export of protons to the cytosol, which partially regulates the stromal pH during photosynthesis. 

Synthesis of ii—Tocopherol and Plastoquinone-9 from Homogentlsate at the Chloroplast Envelope Membrane... 

The reaction mechanism of PQ synthesis equals that of aT synthesis. The synthesis also occurs exclusively at the inner chloroplast envelope membrane /8/ (Fig. 4), however, it can be assumed that either prenylquinone is formed by its own enzyme garniture. 2-Methyl-6-nonaprenyl-(solanosyl-)qulnol is formed from homogentlsate plus nonaprenyl-(solanosyl-)PP. The quinol formed is then methylated by SAM to yield PQH /6/ (Fig. 4). Even if the sequence in PQ synthesis is clarified Homogentlsate — 2-Methyl-6—nonaprenylqulnol —PQH. no data are available for the synthesis of hydroxylated quinones... 

Table 1. Enzymatic reactions of the galactolipid metabolism of spinach chloroplast envelope membranes. 

Fig. 1. Lipid pattern of spinach chloroplast envelope membranes, obtained from thermolysin-treated (left) and untreated (right) chloroplasts. Chromatograms are shown obtained by HPLC analysis only non-acidic lipids were eluted from the aminopropyl-modified silica column (18). Peak labels 1, carotenoids and diacylglycerol 2, unknown 3, MGDG 4, PC 5-6, DGDG 7, PG and sulfolipid (overload) 8, TGDG. Note the small peak 1, the high MGDG peak, and the absence of TGDG in the thermolysin-treated membranes.

Table 2. Localization of enzymes involved in galactolipid metabolism in the chloroplast envelope membranes.

CHARACTERIZATION OF GALACTOSYLTRANSFERASES IN SPINACH CHLOROPLAST ENVELOPE MEMBRANES... 

UDPgal transferase (UDPGT) activity has been localized in the outer chloroplast envelope membrane (OEM) of pea (18 3 plant) (1) and in the inner chloroplast envelope membrane (lEM) of spinach (16 3 plant) (2). 

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