A possible role of prolyl oligopeptidase during Linum usitatissimum (flax) seed development.

Involvement of prolyl oligopeptidases (POPs) in the control of several mammalian peptide hormone signalling pathways has been studied extensively in recent years. POPs are ubiquitous enzymes, but little attention has been paid to understanding their function in plants. Using a cDNA-AFLP approach, two flax (Linum usitatissimum) POP ESTs were identified as being specifically expressed in the early stages of flax seed development. This specific expression was confirmed using real time RT-PCR and in situ hybridisation approaches. Seed expression of Arabidopsis POP genes was measured and showed no specificity. Comparison between results obtained with flax and Arabidopsis is discussed in order to address a hypothetic function for POPs during seed formation. These results provide the first insights into POP gene expression and hypothetical function in plants.

Characterization of an iron-dependent regulatory sequence involved in the transcriptional control of AtFer1 and ZmFer1 plant ferritin genes by iron.

In eukaryotic cells, ferritin synthesis is controlled by the intracellular iron status. In mammalian cells, iron derepresses ferritin mRNA translation, whereas it induces ferritin gene transcription in plants. Promoter deletion and site-directed mutagenesis analysis, combined with gel shift assays, has allowed identification of a new cis-regulatory element in the promoter region of the ZmFer1 maize ferritin gene. This Iron-Dependent Regulatory Sequence (IDRS) is responsible for transcriptional repression of ZmFer1 under low iron supply conditions. The IDRS is specific to the ZmFer1 iron-dependent regulation and does not mediate the antioxidant response that we have previously reported (Savino et al. (1997) J. Biol. Chem. 272, 33319-33326). In addition, we have cloned AtFer1, the Arabidopsis thaliana ZmFer1 orthologue. The IDRS element is conserved in the AtFer1 promoter region and is functional as shown by transient assay in A. thaliana cells and stable transformation in A. thaliana transgenic plants, demonstrating its ubiquity in the plant kingdom.

The control of intracellular glycerol in Saccharomyces cerevisiae influences osmotic stress response and resistance to increased temperature.

Glycerol has been demonstrated to serve as the major osmolyte of Saccharomyces cerevisiae. Consistently, mutant strains gpd1gpd2 and gpp1gpp2, which are devoid of the main glycerol biosynthesis pathway, have been shown to be osmosensitive. In addition, the primary hyperosmotic stress response is affected in these strains. Hog1p phosphorylation turned out to be prolonged and osmostress-induced gene expression is delayed compared with the kinetics observed in wild-type cells. A hog1 deletion strain was previously found to contain lower internal glycerol and therefore displays an osmosensitive phenotype. Here, we show that the osmosensitivity of hog1 is suppressed by growth at 37 degrees C. We reasoned that this temperature-remedial osmoresistance might be caused by a higher intracellular glycerol level at the elevated temperature. This hypothesis was confirmed by measurement of the glycerol concentration, which was shown to be similar for wild type and hog1 cells only at elevated growth temperatures. In agreement with this finding, hog1 cells containing an fps1 allele, encoding a constitutively open glycerol channel, have lost their temperature-remedial osmoresistance. Furthermore, gpd1gpd2 and gpp1gpp2 strains were found to be temperature sensitive. The growth defect of these strains could be suppressed by adding external glycerol. In conclusion, the ability to control glycerol levels influences proper osmostress-induced signalling and the cellular potential to grow at elevated temperatures. These data point to an important, as yet unidentified, role of glycerol in cellular functioning.

Response of Saccharomyces cerevisiae to severe osmotic stress: evidence for a novel activation mechanism of the HOG MAP kinase pathway.

The HOG/p38 MAP kinase route is an important stress-activated signal transduction pathway that is well conserved among eukaryotes. Here, we describe a novel mechanism of activation of the HOG pathway in budding yeast. This mechanism operates upon severe osmostress conditions (1.4 M NaCl) and is independent of the Sln1p and Sho1p osmosensors. The alternative input feeds into the HOG pathway MAPKK Pbs2p and requires activation of Pbs2p by phosphorylation. We show that, upon severe osmotic shock, Hog1p nuclear accumulation and phosphorylation is delayed compared with mild stress. Moreover, both events lost their transient pattern, presumably because of the absence of negative feedback mediated by Ptp2p tyrosine phosphatase, which we found to be localized in the nucleus. Under severe osmotic stress conditions, the delayed nuclear accumulation correlates with a delay in stress-responsive gene expression. Severe osmoshock leads to a situation in which active and nuclear-localized Hog1p is transiently unable to induce transcription of osmotic stress-responsive genes. It also appeared from our studies that the Sho1p osmosensor is less active under severe osmotic stress conditions, whereas the Sln1p/Ypd1p/Ssk1p sensor and signal transducer functions normally under these circumstances.

Iron homeostasis alteration in transgenic tobacco overexpressing ferritin.

Intracellular iron concentration requires tight control and is regulated both at the uptake and storage levels. Our knowledge of the role that the iron-storage protein ferritins play in plants is still very limited. Overexpression of this protein, either in the cytoplasm or the plastids of transgenic tobacco, was obtained by placing soybean ferritin cDNA cassettes under the control of the CAMV 35S promoter. The protein accumulated in 4- and 6-day-old seedlings and in leaves of 3-week-old plants but not in dry seeds or in 2-day-old seedlings, which is consistent with previous reports describing a post-transcriptional control of ferritin amounts during the germination process. Overaccumulated ferritin in leaves was correctly assembled as 24-mers. Transformants were more resistant to methylviologen toxicity, indicating that the transgenic ferritins were functional in vivo. Ferritin overaccumulation in transgenic tobacco leaves leads to an illegitimate iron sequestration. As a consequence, these transgenic plants behave as iron deficient and activate iron transport systems as revealed by an increase in root ferric reductase activity and in leaf iron content.

Conformational changes and in vitro core-formation modifications induced by site-directed mutagenesis of the specific N-terminus of pea seed ferritin.

Plant ferritin has a three-dimensional structure predicted to be very similar to that of animal ferritin. It has, however, an additional specific sequence of 24 amino acids at its N-terminus named extension peptide (EP). In order to determine precisely the interactions between EP and other domains of the pea seed ferritin subunit, three point mutations were performed. The mutated residues were chosen by three-dimensional computer modelling of the pea seed ferritin subunit structure [Lobréaux, Yewdall, Briat and Harrison (1992) Biochem. J. 228, 931-939]. The mutant recombinant proteins were expressed in Escherichia coli and purified to homogeneity; all the mutants were found to be assembled as 24-mers. When Ala-13 was replaced by His, as in mammalian ferritins, ferroxidase activity was significantly reduced. Moreover, in vitro iron-core formation in Pro-X-->Ala, Lys-R-->Glu and Ala-13-->His mutants was increased after denaturation by urea followed by renaturation; this was also observed with the EP deletion mutant (r delta TP/EP). The recombinant ferritins were also analysed using tryptophan fluorescence spectra. The r delta TP/EP, Pro-X-->Ala and Lys-R-->Glu mutants were found to be more susceptible to denaturation by urea than the native r delta TP pea seed ferritin.

Purification and characterization of recombinant pea-seed ferritins expressed in Escherichia coli: influence of N-terminus deletions on protein solubility and core formation in vitro.

Plant ferritin subunits are synthesized as precursor molecules; the transit peptide (TP) in their NH2 extremity, responsible for plastid targeting, is cleaved during translocation to this compartment. In addition, the N-terminus of the mature subunit contains a plant-specific sequence named extension peptide (EP) [Ragland, Briat, Gagnon, Laulhère, Massenet, and Theil, E.C. (1990) J. Biol. Chem. 265, 18339-18344], the function of which is unknown. A novel pea-seed ferritin cDNA, with a consensus ferroxidase centre conserved within H-type animal ferritins has been characterized. This pea-seed ferritin cDNA has been engineered using oligonucleotide-directed mutagenesis to produce DNA fragments (1) corresponding to the wild-type (WT) ferritin precursor, (2) with the TP deleted, (3) with both the TP and the plant specific EP sequences deleted and (4) containing the TP but with the EP deleted. These four DNA fragments have been cloned in an Escherichia coli expression vector to produce the corresponding recombinant pea-seed ferritins. Expression at 37 degrees C led to the accumulation of recombinant pea-seed ferritins in inclusion bodies, whatever the construct introduced in E. coli. Expression at 25 degrees C in the presence of sorbitol and betaine allowed soluble proteins to accumulate when constructs with the TP deleted were used; under this condition, E. coli cells transformed with constructs containing the TP were unable to accumulate recombinant protein. Recombinant ferritins purified from inclusion bodies were found to be assembled only when the TP was deleted; however assembled ferritin under this condition had a ferroxidase activity undetectable at acid pH. On the other hand, soluble recombinant ferritins with the TP deleted and expressed at 25 degrees C were purified as 24-mers containing an average of 40-50 iron atoms per molecule. Despite the conservation in the plant ferritin subunit of a consensus ferroxidase centre, the iron uptake activity in vitro at pH 6.8 was found to be lower than that of the recombinant human H-ferritin, though it was much more active than the recombinant human L-ferritin. The recombinant ferritin with both the TP and the EP deleted (r delta TP/EP) assembled correctly as a 24-mer; it has slightly higher ferroxidase activity and decreased solubility compared with the wild-type protein with the TP deleted (r delta TP). In addition, on denaturation by urea followed by renaturation by dialysis the r delta TP/EP protein showed a 25% increase in core-formation in vitro compared with the r delta TP protein.(ABSTRACT TRUNCATED AT 400 WORDS)

Purification, characterization and function of bacterioferritin from the cyanobacterium Synechocystis P.C.C. 6803.

Storage and buffering of iron is achieved by a class of proteins, the ferritins, widely distributed throughout the living kingdoms. All ferritins have in common their three-dimensional structure and their ability to store large amounts of iron in their central cavity. However, eukaryotic ferritins from plants and animals and bacterioferritins have no sequence similarity, and besides non-haem iron bacterioferritins contain haem residues whereas eukaryotic ferritins do not. In this paper we report the first purification and characterization of a bacterioferritin from a cyanobacterium. It has a molecular mass of 400 kDa and is built up from 19 kDa subunits. Its N-terminal sequence shows 73% identity with that of the Escherichia coli bacterioferritin subunit. It contains 2300 atoms of iron and 1500 molecules of phosphate per ferritin molecule and 0.25 haem residue per subunit; the alpha-peak of the cytochrome has its maximum at 559 nm. In contrast with what is known for eukaryotic ferritins, we found that bacterioferritin from Synechocystis is not inducible by iron under the conditions that we have tested and that it has a constant concentration whatever the iron status of the cells, even at very low iron concentration. Bacterioferritin from Synechocystis P.C.C. 6803 is fully assembled in vivo and it is shown by labelling with 59Fe that it is able to load iron in vitro as well as in vivo. Bacterioferritin from Synechocystis is shown to have an iron-buffering function while the bulk of cellular iron is found associated with a pool of low-molecular-mass electronegative molecules. The role of Synechocystis bacterioferritin in iron metabolism is discussed.