Chloroplast proteins

The Vepex process developed in Hungary (Table 9) involves disintegration of plant materials followed by double screw pressing to maximize juice production. Green chloroplastic protein is removed by direct steam-injection heat treatment at 82°C with the addition of flocculents and centrifugation. The white protein fraction is separated from the chlorophyU-free process juice by direct steam injection at 80°C, followed by centrifugation and drying (94). 

The synthesis of virtually all proteins in a cell begins on ribosomes in the cytosol (except a few mitochondrial, and in the case of plants, a few chloroplast proteins that are synthesized on ribosomes inside these organelles). The fate of a protein molecule depends on its amino acid sequence, which can contain sorting signals that direct it to its corresponding organelle. Whereas proteins of mitochondria, peroxisomes, chloroplasts and of the interior of the nucleus are delivered directly from the cytosol, all other organelles receive their set of proteins indirectly via the ER. These proteins enter the so-called secretory pathway (Fig. 1). 

Chloroplast protein synthesis is controlled largely at the post-transcriptional level [20,21] and can be repressed by the inclusion of antibiotics such as streptomycin in the sprouting medium. Streptomycin binds to the 16S rRNA and causes the ribosome to misread the mRNA sequence, producing incorrect and non-functional proteins [22]. 

Hirsch S, Muckel E, Heemeyer F, vonHeijne G, Sool J. A receptor component of the chloroplast protein translocation machinery. Science 1994 266 1989-1992. 

SchnellDJ, KesslerF, BlobelG. Isolation of components ofthe chloroplast protein import machinery. Science 1994 266 1007-1012. 

Most chloroplastic proteins, except those of the outer envelope membrane, have an N-terminal extension that is usually cleaved during matu-... 

Cline, K., and Henry, R. (1996). Import and routing of nucleus-encoded chloroplast proteins. Annu. Rev. Cell Dev. Biol. 12, 1-26. 

Like mitochondria, chloroplasts contain their own DNA and protein-synthesizing machinery. Some of the polypeptides of chloroplast proteins are encoded by chloroplast genes and synthesized in the chloroplast others are encoded by nuclear genes, synthesized outside the chloroplast, and imported (Chapter 27). When plant cells grow and divide, chloroplasts give rise to new... 

Ellis, R.J. (1981). Chloroplast proteins Synthesis, transport, and assembly. Ann. Rev. Plant Physiol., 32 111-137. 

Most mitochondria and chloroplast proteins are made on free cytosolic ribosomes, released into the cytosol and then taken up into the organelle. Uptake into the mitochondrial matrix requires a matrix-targeting sequence and occurs at sites where the outer and inner mitochondrial membranes come into contact. The process is mediated by hsp70 and hsp60 proteins and requires both ATP hydrolysis and an electrochemical gradient across the inner mitochondrial membrane. Targeting of proteins to other compartments of mitochondria or chloroplasts requires two signals. 

Allen, J.F., Bennett,J., Steinback, K.E. and Amtzen, C.J. 1981. Chloroplast protein phosphorylation couples plastoquinone redox changes to distribution of excitation energy between the two photosystems. Nature (Lond) 291.25-29. 

Prior to 1961, all the factors that catalyzed the photoreduction of pyridine nucleotides had been isolated from either chloroplasts or leaves. The green plant was considered the only source of these proteins. However, in 1961 the association of these proteins with green plants ceased to be unique when Losada, Whatley, and Arnon (64) isolated a similar protein from a photosynthetic bacterium. This bacterial protein replaced the chloroplast protein in the photoreduction of TPN by illuminated spinach chloroplast fragments. 

A third group of OMTs are the protein carboxyl MTs, whose molecular mass is approximately 27 kDa 43 These proteins are found in almost all organisms, and they serve to repair damaged proteins. In plants, they have been shown to be important during stress and in seed viability.14. Their sequences are related to the animal glycine N-methyltransferases (NMTs) and die RNA/DNA MTs. Furthermore, the plant protein carboxyl MTs share no similarity to the other plant MTs involved in specialized metabolism. The enzyme that methylates a carboxyl functionality of Mg-protoporphyrin IX to produce a methylester constitutes a fourth type of plant OMT. This chloroplastic protein, whose mature form has a molecular mass of 31 kDa, is related to MTs with similar functions from photosynthetic bacteria, but is not closely related to any other plant OMTs.6... 

Sundberg, E., Slagter,J., Fridborg, I., Cleary, S., Robinson, C., and Coupland, G. (1997). Albino3, an Arabidopsis nuclear gene essential for chloroplast differentiation, encodes a chloroplast protein that shows homology to proteins present in bacterial membranes and yeast mitochondria. Plant Cell 9, 717-730. 

Falkowski, P. G., Sukenik, A., and Herzig, R. (1989). Nitrogen limitation in Isochrysis galbana (Haptophyceae). II. Relative abundance of chloroplast proteins. J. Phycol. 25, 471-478. 

Whether the signal sequence alone is sufficient to specify localization of a protein is unclear. The signal sequences of some mitochondrial and chloroplast proteins, when linked to a soluble, cytoplasmic protein, can... 

The mechanism involved in the posttranslational transport of proteins into chloro-plasts has so far received only limited attention. However, the import of plastocyanin into the thylakoid lumen has been determined to require two import signals near the N-terminal of the newly synthesized protein. Assuming that chloroplast protein import resembles the import process for mitochondria, suggest a reasonable hypothesis to explain how plastocyanin (a lumen protein associated with the inner surface of the thylakoid membrane) is transported and processed. What enzymatic activities and transport structures do you expect are involved in this process ... 

MOLECULAR CHAPERONES Molecular chaperones apparently assist unfolded proteins in two ways. First, during a finite time between synthesis and folding, proteins must be protected from inappropriate protein-protein interactions. Some proteins must remain unfolded until they are inserted in an organelle membrane, for example, certain mitochondrial and chloroplast proteins. Second, proteins must fold rapidly and precisely into their correct conformations. Some must be assembled into multisubunit complexes. Investigations of protein folding in a variety of organisms reveal that two major molecular chaperone classes are involved in protein folding. 

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