Envelope, chloroplast

The carotenoids are located in photosynthetic pigment-protein complexes (PPCs) in the thylakoid membranes (Young, 1993), with minor amounts in the chloroplast envelope (Joyard et al, 1991) and the envelope of amyloplasts (Fishwick and Wright, 1980). In all plastid envelope membranes, violaxanthin is the major carotenoid. Carotenes are also found in plastoglobuli (Lichtenthaler and Peveling, 1966). 

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... 

FROELICH, J.E., ITOH, A., HOWE, G.A., Tomato allene oxide synthase and fatty acid hydroperoxide lyase, two cytochrome P450s involved in oxylipin metabolism, are targeted to different membranes of chloroplast envelope., Plant Physiol., 2001,125,306-317. 

Heber, U., and Heldt, H. W. 1981. The chloroplast envelope Structure function and role in leaf metabolism. Ann. Rev. Plant Physiol 25, 393-421. 

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. 

Plant plastocyanins are synthesized in the cytosol as 160-170-ammo acid precursor polypeptides consisting of a 60-70-residue transit peptide followed by a 97 99-amino acid mature protein. The transit peptide imports the precursor plastocyanin molecule across the chloroplast envelope and thylakoid membranes to its final destination in the thylakoid lumen, where it shuttles electrons by accepting them from the membrane bound cytochrome / (cyt /) of the cyt b6/f complex and donating them to the photooxidized reaction center P700-I- of photosystem I. Cyanobacterial plastocyanins use an 30-amino acid leader seqnence for thylakoid membrane translocation. Currently, there are more than 100 plant and cyanobacterial plastocyanin sequences that are available either by direct protein sequencing or deduced from the nucleotide sequences of their genes. 

Rm = resistance of mesophyll cells containing mainly 3 components (compare Fig. 3) Rcw = cell wall, Rpi = plasmalemma and / Cyt = cytoplasma. Rm = Rqw + Rpi + Rcyt-/ Ch = resistance of chloroplasts containing 2 components Rqe = chloroplast envelope and Rstr = stroma including the photosynthetic apparatus of CO2 incorporation. 

Figure 9. Change in chloroplast size on addition of various substances. Halo-genides were added in 30 mM increments, which resulted in shrinkage. These additions were followed by valinomycin (val) at a concentration of 2 /xM, which resulted in expansion. Changes in chloroplast size at 20°C were monitored by changes in the apparent absorbance of the chloroplast suspension at 535 nm (43). Note different slopes of the absorbance decrease seen on addition of valinomycin, which increases the K+ permeability of the chloroplast envelope. As in the presence of the antibiotic, K+ diffusion is not limiting the rate of salt uptake, different slopes indicate different anion fluxes. For experimental conditions see Ref. 91.

PaO is intriguingly specific for Pheo a (5a) and is located in the chloroplast envelope. It catalyzes the remarkable transformation of 5a into (a bound form of) RCC (11) (5). Besides the incorporation of two oxygen atoms, the ring opening at the newly oxygenated sites appears to achieve, all in this step, the formation of two carbonyl functions and the saturation of the eastern (3-meso position. The mechanism of the hypothetical isomerization of the primary enzymatic oxygenation product to the ring-opened (enzyme-bound form of) 11 has not been clarified. 

Matile P, Schellenberg M, Vicentini F (1997) Localization of Chlorophyllase in the Chloroplast Envelope. Planta 201 96... 

Chen, K., X. Chen, and D. J. Schnell. 2000. Mechanism of protein import across the chloroplast envelope. Biochem. Soc. Trans. 28 485-491. 

MATILE p, SCHELLENBERG M AND viCENTiNi F. 1997. Localization of chlorophyllase in the chloroplast envelope. Planta 201 96-99. 

Lipid trafficking across the chloroplast envelope is a major flux. 106... 

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. 

Violaxanthin and zeaxanthin undergoing interconversion in the xanthophyll cycle have been found to be present not only in a fraction of pigments mobile within the thylakoid membrane but also to remain in a dynamic equilibrium between the thylakoids, where the main xanthophyll cycle enzymes are localized, and the chloroplast envelope (Siefermaim-Harms et al., 1978)... 

Siefermann-Harms D, Joyard J and Douce R (1978) Light-induced changes of the carotenoid levels in chloroplast envelopes. Plant Physiol 61 530-533... 

Since the stress stimulus is perceived by the plasma membrane (see above), there must be transmission of a signal from the plasma membrane to the nucleus, causing transcription and synthesis of cytoplasmic protein(s). The cleavage enzyme must then act in the plastid, the sole location of carotenoids in green plants. Since carotenoids are present in the chloroplast envelope [168], the cleavage enzyme could also be cytosolic, with the cleavage of xanthophylls taking place at the surface of the envelope. 

Between photosystem II submembrane fractions which must be isolated and whole cells which must be cultivated, many immobilization works (28 on 45 references cited) employed chloroplasts and thylakoids intact chloroplasts, mixtures of chloroplasts and photosynthetic membranes, or specially, thylakoids alone. To obtain these photosynthetic membranes, the chloroplasts were subjected to osmotic shock by briefly placing them in a hypotonic medium. This procedure ruptures the chloroplast envelope and releases the stroma. The stripped chloroplasts were then returned to an isotonic medium. The interest of this photosynthetic material is justified by a direct contact between the reaction sites and the operation medium. The photosyntheric preparations were often obtained from spinach leaves. 

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. 

Following the demonstration by Douce and his colleagues that the acylation of glycerol-3-phosphate in the chloroplast is catalyzed by an enzyme of the stroma and the second acylation to form PA is catalyzed by an enzyme of the chloroplast envelope, further studies concentrated on the characterization of these reactions. Bertrams and Heinz (22) and Frentzen etal (2 ) have delineated the fatty acyl specificity for the acylation of glyoerol-3-phosphate by the soluble enzyme from the chloroplast stroma. 

There was a strong preference for oleate over palmitate by the enzymes from spinach and pea. The specificity was the same both for acyl-CoAs and for acyl-ACPs although the latter is almost certainly the physiological substrate. This substrate specificity was reversed when the requirements for the second acylation reaction were studied, palmitate being almost exclusively used in the conversion of 1-oleyl-G-3-P to PA by enzymes localized on the chloroplast envelope. These specificities are entirely consistent with what we know of the fatty acid distribution of complex lipids of these two species. 

Table II. PG synthesis by chloroplast envelopes. Variation in components of the reaction mixtures causes the accumulation of specific intermediates. Mercuric ions inhibit PGP phosphatase. [Data from Andrews and Mudd (30)1...

---