Language：English   Publisher：AMER SOC PLANT BIOLOGISTS  

In higher plants, fat-storing seeds utilize storage lipids as a source of energy during germination. To enter the beta-oxidation pathway, fatty acids need to be activated to acyl-coenzyme As (CoAs) by the enzyme acyl-CoA synthetase (ACS; EC 6.2.1.3). Here, we report the characterization of an Arabidopsis cDNA clone encoding for a glyoxysomal acyl-CoA synthetase designated AtLACS6. The cDNA sequence is 2,106 bp long and it encodes a polypeptide of 701 amino acids with a calculated molecular mass of 76,617 D. Analysis of the amino-terminal sequence indicates that acyl-CoA synthetase is synthesized as a larger precursor containing a cleavable amino-terminal presequence so that the mature polypeptide size is 663 amino acids. The presequence shows high similarity to the typical PTS2 (peroxisomal targeting signal 2). The AtLACS6 also shows high amino acid identity to prokaryotic and eukaryotic fatty acyl-CoA synthetases. Immunocytochemical and cell fractionation analyses indicated that the AtLACS6 is localized on glyoxysomal membranes. AtLACS6 was overexpressed in insect cells and purified to near homogeneity. The purified enzyme is particularly active on long-chain fatty acids (C16:0). Results from immunoblot analysis revealed that the expression of both AtLACS6 and beta-oxidation enzymes coincide with fatty acid degradation. These data suggested that AtLACS6 might play a regulatory role both in fatty acid import into glyoxysomes by making a complex with other factors, e.g. PMP70, and in fatty acid beta-oxidation activating the fatty acids.

DOI： 10.1104/pp.012955

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Language：English   Publisher：OXFORD UNIV PRESS  

Peroxisomes were visualized in living cells of various tissues in transgenic Arabidopsis by green fluorescent protein (GFP) through the addition of the peroxisomal targeting signal 1 (PTS1) or PTS2. The observation using confocal laser scanning microscopy revealed that the GFP fluorescence signals were detected as spherical spots in all cells of two kinds of transgenic plants. Immunoelectron microscopic analysis using antibodies against the peroxisomal marker protein, catalase, showed the presence of GFP in peroxisomes, confirming that GFP was correctly transported into peroxisomes by PTS1 or PTS2 pathways. It has been also revealed that peroxisomes are motile organelles whose movement might be caused by cytoplasmic flow. The movement of peroxisomes was more prominent in root cells than that in leaves, and divided into two categories: a relatively slow, random, vibrational movement and a rapid movement. Treatment with anti-actin and anti-tubulin drugs revealed that actin filaments involve in the rapid movement of peroxisomes. Moreover, abnormal large peroxisomes are present as clusters at the onset of germination, and these clusters disappear in a few days. Interestingly, tubular peroxisomes were also observed in the hypocotyl. These findings indicate that the shape, size, number and movement of peroxisomes in living cells are dynamic and changeable rather than uniform.

DOI： 10.1093/pcp/pcf037

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：OXFORD UNIV PRESS  

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Language：English   Publisher：Humana Press  

Plant peroxisomes contain at least four proteins, namely, citrate synthase, malate dehydrogenase, long-chain acyl-CoA oxidase, and 3-ketoacyl-CoA thiolase, which are synthesized as large precursors with an N-terminal cleavable presequence. Each presequence has a conserved domain (R[I/L/Q]-X5-HL) that is homologous to peroxisomal targeting signal 2 from mammals and yeasts. In addition, a cysteine residue is found at the C-terminal ends of the presequences, whose function has not yet been described. The authors analyzed the function of the presequences and the conserved amino acids using transgenic Arabidopsis plants, which accumulate β-glucuronidase carrying the presequence of the peroxisomal proteins from plants. Immunological and immunocytochemical studies on the transgenic plants showed that a conserved sequence in the extrapeptides is essential for targeting to peroxisomes, and a cysteine residue at the cleavage site is involved in the processing of the presequence. These results suggest that the presequences of the peroxisomal proteins function as targeting signals, and are necessary for the recognition of the processing.

DOI： 10.1385/CBB:32:1-3:269

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Language：English   Publisher：JAPANESE SOC PLANT PHYSIOLOGISTS  

Employing transgenic Arabidopsis plants, we analyzed the mechanism for the translocation of peroxisomal proteins from the cytosol into the matrix that is mediated by the N-terminal targeting signal. A hybrid Arabidopsis variety was generated which accumulates two kinds of originally bacterial proteins, P-glucuronidase (GUS) and a GUS chimeric protein designated as CS-Delta C42-GUS, that carries the N-terminal targeting signal for glyoxysomal citrate synthase. Because the CS-Delta C42-GUS is targeted to peroxisomes but had never been observed to be processed to produce the mature protein, it can be distinguished from the GUS protein by its molecular size. Cell fractionation analyses showed that the native GUS protein, although lacking the targeting signal, was co-localized with the CS-Delta C42-GUS protein in the peroxisomes of the hybrid plant. It is suggested that the native GUS protein forms oligomeric structures with the peroxisome-targeted chimeric proteins and can therefore be transported into peroxisomes. Sucrose density gradient centrifugation revealed that the native GUS and the chimeric GUS indeed are present both as a dimer and a tetramer in the Arabidopsis hybrid variety.

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Language：English   Publisher：AMER SOC BIOCHEMISTRY MOLECULAR BIOLOGY INC  

A cDNA clone for pumpkin acyl-CoA oxidase (EC 1.3.3.6; ACOX) was isolated from a lambda gt11 cDNA library constructed from poly(A)(+) RNA extracted from etiolated cotyledons, The inserted cDNA clone contains 2313 nucleotides and encodes a polypeptide of 690 amino acids, Analysis of the amino-terminal sequence of the protein indicates that the pumpkin acyl-CoA oxidase protein is synthesized as a larger precursor containing a cleavable amino-terminal presequence of 45 amino acids. This presequence shows high similarity to the typical peroxisomal targeting signal (PTS2). Western blot analysis following cell fractionation in a sucrose gradient revealed that ACOX is localized in glyoxysomes, A partial purification of ACOX from etiolated pumpkin cotyledons indicated that the ACOX cDNA codes for a long chain acyl-CoA oxidase, The amount of ACOX increased and reached to the maximum activity by day 5 of germination but decreased about 4-fold on the following days during the subsequent microbody transition from glyoxysomes to leaf peroxisomes. By contrast, the amount of mRNA was already high at day 1 of germination, increased by about 30% at day 3, and faded com pletely by day 7. These data indicated that the expression pattern of ACOX was very similar to that of the glyoxysomal enzyme 3-ketoacyl-CoA thiolase, another marker enzyme of the beta-oxidation spiral, during germination and suggested that the expression of each enzyme of P-oxidation is coordinately regulated.

DOI： 10.1074/jbc.273.14.8301

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Language：English   Publisher：JAPANESE SOC PLANT PHYSIOLOGISTS  

Glyoxysomal malate dehydrogenase (gMDH) is an enzyme of the glyoxylate cycle that participates in degradation of storage oil, We have cloned a cDNA for gMDH from etiolated pumpkin cotyledons that encodes a polypeptide consisting of 356 amino acid residues, The nucleotide and N-terminal amino acid sequences revealed that gMDH is synthesized as a precursor with an N-terminal extrapeptide, The N-terminal presequence of 36 amino acid residues contains two regions homologous to those of other microbody proteins, which are also synthesized as large precursors, To investigate the functions of the N-terminal presequence of gMDH, we generated transgenic Arabidopsis that expressed a chimeric protein consisting of beta-glucuronidase and the N-terminal region of gMDH. Immunological and immunocytochemical studies revealed that the chimeric protein was imported into microbodies such as glyoxysomes and leaf peroxisomes and was then subsequently processed, Site-directed mutagenesis studies showed that the conserved amino acids in the N-terminal presequence, Arg-10 and His-17, function as recognition sites for the targeting to plant microbodies, and Cys-36 in the presequence is responsible for its processing. These results correspond to those from the analyses of glyoxysomal citrate synthase (gCS), which was also synthesized as a large precursor, suggesting that common mechanisms that can recognize the targeting or the processing of gMDH and gCS function in higher plant cells.

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：AMER SOC PLANT PHYSIOLOGISTS  

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Language：English   Publisher：AMER SOC PLANT PHYSIOLOGISTS  

Glyoxysomal citrate synthase in pumpkin is synthesized as a precursor that has a cleavable presequence at its N-terminal end. To investigate the role of the presequence in the transport of the protein to the microbodies, we generated transgenic Arabidopsis plants that expressed beta-glucuronidase with the N-terminal presequence of the precursor to the glyoxysomal citrate synthase of pumpkin. Immunogold labeling and cell fractionation studies showed that the chimeric protein was transported into microbodies and subsequently was processed. The chimeric protein was transported to functionally different microbodies, such as glyoxysomes, leaf peroxisomes, and unspecialized microbodies. These observations indicated that the transport of glyoxysomal citrate synthase is mediated by its N-terminal presequence and that the transport system is functional in all plant microbodies. Site-directed mutagenesis of the conserved amino acids in the presequence caused abnormal targeting and inhibition of processing of the chimeric protein, suggesting that the conserved amino acids in the presequence are required for recognition of the target or processing.

DOI： 10.1105/tpc.8.9.1601

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Language：English   Publisher：BLACKWELL SCIENCE LTD  

Malate synthase is a glyoxysome-specific enzyme. The carboxy-terminal tripeptide of the enzyme is Ser-Arg-Leu (SRL), which is known to function as a peroxisomal targeting signal in mammalian cells. To analyze the function of the carboxy-terminal amino acids of pumpkin malate synthase in plant cells, a chimeric gene was con structed that encoded a fusion protein which consisted of beta-glucuronidase and the carboxyl terminus of the enzyme. The fusion protein was expressed and accumulated in transgenic Arabidopsis that had been transformed with the chimeric gene. Immunocytochemical analysis of the transgenic plants revealed that the carboxy-terminal five amino acids of pumpkin malate synthase were sufficient for transport of the fusion protein into glyoxysomes in etiolated cotyledons, into leaf peroxisomes in green cotyledons and in mature leaves, and into unspecialized microbodies in roots, although the fusion protein was no longer transported into microbodies when SRL at the carboxyl terminus was deleted. Transport of proteins into glyoxysomes and leaf peroxisomes was also observed when the carboxy-terminal amino acids of the fusion protein were changed from SRL to SKL, SRM, ARL or PRL. The results suggest that tripeptides with S, A or P at the -3 position, K or R at the -2 position, and L or M at the carboxyl terminal position can function as a targeting signal for three kinds of plant microbody.

DOI： 10.1046/j.1365-313X.1996.10020225.x

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Language：English   Publisher：KLUWER ACADEMIC PUBL  

A cDNA clone for 3-ketoacyl-CoA thiolase (EC 2.3.1.16) was isolated from a lambda gt11 cDNA library constructed from the poly(A)(+) RNA of etiolated pumpkin cotyledons. The cDNA insert contained 1682 nucleotides and encoded 461 amino acid residues. A study of the expression in vitro of the cDNA and analysis of the amino-terminal sequence of the protein indicated that pumpkin thiolase is synthesized as a precursor which has a cleavable amino-terminal presequence of 33 amino acids. The amino-terminal presequence was highly homologous to typical amino-terminal et proteins to microbodies. Immunoblot analysis showed that the amount of thiolase increased markedly during germination but decreased dramatically during the light-inducible transition of microbodies from glyoxysomes to leaf peroxisomes. By contrast, the amount of mRNA increased temporarily during the early stage of germination. In senescing cotyledons, the levels of the thiolase mRNA and protein increased again with the reverse transition of microbodies from leaf peroxisomes to glyoxysomes, but the pattern of accumulation of the protein was slightly different from that of malate synthase. These results indicate that expression of the thiolase is regulated in a similar manner to that of other glyoxysomal enzymes, such as malate synthase and citrate synthase, during seed germination and post-germination growth. By contrast, during senescence, expression of the thiolase is regulated in a different manner from that of other glyoxysomal enzymes.

DOI： 10.1007/BF00019471

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Language：English   Publisher：SPRINGER  

A cDNA clone for glyoxysomal citrate synthase (gCS) was isolated from a lambda gt11 cDNA library prepared from etiolated pumpkin cotyledons. The cDNA of 1989 bp consisted of a 1548 bp open reading frame that encoded 516 amino acid residues. The deduced amino acid sequence of gCS did not have a typical peroxisomal targeting signal at its carboxyl terminal. A study of expression in vitro of the cDNA and an analysis of the amino-terminal sequence of the citrate synthase indicated that gCS is synthesized as a larger precursor that has a cleavable amino-terminal presequence of 43 amino acids. The predicted amino-terminal sequence of pumpkin gCS was highly homologous to those of other microbody enzymes, such as 3-ketoacyl-CoA thiolase of rat and malate dehydrogenase of watermelon that are also synthesized as precursors of higher molecular mass.
Immunoblot analysis showed that the level of gCS protein increased markedly during germination and decreased rapidly during the light-induced transition of microbodies from glyoxysomes to leaf peroxisomes. By contrast, the level of mRNA for gCS reached a maximum earlier than that of the protein and declined even in darkness. The level of the mRNA was low during the microbody transition. These results indicate that the accumulation of the gCS protein does not correspond to that of the mRNA and that degradation of gCS is induced during the microbody transition, suggesting that post-transcriptional regulation plays an important role in the microbody transition.

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Language：English   Publisher：JAPANESE SOC PLANT PHYSIOLOGISTS  

Intravacuolar pigmented structures occurred in anthocyanin-producing cultured cells of sweet potato (Ipomoea batatas) were characterized. Formation of the pigmented structures in sweet potato cells was induced by transfer of callus cultured in 2,4-D containing agar medium into 2,4-D free liquid medium under continuous illumination. These structures were found in the vacuoles. The pigmented structures were isolated from the protoplasts by precipitation in 60% (w/w) sucrose after centrifugation. Electron microscopic observations of the anthocyanin-containing cultured cells showed these structures had neither membrane boundary nor internal structures, and were found as strongly osmiophilic globules in vacuoles. Numerous small osmiophilic globules were observed in central vacuoles at the early stage of anthocyanin accumulation, but not found in cytoplasm. Similar pigmented structures in vacuoles were also formed by treatment with neutral red. These observations indicate that these pigmented structure is the high density and insoluble globules highly concentrated with anthocyanin, which was synthesized in cytoplasm and transported to the central vacuoles.

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