GTP-binding proteins in plants

The Arf-GAP Proteins AoGcs1 and AoGts1 Regulate Mycelial Development, Endocytosis, and Pathogenicity in Arthrobotrys oligospora

Small GTPases from the ADP-ribosylation factor (Arf) family and their activating proteins (Arf-GAPs) regulate mycelial development, endocytosis, and virulence in fungi. Here, we identified two orthologous Arf-GAP proteins, AoGcs1 and AoGts1, in a typical nematode-trapping fungus Arthrobotrys oligospora. The transcription of Aogcs1 and Aogts1 was highly expressed in the sporulation stage. The deletion of Aogcs1 and Aogts1 caused defects in DNA damage, endocytosis, scavenging of reactive oxygen species, lipid droplet storage, mitochondrial activity, autophagy, serine protease activity, and the response to endoplasmic reticulum stress. The combined effects resulted in slow growth, decreased sporulation capacity, increased susceptibility to chemical stressors and heat shock, and decreased pathogenicity of the mutants compared with the wild-type (WT) strain. Although deletion of Aogcs1 and Aogts1 produced similar phenotfypic traits, their roles varied in conidiation and proteolytic activity. The ΔAogts1 mutant showed a remarkable reduction in conidial yield compared with the WT strain but not in proteolytic activity; in contrast, the ΔAogcs1 mutant showed an increase in proteolytic activity but not in sporulation. In addition, the growth of ΔAogcs1 and ΔAogts1 mutants was promoted by rapamycin, and the ΔAogts1 mutant was sensitive to H-89. Collectively, the ΔAogts1 mutant showed a more remarkable difference compared with the WT strain than the ΔAogcs1 mutant. Our study further illustrates the importance of Arf-GAPs in the growth, development, and pathogenicity of nematode-trapping fungi.

Era-like GTP protein gene expression in rice

The mutations are genetic changes in the genome sequences and have a significant role in biotechnology, genetics, and molecular biology even to find out the genome sequences of a cell DNA along with the viral RNA sequencing. The mutations are the alterations in DNA that may be natural or spontaneous and induced due to biochemical reactions or radiations which damage cell DNA. There is another cause of mutations which is known as transposons or jumping genes which can change their position in the genome during meiosis or DNA replication. The transposable elements can induce by self in the genome due to cellular and molecular mechanisms including hypermutation which caused the localization of transposable elements to move within the genome. The use of induced mutations for studying the mutagenesis in crop plants is very common as well as a promising method for screening crop plants with new and enhanced traits for the improvement of yield and production. The utilization of insertional mutations through transposons or jumping genes usually generates stable mutant alleles which are mostly tagged for the presence or absence of jumping genes or transposable elements. The transposable elements may be used for the identification of mutated genes in crop plants and even for the stable insertion of transposable elements in mutated crop plants. The guanine nucleotide-binding (GTP) proteins have an important role in inducing tolerance in rice plants to combat abiotic stress conditions.

Unoccupied aerial systems discovered overlooked loci capturing the variation of entire growing period in maize

Traditional phenotyping methods, coupled with genetic mapping in segregating populations, have identified loci governing complex traits in many crops. Unoccupied aerial systems (UAS)-based phenotyping has helped to reveal a more novel and dynamic relationship between time-specific associated loci with complex traits previously unable to be evaluated. Over 1,500 maize (Zea mays L.) hybrid row plots containing 280 different replicated maize hybrids from the Genomes to Fields (G2F) project were evaluated agronomically and using UAS in 2017. Weekly UAS flights captured variation in plant heights during the growing season under three different management conditions each year: optimal planting with irrigation (G2FI), optimal dryland planting without irrigation (G2FD), and a stressed late planting (G2LA). Plant height of different flights were ranked based on importance for yield using a random forest (RF) algorithm. Plant heights captured by early flights in G2FI trials had higher importance (based on Gini scores) for predicting maize grain yield (GY) but also higher accuracies in genomic predictions which fluctuated for G2FD (-0.06∼0.73), G2FI (0.33∼0.76), and G2LA (0.26∼0.78) trials. A genome-wide association analysis discovered 52 significant single nucleotide polymorphisms (SNPs), seven were found consistently in more than one flights or trial; 45 were flight or trial specific. Total cumulative marker effects for each chromosome's contributions to plant height also changed depending on flight. Using UAS phenotyping, this study showed that many candidate genes putatively play a role in the regulation of plant architecture even in relatively early stages of maize growth and development.

A maize ADP-ribosylation factor ZmArf2 increases organ and seed size by promoting cell expansion in Arabidopsis

ADP-ribosylation factors (ARFs) are small GTP-binding proteins that regulate a wide variety of cell functions. Previously, we isolated a new ARF, ZmArf2, from maize (Zea mays). Sequence and expression characteristics indicated that ZmArf2 might play a critical role in the early stages of endosperm development. In this study, we investigated ZmArf2 function by analysis of its GTP-binding activity and subcellular localization. We also over-expressed ZmArf2 in Arabidopsis and measured organ and cell size and counted cell numbers. The expression levels of five organ size-associated genes were also determined in 35S::ZmArf2 transgenic and wild-type plants. Results showed that the recombinant ZmArf2 protein purified from Escherichia coli exhibited GTP-binding activity. Subcellular localization revealed that ZmArf2 was localized in the cytoplasm and plasma membrane. ZmArf2 over-expression in Arabidopsis showed that 35S::ZmArf2 transgenic plants were taller and had larger leaves and seeds compared to wild-type plants, which resulted from cell expansions, not an increase in cell numbers. In addition, three cell expansion-related genes, AtEXP3, AtEXP5 and AtEXP10, were upregulated in 35S::ZmArf2 transgenic lines, while the expression levels of AtGIF1 and AtGRF5, were unchanged. Collectively, our studies suggest that ZmArf2 has an active GTP-binding function, and plays a crucial role in growth and development in Arabidopsis through cell expansion mediated by cell expansion genes.

Molecular characterization and identification of target protein of an important vesicle trafficking gene AlRab7 from a salt excreting halophyte Aeluropus lagopoides

The endomembrane system plays an important role during cellular adaptation of the plants with the extracellular environment. The small GTP-binding protein Rab7 located at the vacuolar membrane regulates the vesicle fusion with the vacuole and thereby helps in recycling of the molecules. This is the first report on isolation and characterization of AlRab7 gene from the halophyte plant, Aeluropus that extrudes NaCl through salt glands and grows luxuriantly throughout the year at the Gujarat coast, India. The AlRab7 encodes a protein with 206 amino acids, and a highly conserved effector-binding domain and four nucleotide-binding domains. The in silico analysis predicts the presence of the prenylation site for Rab geranylgeranyltransferase 2 and the Rab escort protein site. The C-terminal two cysteine residues in -XCC sequence are present for membrane attachment. Transcript expression of the AlRab7 gene was differentially regulated by different environmental stimuli such as dehydration, salinity, and hormone abscisic acid (ABA). The recombinant Escherichia coli cells showed improved growth in Luria Bertani medium supplemented with NaCl, KCl, mannitol, ABA, and indole-3-acetic acid. A novel Rab7 interacting partner AlRabring7 was identified by yeast two-hybrid screening.

Apple hypanthium firmness: new insights from comparative proteomics

Fruit firmness constitutes an important textural property and is one of the key parameters for estimating ripening and shelf life, which has a major impact on commercialization. In order to decipher the mechanisms related to firmness of apples (Malus × domestica Borkh.), two-dimensional gel electrophoresis (2-DE) was used to compare the total proteome of high and low firmness phenotypes from apple hypanthia of a 'Golden Delicious' × 'Dietrich' population. A total of 36 differentially regulated protein spots were positively identified by matrix assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry (MS) and then validated against the Malus expressed sequence tags (EST) database. The findings of this study indicated a lower expression of ethylene biosynthesis related proteins in the high firmness phenotype, which could be linked to the slowing down of the ripening and softening processes. The reduced accumulation of proteins involved in ethylene biosynthesis juxtaposed to the upregulation of a transposase and a GTP-binding protein in the high firmness phenotype. The results also showed higher expression of cytoskeleton proteins in the high firmness phenotype compared to the low firmness phenotype, which play a role in maintaining cell structure and possibly fruit integrity. Finally, a number of proteins involved in detoxification and defense were expressed in fruit hypanthium. This proteomic study provides a contribution towards a better understanding of regulatory networks involved in fruit hypanthium firmness and/or softening, which could be instrumental in the development of improved fruit quality.

Tomato Rab11a characterization evidenced a difference between SYP121-dependent and SYP122-dependent exocytosis

The regulatory functions of Rab proteins in membrane trafficking lie in their ability to perform as molecular switches that oscillate between a GTP- and a GDP-bound conformation. The role of tomato LeRab11a in secretion was analyzed in tobacco protoplasts. Green fluorescent protein (GFP)/red fluorescent protein (RFP)-tagged LeRab11a was localized at the trans-Golgi network (TGN) in vivo. Two serines in the GTP-binding site of the protein were mutagenized, giving rise to the three mutants Rab11S22N, Rab11S27N and Rab11S22/27N. The double mutation reduced secretion of a marker protein, secRGUS (secreted rat beta-glucuronidase), by half, whereas each of the single mutations alone had a much smaller effect, showing that both serines have to be mutated to obtain a dominant negative effect on LeRab11a function. The dominant negative mutant was used to determine whether Rab11 is involved in the pathway(s) regulated by the plasma membrane syntaxins SYP121 and SYP122. Co-expression of either of these GFP-tagged syntaxins with the dominant negative Rab11S22/27N mutant led to the appearance of endosomes, but co-expression of GFP-tagged SYP122 also labeled the endoplasmic reticulum and dotted structures. However, co-expression of Rab11S22/27N with SYP121 dominant negative mutants decreased secretion of secRGUS further compared with the expression of Rab11S22/27N alone, whereas co-expression of Rab11S22/27N with SYP122 had no synergistic effect. With the same essay, the difference between SYP121- and SYP122-dependent secretion was then evidenced. The results suggest that Rab11 regulates anterograde transport from the TGN to the plasma membrane and strongly implicate SYP122, rather than SYP121. The differential effect of LeRab11a supports the possibility that SYP121 and SYP122 drive independent secretory events.

Constitutive overexpression of a stress-inducible small GTP-binding protein PgRab7 from Pennisetum glaucum enhances abiotic stress tolerance in transgenic tobacco

The Rab GTPases are important components of endocytic network in plant cells. Endocytosis participates in the cell's reaction to extracellular stimuli by desensitizing, down-regulating or recycling receptors and membrane proteins. Rab7 is a small GTP-binding protein involved in intracellular vesicle trafficking from late endosome to the vacuole. We have isolated Rab7 cDNA from Pennisetum glaucum, a relatively drought-stress tolerant food grain crop grown commonly in India, during cDNA-subtractive hybridization of dehydration-stress treated plants. The PgRab7 ORF, encoding 207 aminoacids, was over-expressed in E. coli. The recombinant PgRab7 protein showed GTP-binding and GTPase activity. Transcript expression of PgRab7 gene was differentially up-regulated by different environmental stimuli such as cold, dehydration and NaCl and also by a plant hormone IAA. Overexpression of PgRab7 gene enhanced tolerance to NaCl and mannitol in transgenic tobacco. Transgenic plants also had increased alkaline phosphatase (ALP) activity. These results show that PgRab7 is a potential candidate gene for developing both salinity and dehydration tolerance in planta.

Biochemical character of the purified OsRAA1, a novel rice protein with GTP-binding activity, and its expression pattern in Oryza sativa

OsRAA1, as previously reported, is a novel conserved protein in plants and plays an important role in rice root development. Overexpression of OsRAA1 results in reduced growth of primary roots and an increased number of adventitious roots. The biochemical functions and expression patterns of OsRAA1, however, remain poorly understood. To obtain purified OsRAA1 for biochemical analysis, the coding region was amplified by RT-PCR and expressed as a fusion protein with glutathione S-transferase in Escherichia coli. The antibodies to OsRAA1 were prepared by a synthetic 15-residue peptide (YYEDPSLFQFHKRGS) cross-linked with bovine serum albumin. Results of isotope labeling experiments suggested that OsRAA1 had binding activities with [alpha-32P]-GTP. The immunoprecipitation data showed that OsRAA1 had tissue-specific expression in roots and spikes rather than young shoots and leaves, which was consistent with its transcriptional expression. Our results indicate that OsRAA1 GTP-binding activity may contribute to the regulation mechanism of root development mediated by OsRAA1.

Pollen Development and Tube Growth are Affected in the Symbiotic Mutant of Lotus japonicus, crinkle

The symbiotic mutant of Lotus japonicus, crinkle (crk), exhibits abnormal nodulation and other alterations in the root hairs, trichomes, and seedpods. Defective nodulation in crk mutant is due to the arrested infection thread growth from the epidermis into the cortex. Here, we describe that crk is also affected in male fertility that causes the production of small pods with few seeds. Under in vitro conditions, pollen germination and tube growth were markedly reduced in the crk mutant. A swollen tip phenotype with disorganized filamentous actin (F-actin) was observed in the mutant pollen tubes after prolonged in vitro culture. During pollen development, the striking difference noted in the mutant was the small size of the microspores that remained spherical. Histological examination of ovule development, as well as outcrosses of the mutant as female to wild type as male, showed no evidence of abnormality in the female gametophyte development. Based on these findings, the Crk gene, aside from its role in the infection process during nodulation, is also involved in male gametophyte development and function. Therefore, this gene represents a connection between nodule symbiosis, polar tip growth, and other plant developmental processes.

A rac-like small G-protein from Brassica campestris activates a PKC-dependent phospholipase D

A cDNA clone encoding a rac-like small GTP binding protein was isolated from a cDNA library of Chinese cabbage (Brassica campestris L. ssp. pekinensis) flower buds and named Brac1. The Brac1 cDNA contains an open reading frame encoding 198 amino acid residues with an estimated molecular mass of 21,690 Da and this coding region has conserved residues and motifs unique to the Rho subfamily of proteins. The deduced amino acid sequence of the Brac1 protein is closely related to that of Arabidopsis thaliana Arac3 (91%), but it shares relatively little homology with other members of the Ras superfamily (about 30% identity). To further characterize Brac1, a pGBrac1 expression vector consisting of PCR-amplified Brac1 cDNA plus glutathione S-transferase (GST) and pBKS(+)II was used to purify the protein. Using a PEI-cellulose/TLC plate, GTPase activity of this protein was confirmed and competition binding studies, using the guanine nucleotides, ATP, UTP and CTP, revealed that the di- and triphosphate forms of guanine nucleotides strongly bind Brac1. Membrane-bound PLD activity was synergistically enhanced by Brac1 in the presence of protein kinase C, but not in the presence of ARF (ADP-ribosylation factor). Genomic analysis indicated that Brac1 belongs to a multigene family. Brac1 transcripts were expressed in all the organs of Brassica, but were especially prevalent in flower buds.

Molecular and biochemical analyses of OsRab7, a rice Rab7 homolog

Rab7 is a small GTP-binding protein important in early to late endosome/lysosome vesicular transport in mammalian cells. We have isolated a Rab7 cDNA clone, OsRab7, from a cold-treated rice cDNA library by the subtraction screening method. The cDNA encodes a polypeptide of 206 amino acids with a calculated molecular mass of about 23 kDa. Its predicted amino acid sequence shows significantly high identity with the sequences of other Rab7 proteins. His-tagged OsRab7 bound to radiolabeled GTPgammaS in a specific and stoichiometric manner. Biochemical and structural properties of the Rab7 wild type (WT) protein were compared to those of Q67L and T22N mutants. The detergent 3-([3-cholamidopropyl]dimethylammonio)-1-propane sulfonate (CHAPS) increased the guanine nucleotide binding and hydrolysis activities of Rab7WT. The OsRab7Q67L mutant showed much lower GTPase activity compared to the WT protein untreated with CHAPS, and the T22N mutant showed no GTP binding activity at all. The OsRab7Q67L mutant was constitutively active for guanine nucleotide binding while the T22N mutant (dominant negative) showed no guanine nucleotide binding activity. When bound to GTP, the Rab7WT and the Q67L mutants were protected from tryptic proteolysis. The cleavage pattern of the Rab7T22N mutant, however, was not affected by GTP addition. Northern and Western blot analyses suggested that OsRab7 is distributed in various tissues of rice. Furthermore, expression of a rice Rab7 gene was differentially regulated by various environmental stimuli such as cold, NaCl, dehydration, and ABA. In addition, subcellular localization of OsRab7 was investigated in the Arabidopsis protoplasts by a double-labeling experiment using GFP-fused OsRab7 and FM4-64. GFP-OsRab7 is localized to the vacuolar membrane, suggesting that OsRab7 is implicated in a vesicular transport to the vacuole in plant cells.

Arf1 GTPase plays roles in the protein traffic between the endoplasmic reticulum and the Golgi apparatus in tobacco and Arabidopsis cultured cells

Arf GTPases are known to be key regulators of vesicle budding in various steps of membrane traffic in yeast and animal cells. We cloned the Arabidopsis Arf1 homologue, AtArf1, and examined its function. AtArf1 complements yeast arf1 arf2 mutants and its GFP-fusion is localized to the Golgi apparatus in plant cells like its animal counterpart. The expression of dominant negative mutants of AtArf1 in tobacco and Arabidopsis cultured cells affected the localization of co-expressed GFP-tagged proteins in a variety of ways. AtArf1 Q71L and AtArf1 T31N, GTP- and GDP-fixed mutants, respectively, changed the localization of a cis-Golgi marker, AtErd2-GFP, from the Golgi apparatus to the endoplasmic reticulum but not that of GFP-AtRer1B or GFP-AtSed5. GFP-AtRer1B and GFP-AtSed5 were accumulated in aberrant structures of the Golgi by AtArf1 Q71L. A soluble vacuolar protein, sporamin-GFP, was also located to the ER by AtArf1 Q71L. These results indicate that AtArf1 play roles in the vesicular transport between the ER and the Golgi and in the maintenance of the normal Golgi organization in plant cells.

Involvement of GTP-binding protein in the induction of phytoalexin biosynthesis in cultured carrot cells

Biosynthetic activity of carrot phytoalexin 6-methoxymellen was induced in cell suspension culture by the treatment with oligogalacturonide elicitor; however, the elicitor-induced activity appreciably reduced in the presence of suramin, a potent inhibitor of GTP-binding proteins. In contrast, addition of G-protein activators, such as mastoparan or GTP-gamma-S, to carrot cell culture triggered 6-methoxymellein production even in the absence of uronide elicitor. An appreciable GTPase activity was found in purified plasma membrane of cultured carrot cells, and the hydrolytic activity was significantly increased by the addition of elicitor. Carrot plasma membrane was capable of associating with GTP-gamma-S, and the binding ability was markedly increased in the presence of elicitor. However, the binding activity markedly decreased when the membrane preparation was pre-incubated with GTP but not with ATP. These observations strongly suggest that a certain GTP-binding protein located at plasma membrane of cultured carrot cells plays an important role in the oligogalacturonide elicitor-induced 6-methoxymellein production.

Functional implications of protein isoprenylation in plants

Plant protein isoprenylation has received considerable attention in the past decade. Since the initial discovery of isoprenylated plant proteins and their respective protein isoprenyltransferases, several research groups have endeavored to understand the physiological significance of this process in plants. Various experimental approaches, including inhibitor studies, systematic methods of protein identification, and mutant analyses in Arabidopsis thaliana, have enabled these groups to elucidate important roles for isoprenylated proteins in cell cycle control, signal transduction, cytoskeletal organization, and intracellular vesicle transport. This article reviews recent progress in understanding the functional implications of protein isoprenylation in plants.

A dominant negative mutant of sar1 GTPase inhibits protein transport from the endoplasmic reticulum to the Golgi apparatus in tobacco and Arabidopsis cultured cells

Protein secretion plays an important role in plant cells as it does in animal and yeast cells, but the tools to study molecular events of plant secretion are very limited. We have focused on the Sar1 GTPase, which is essential for the vesicle formation from the endoplasmic reticulum (ER) in yeast, and have previously shown that tobacco and Arabidopsis SAR1 complement yeast sar1 mutants. In this study, we have established a transient expression system of GFP-fusion proteins in tobacco and Arabidopsis cultured cells. By utilizing confocal laser scanning microscopy, we demonstrate that a dominant negative mutant of Arabidopsis Sar1 inhibits the ER-to-Golgi transport of Golgi membrane proteins, AtErd2 and AtRer1B, and locates them to the ER. The same mutant Sar1 also blocks the exit from the ER of a vacuolar storage protein, sporamin. These results not only provide the first evidence that the Sar1 GTPase functions in the ER-to-Golgi transport in plant cells, but also prove that conditional expression of dominant mutants of secretory machinery can be a useful tool in manipulating vesicular trafficking.

Nicotiana tabacum cDNAs encoding alpha and beta subunits of a heterotrimeric GTP-binding protein isolated from hairy root tissues

Heterotrimeric GTP-binding proteins (G-proteins) play important roles in signal transduction pathways in eukaryotic cells. Through differential screening of a hairy root cDNA library of tobacco (Nicotiana tabacum L.) against transcripts from non-root tissues of normal cuttings, we obtained a partial cDNA clone that showed abundant expression and high homology to the alpha subunit gene of plant G-protein. After RACE-PCR, a full-length cDNA clone was obtained, which was 1,677-bp in length and contained an open reading frame encoding a protein of 384 amino acids. A cDNA clone encoding a beta subunit of G-protein was also isolated from the same cDNA library based on PCR amplification and library screening. The clone was 1,600-bp in length and contained an open reading frame encoding 377 amino acids. The deduced amino acid sequences of these clones showed high homology (75.5 to 99.8% amino acid identity) with alpha and beta subunits of other plant G-proteins. Genomic Southern blot analysis showed that the amphidiploid tobacco genome possessed two major copies of both alpha and beta subunit genes and some minor homologous copies. Northern blot analysis showed that the transcript of alpha subunit gene was abundant in the root tissues, particularly in the hairy root tissues. In contrast, the level of expression of the beta subunit gene was equivalent in all the tissues studied. Possible function of tobacco G-protein was discussed.

Isolation and sequencing of a cDNA clone encoding a 20-kDa protein with trypsin inhibitory activity

The amino acid sequence of a novel trypsin inhibitor (p20) was completed by the molecular cloning of the protein in cultured soybean cells. The clone nucleotide contains an open reading frame encoding a polypeptide of 206 amino acids that shows 45-50% sequence homology to members of the Kunitz-type trypsin inhibitor family. The p20 transcript is expressed in the roots, stems and leaves of soybean seedlings. DNA gel blot analyses show that the p20 in soybean is encoded by a single gene, and that this gene may not contain an intron.

cDNA cloning and gene expression of three small GTP-binding proteins in defense response of chickpea

The cDNA clones encoding rab type (INR134 and ELR19) and rac type (ELR26) small GTP-binding proteins were isolated from Ascochyta rabiei-inoculated chickpea leaves and the elicitor-treated cell cultures. Rac type ELR26 showed enhanced expression in inoculated leaves indicating correlation with the defense response.

Rice gibberellin-insensitive dwarf mutant gene Dwarf 1 encodes the alpha-subunit of GTP-binding protein

A rice Dwarf 1 gene was identified by using a map-based cloning strategy. Its recessive mutant allele confers a dwarf phenotype. Linkage analysis revealed that a cDNA encoding the alpha-subunit of GTP-binding protein cosegregated with d1 in 3,185 d1 segregants. Southern hybridization analysis with this cDNA as a probe showed different band patterns in several d1 mutant lines. In at least four independent d1 mutants, no gene transcript was observed by Northern hybridization analysis. Sequencing analysis revealed that an 833-bp deletion had occurred in one of the mutant alleles, which resulted in an inability to express GTP-binding protein. A transgenic d1 mutant with GTP-binding protein gene restored the normal phenotype. We conclude that the rice Dwarf 1 gene encodes GTP-binding protein and that the protein plays an important role in plant growth and development. Because the d1 mutant is classified as gibberellin-insensitive, we suggest that the GTP-binding protein might be associated with gibberellin signal transduction.

Light regulation of nitrate reductase gene expression in maize involves a G-protein

This paper reports three lines of evidence to demonstrate the presence of heterotrimeric G-proteins in maize and their involvement in the regulation of nitrate reductase gene expression by light: (1) Southern blot analysis of maize genomic DNA using a human Ha-ras cDNA probe revealed specific bands indicating the presence of G-protein (alpha subunit) gene(s) in maize. Northern blot analysis of maize total RNA using the same probe revealed that the putative Galpha gene(s) is transcriptionally active. (2) Western blots containing purified plasma membrane proteins from maize leaves showed specific binding of gamma [35S]-labeled GTP in a red light-dependent manner, indicating the involvement of G-proteins in mediating the light signal. The size of the putative Galpha gene product (approximately 45 kDa) indicates that it may be a heterotrimeric G-protein. (3) Cholera toxin mimicked the effect of red light to enhance the transcript levels of nitrate reductase (NR), indicating that G-proteins may mediate light regulation of NR gene expression.

Effects of mevinolin on cell cycle progression and viability of tobacco BY-2 cells

Mevinolin, an inhibitor of 3-hydroxy-3-methylglutaryl-CoA reductase, was used to study the importance of mevalonic acid (MVA) for cell cycle progression of tobacco (Nicotiana tabacum L.) BY-2 cells. After treatment with 5 microM mevinolin, the cell cycle progression was completely blocked and two cell populations accumulated (80% in phase G0/G1 and 20% in G2/M). The arrest could be released by subsequent addition of MVA. Effects were compared to those caused by aphidicolin, an inhibitor of alpha-like DNA polymerases that blocks cell cycle at the entry of the S phase. The 80% proportion of mevinolin-treated TBY-2 cells was clearly arrested before the aphidicolin-inducible block. By the aid of a double-blocking technique, it was shown that the mevinolin-induced cell arrest of highly synchronized cells was due to interaction with a control point located at the mitotic telophase/entry G1 phase. Depending on the developmental stage, mevinolin induced rapid cell death in a considerable percentage of cells. Mevinolin treatment led to a partial synchronization, as shown by the increase in mitotic index. The following decrease was correlated with the above-mentioned induction of cell death.

AtGDI2, a novel Arabidopsis gene encoding a Rab GDP dissociation inhibitor

The GTPase cycle of Rab/Ypt proteins is strictly controlled by several classes of regulators to ensure their proper roles in membrane traffic. GDP dissociation inhibitor (GDI) is known to play essential roles in regulating nucleotide states and subcellular localizations of Rab/Ypt proteins. To obtain further knowledge on this regulator molecule in plants, we isolated and characterized two genes of Arabidopsis thaliana that encode different GDIs. AtGDI1 has been identified by a novel functional cloning in yeast [Ueda et al. (1996) Plant Cell, 8, 2079-2091] and AtGDI2 was isolated by cross-hybridization in this study. AtGDI2, as well as AtGDI1, complements the yeast sec19/gdi1 mutant, indicating that they can replace the function of yeast GDI. Evidence is shown that both AtGDI1 and AtGDI2 can interact with Ara4, an Arabidopsis Rab protein, in the yeast ypt1 mutant cells. AtGDI2 is ubiquitously expressed in Arabidopsis tissues with some difference from AtGDI1 in expression level. Genomic DNA hybridization using specific probes reveals the presence of one more GDI gene in Arabidopsis. This may imply differentiated roles of GDI in higher plants.

An expression cloning method to identify monomeric GTP-binding proteins by GTP overlay

We have developed a method for identifying monomeric GTP-binding proteins that is based on probing plasmid expression libraries with [alpha-32P]GTP. The method involves the production of nitrocellulose replica filter lifts from a plasmid cDNA expression library and treatment of the filters with chloroform vapor to lyse the Escherichia coli and to denature and inactivate endogenous E. coli GTP-binding proteins, thus allowing the direct identification of cDNA clones which encode Ras-like small GTP-binding proteins by ligand blotting. Using this procedure we have cloned a series of small Ras-like GTP-binding proteins from human retina. The method relies on a functional test, ligand specificity of the expressed proteins, to identify candidate molecules. This results in the isolation of predominantly full-length cDNA clones without relying on DNA sequence similarity. Thus, this method may be particularly useful for the cloning of novel Ras-related GTP-binding proteins which share limited sequence similarity with previously identified members of the Ras superfamily.

Sequence analysis of a 24-kb contiguous genomic region at the Arabidopsis thaliana PFL locus on chromosome 1

As part of the European Union program of European Scientist Sequencing Arabidopsis (ESSA), the DNA sequence of a 24.053-bp insert of cosmid clone CC17J13 was determined. The cosmid is located on chromosome 1 at the PFL locus (position 30 cM). Analysis of the sequence and comparison to public databases predicts seven genes in this area, thus approximately one gene every 3.3 kb. Three cDNAs corresponding to genes in this region were also sequenced. The homologies and/or possible functions of the (putative) genes are discussed. Proteins encoded by genes in this region include a polyadenylate-binding protein (PAB-3) and a GTP-binding protein (Rab7) as well as a novel protein, possibly involved in double-stranded RNA unwinding and apoptosis. Intriguingly, the gene encoding the PAB-3 protein, which is very specifically expressed, is flanked by putative matrix attachment regions.

Guanine-nucleotide binding and hydrolyzing kinetics of ORrab2, a rice small GTP-binding protein expressed in Escherichia coli

The ORrab2 gene encodes a GTP-binding protein of 23.169 kDa. The deduced amino acid sequence shows that ORrab2 has the motifs conserved among small GTP-binding proteins in plants and that it shares sequence identity with Atrab2 (93.0%), Hrab2 (85.2%), Hrab4 (51.9%), Hrab1 (46.2%), YPT (40.7%), Hrab3B (40.0%), Hrab3A (38.1%), SEC4 (38.1%), Hrab5 (34.3%) and Hrab6 (32.4%). To analyze the biochemical properties of this protein, an ORrab2 cDNA was overexpressed in Escherichia coli and the protein purified by Ni2+-nitrilotriacetic acid agarose and hydroxyapatite column chromatography. The molecular mass of the protein bearing a His-tag is approximately 28.2 kDa. The guanine-nucleotide binding and hydrolyzing activity of ORrab2 increased with non-ionic C12E10 (polyoxyethylene 10-lauryl ether) and ionic Chaps detergent treatment. ORrab2 bound maximally 1.03 mol of [gamma-35S]GTP[S]/mol of protein with a Kd value of 56.83 nM. The ratios k(off GDP)/k(off GTP) of ORrab2 were 3.63 for the control, 3.7 in the presence of C12E10, and 3.83 with Chaps, indicating that ORrab2 has a higher affinity for GTP than GDP. The rate (k(cat)) of Pi release against [gamma-32P]GTP bound ORrab2 in a steady state and the rate of hydrolysis of [gamma-32P]GTP (kGTPase) were calculated to be 432 x 10(-4) +/- 8 x 10(-4) min(-1) and 172 x 10(-4) +/- 2 x 10(-4) min(-1), respectively, in the presence of 0.1% C12E10 and 1 mM MgSO4.

Ca2+-independent and Ca2+/GTP-binding protein-controlled exocytosis in a plant cell

Exocytosis allows the release of secretory products and the delivery of new membrane material to the plasma membrane. So far, little is known about the underlying molecular mechanism and its control in plant cells. We have used the whole-cell patch-clamp technique to monitor changes in membrane capacitance to study exocytosis in barley aleurone protoplasts. To investigate the involvement of Ca2+ and GTP-binding proteins in exocytosis, protoplasts were dialyzed with very low (<2 nM) and high (1 microM) free Ca2+ and nonhydrolyzable guanine nucleotides guanosine 5'-gamma-thio]triphosphate (GTP[gammaS]) or guanosine 5'-[beta-thio]diphosphate (GDP[betaS]). With less than 2 nM cytoplasmic free Ca2+, the membrane capacitance increased significantly over 20 min. This increase was not altered by GTP[gammaS] or GDP[betaS]. In contrast, dialyzing protoplasts with 1 microM free Ca2+ resulted in a large increase in membrane capacitance that was slightly reduced by GTP[gammaS] and strongly inhibited by GDP[betaS]. We conclude that two exocytotic pathways exist in barley aleurone protoplasts: one that is Ca2+-independent and whose regulation is currently not known and another that is stimulated by Ca2+ and modulated by GTP-binding proteins. We suggest that Ca2+-independent exocytosis may be involved in cell expansion in developing protoplasts. Ca2+-stimulated exocytosis may play a role in gibberellic acid-stimulated alpha-amylase secretion in barley aleurone and, more generally, may be involved in membrane resealing in response to cell damage.

Tobacco cells contain a protein, immunologically related to the neutrophil small G protein Rac2 and involved in elicitor-induced oxidative burst

Suspension-cultured cells of Nicotiana tabacum generated active oxygen species (AOS) when they were treated with the proteinaceous elicitor, cryptogein. This response was blocked by diphenylene iodonium, an inhibitor of the neutrophil NADPH oxidase. When microsomal extracts of tobacco cells were probed with an antibody directed against the human small G protein Rac2, two immunoreactive proteins were detected at 18.5 and 20.5 kDa. The same experiment performed with cytosolic extracts of tobacco cells led to the observation of a strong immunoreactive protein at 21.5 kDa only in the cryptogein-treated cells. The appearance of this cytosolic protein was related to the production of AOS by the elicited cells. These results provide evidence for the possible involvement of small G proteins, homologous to the neutrophil Rac2 protein, in the regulation of the elicitor-induced oxidative burst in plant.

A homolog of the mammalian GTPase Rab2 is present in Arabidopsis and is expressed predominantly in pollen grains and seedlings

Vesicle traffic between the endoplasmic reticulum and the Golgi apparatus in mammals requires the small GTP-binding protein Rab2, but Saccharomyces cerevisiae appears not to have a Rab2 homolog. Here it is shown that the higher plant, Arabidopsis thaliana, contains a gene, At-RAB2, whose predicted product shares 79% identity with human Rab2 protein. Transgenic plants containing fusions between beta-glucuronidase and sequences upstream of At-RAB2 demonstrated histochemical staining predominantly in maturing pollen and rapidly growing organs of germinating seedlings. beta-glucuronidase activity in pollen is first detectable at microspore mitosis and increases thereafter. In this respect, the promoter of At-RAB2 behaves like those of class II pollen-specific genes, whose products are often required after germination for pollen tube growth. Seedling germination and pollen tube growth are notable for their unusually high rates of cell wall and membrane biosynthesis. These results are consistent with a role for At-RAB2 in secretory activity.

A rab11-like gene is developmentally regulated in ripening mango (Mangifera indica L.) fruit

A full-length cDNA clone from mango (Mangifera indica L.) fruit has homology to the rab11/YPT3 class of small GTPases. The corresponding mRNA is expressed in fruit, only during ripening. The likely involvement of this RabX protein in trafficking cell-wall modifying enzymes through the trans-Golgi network is discussed.

Isolation and analysis of the soybean SGA2 gene (cDNA), encoding a new member of the plant G-protein family of signal transducers

We have isolated a cDNA clone from Glycine max, named SGA2, coding for a G alpha-subunit protein. The encoded polypeptide, SG alpha2, shows a molecular mass of 45 kDa and contains most of the conserved regions involved in guanine nucleotide binding and hydrolysis. Comparison at the nucleotide and amino acid sequence levels with the other plant G alpha's shows a high degree of conservation (>85% similarity). Phylogenetic analysis of these plant genes with the other G alpha's from different species clearly indicate that those proteins represent a new member of the heterotrimeric G-protein family, named Gp. Tissue localization of SGA2 transcripts in root, stem and leaf organs shows that this gene is widely expressed throughout the plant although it is most abundant in the vascular tissues of all these organs. Furthermore, the transcript is more abundant in young tissues and organ primordia than mature tissues. The high degree of sequence conservation among the plant G alpha's and the differences to other species of other kingdoms, suggest that plant G proteins may function in specialized signalling processes.

Identification and characterization of a lower plant Ypt/Rab guanosine dissociation inhibitor (GDI)

The cDNA encoding a Ypt/Rab guanosine dissociation inhibitor (Ypt-GDI) was isolated from the multicellular green alga Volvox carteri, representing the first complete plant gdi gene described. The gdiV1 gene occurs as a single copy in the algal genome, indicating that its product regulates all YptV proteins from Volvox. The derived GDI protein (GDIV1p) shows high similarity to animal and fungal GDIs. A specific antibody developed against GDIV1p detected the protein throughout the whole Volvox life-cycle. GDIV1p was localized in the cytoplasm and in the algal flagellum. This is in line with earlier findings of a dual localization of Ypt proteins both in the cell body and in the motility organelle, and indicates a novel role of the GDI/Ypt system, possibly in intraflagellar transport.

A low-molecular-mass GTP-binding protein in the cytosol of germinated wheat embryos

A low-molecular-mass protein able to bind GTP in both native and SDS-denaturating conditions was detected in the cytosol of embryos from wheat (Triticum aestivum L.) seeds germinated for 40 h. The protein fulfilled most of the distinguishing criteria common to eukaryotic small GTP-binding proteins. It retained the ability to bind GTP after SDS/PAGE and nitrocellulose blotting. The protein eluted from Sephadex G-200 gel filtration with a Ve/Vo value corresponding to a molecular mass of 18 kDa, whereas on SDS/PAGE the molecular mass was 20 kDa. The native protein, which showed an intrinsic GTPase activity highly sensitive to NaF, bound the guanine nucleotide with high specificity and with a relatively high affinity (Kd approximately 85 nM). The GTP-binding protein was not detectable in other subcellular fractions; in the microsomal fraction, two other peptides of low molecular mass (23.5 and 21.5 kDa) with GTP-binding activity were detected. These results indicate that in the cytosolic fraction of germinating wheat embryos there is a 20-kDa protein which is biochemically similar to the known small GTP-binding proteins that currently have been detected almost exclusively in the membrane fraction of plant material.

G proteins regulate dihydropyridine binding to moss plasma membranes

The role of calcium as an activator and regulator of many biological processes is linked to the ability of the cell to rapidly change its cytoplasmic calcium levels. Calcium acts as an intracellular messenger in hormone-induced bud formation during the development of the moss Physcomitrella patens. Calcium transport and ligand binding studies have implicated plasma membrane-localized 1, 4-dihydropyridine (DHP)-sensitive calcium channels in this increase in cellular calcium. To understand the regulation of the moss calcium channel, we investigated the involvement of GTP binding regulatory proteins (G proteins). Guanosine 5'-(gamma-thio)triphosphate (GTPgammaS), a nonhydrolyzable GTP analog that locks G proteins into their active state, stimulated DHP binding to high affinity receptors in the moss plasma membrane. DHP binding was measured as the ability of the DHP agonist Bay K8644 or the DHP antagonist nifedipine to compete with the DHP arylazide [3H]azidopine for binding to moss plasma membranes. G protein stimulation of binding was seen when competition was carried out with either nifedipine or Bay K8644. G proteins regulated the rates of association and dissociation of bound [3H]azidopine, and stimulation was dependent on GTPgammaS concentration. Guanosine 5'-(beta-thio)diphosphate, a GDP analog that locks G proteins into their inactivated state, did not affect the dose dependence of either the agonist or the antagonist. These results suggest that G proteins may act via a membrane-delimited pathway to regulate calcium channels in the moss plasma membrane.

Identification and isoprenylation of plant GTP-binding proteins

To identify isoprenylated plant GTP-binding proteins, Arabidopsis thaliana and Nicotiana tabacum cDNA expression libraries were screened for cDNA-encoded proteins capable of binding [32P]GTP in vitro. ATGB2, an Arabidopsis homologue of the GTP-binding protein Rab2, was found to bind GTP in vitro and to be a substrate for a geranylgeranyl:protein transferase (GGTase) present in plant extracts. The carboxyl terminus of this protein contains a -GCCG sequence, which has not previously been shown to be recognized by any prenyl:protein transferase (PTase), but which most closely resembles that isoprenylated by the type II GGTase (-XXCC, -XCXC, or -CCXX). In vitro geranylgeranylation of an Arabidopsis Rab1 protein containing a carboxyl-terminal-CCGQ sequence confirmed the presence of a type II GGTase-like activity in plant extracts. Several other proteins were also identified by in vitro GTP binding, including Arabidopsis and tobacco homologues of Rab11, ARF (ADP-ribosylation factor) and Sar proteins, as well as a novel 22 kDa Arabidopsis protein (ATG81). This 22 kDa protein had consensus GTP-binding motifs and bound GTP with high specificity, but its structure was not closely related to that of any known GTP-binding protein (it most resembled proteins within the ARF/Sar and G protein alpha-subunit superfamilies).

Cloning and characterization of a cDNA encoding a Rab1-like small GTP-binding protein from Petunia hybrida

The cloning of small GTP-binding proteins from Petunia hybrida was performed using a PCR-based strategy. Degenerate primers were designed from the DTAGQE and FMETSA consensus sequences. Three different cDNAs were amplified. The deduced polypeptide sequences PhPCRGP1 and PhPCRGP2 were homologous to RB11_HUMAN and PhPCRGP3 to RAB1A_HUMAN. Using PhPCRGP3 as a probe, 8 identical clones were selected from a Petunia leaf cDNA library. They all encode the same 22.5 kDa polypeptide, PhRAB1, able to bind GTP in vitro and 72% identical to RAB1A_HUMAN. Hybridizable mRNAs encoding PhRAB1 accumulated preferentially in opened flowers.

Isolation of an additional soybean cDNA encoding Ypt/Rab-related small GTP-binding protein and its functional comparison to Sypt using a yeast ypt1-1 mutant

We have previously reported the isolation of a gene from a soybean cDNA library encoding a Ypt/Rab-related small GTP-binding protein, Sypt. Here, we report the isolation of a second Ypt/Rab-related gene, designated Srab2, from the same soybean cDNA library. And we compare the in vivo function of the two soybean genes utilizing a yeast ypt1-1 mutant. The Srab2 gene encodes 211 amino acid residues with a molecular mass of 23 169 Da. The deduced amino acid sequence of the Srab2 is closely related to the rat (76%) and human (75%) Rab2 proteins, but it shares relatively little homology to Sypt (46%) and Saccharomyces cerevisiae ypt proteins (41%). Genomic Southern blot analysis using the cDNA insert of Srab2 revealed that it belongs to a multigene family in the soybean genome. The protein encoded by Srab2 gene, when expressed in Escherichia coli, disclosed a GTP-binding activity. The expression pattern of the Srab2 gene is quite different from that of the Sypt gene. The Srab2 gene is predominantly expressed in the plumule region, while expression was very low in the other areas in soybean seedlings. On the other hand, the Sypt mRNA is not detectable in any tissues of soybean seedlings grown in the dark. However, light significantly suppressed the Srab2 gene expression, but enhanced the transcript levels of the Sypt gene in leaf and, at even higher levels, in root tissues. When the Srab2 and Sypt genes are introduced separately into a S cerevisiae defective in vesicular transport function, the Srab2 gene cannot complement the temperature-sensitive yeast ypt1-1 mutation at all, in contrast to the Sypt gene. In conclusion, the difference of functional complementation of the yeast mutation together with differential expression of the two genes suggest that the in vivo roles of the Srab2 and Sypt genes may be different in soybean cells.

Small G proteins of two green algae are localized to exocytic compartments and to flagella

The Ypt/Rab proteins are small GTPases, which belong to the Ras superfamily and have been shown to be involved in endo- and exocytosis in mammalian cells and yeast. Using affinity-purified antibodies specific for four Ypt proteins, namely Ypt1p, Ypt4p, Ypt5p and Ypt6p, of the multicellular green alga Volvox carteri (YptVp) and its close unicellular relative Chlamydomonas reinhardtii (YptCp), we examined the abundance of the corresponding antigens during the asexual life cycle of Volvox, and their intracellular localization. The YptV proteins were found in all stages throughout the asexual life cycle and are tightly associated with intracellular membranes. Indirect immunofluorescence revealed that YptV4p, YptV5p and YptV6p are present in perinuclear regions of the cell, indicating an association with the Golgi region. Golgi localization of YptV4p and YptV6p in Volvox was confirmed by immunogold electron microscopy. In contrast, we found Ypt1p associated with the contractile vacuole in both V. carteri and C. reinhardtii. Furthermore, the YptV proteins were also detected along the entire length of the flagella of somatic Volvox cells. This flagellar location was substantiated by western blot analysis of extracts prepared from isolated flagella of both algae. While localization to exocytic compartments is in agreement with the established Ypt/Rab function in intracellular vesicle transport of eukaryotic cells, presence in the algal flagellum is the first hint of a possible role for small G proteins also in motility organelles.

Partial purification and characterization of a calcium-dependent protein kinase in rice leaves

A protein from rice leaves, which was partially purified by sequential chromatography on DE52, MONO-Q and Superose 12, presented calcium-dependent protein kinase (CDPK) activity. This protein kinase phosphorylated the substrate, histone III-S, in a Ca(2+)-dependent manner and the half-maximum concentration of Ca2+ to protein kinase activity (EC50) was 1 microM. This phosphorylation was independent of phosphatidylserine and a phorbol ester. The apparent M(r) of the protein kinase, as determined by phosphorylation in SDS-polyacrylamide gel containing histone III-S, was 45 k. This kinase was found to react differently from other protein kinases, such as protein kinase C from rat brain or CDPK from soybean leaves, owing to the absence of a phospholipid or phorbol ester dependency. CDPK phosphorylated three endogenous proteins as detected by in vitro phosphorylation on two-dimensional PAGE.

Guanosine triphosphate-binding proteins on the plasmalemma of spinach leaf cells

The molecular mechanism of light perception through phytochrome is not well understood. This red-light photosensor has been implicated in various physiological processes, including the photoinduction of flowering. A few recent studies have shown that phytochrome initiates signal transduction chains via guanosine triphosphate (GTP)-binding proteins (G-proteins). We show here by different approaches that G-proteins exist in spinach (Spinacia oleracea L. cv. Nobel). Binding of GTP on the plasmalemma has been partially characterized and its possible regulation by red light examined by in-vitro assays. These experiments indicate a clear regulation of GTP binding by red light and also by Mastoparan. At least three G-proteins or protein subunits were found to be associated with the plasmalemma of leaf cells. The use of an antibody raised against an animal Gβ subunit confirmed the presence of heterotrimeric G-proteins. Separation of a crude membrane extract by free-flow electrophoresis also showed that some G-proteins could exist on the tonoplast.

Characterization and subcellular localization of a small GTP-binding protein (Ara-4) from Arabidopsis: conditional expression under control of the promoter of the gene for heat-shock protein HSP81-1

Small GTP-binding proteins belonging to the rab/YPT family play key roles at various steps in intracellular transport pathways in yeast and mammalian cells. Many members of rab/YPT family have been isolated from plants to date. However, detailed information about the localization and function of the gene products remains limited, even though intracellular transport is likely to be involved in important phenomena such as cell elongation, transport of storage proteins, determination and maintenance of cell polarity and intercellular signal transduction. We have attempted to establish transgenic Arabidopsis plants that overexpress ARA-4, a rab/YPT homologue in order to analyze the function and the localization of the gene product. For overexpression and also for regulation of the expression of this gene, the promoter of the gene for HSP81-1 was employed to drive the transcription of ARA-4 in transgenic plants. The response of the introduced genes to heat shock was analyzed. Upon heat-shock treatment, the ARA-4 gene was efficiently transcribed and translated. The induction of ARA-4 by heat shock was transient, and at least two distinct forms of this protein were found in membrane and cytosolic fractions from transgenic plants. Prolonged incubation after heat shock reduced the amount of the cytosolic form of the induced protein, and the cytosolic form of the protein thus probably represents the unprocessed precursor. Using transgenic plants, we determined the subcellular localization of the product of ARA-4. The protein was predominantly localized on Golgi-derived vesicles, Golgi cisternae and the trans-Golgi network.

Comparison of the expression patterns of genes coding for wheat gluten proteins and proteins involved in the secretory pathway in developing caryopses of wheat

The synthesis of gluten proteins in the developing caryopsis of wheat is highly coordinated, with mRNAs for the various groups being detected from 11 days after anthesis, and the proteins from about 14 days. In contrast, the levels of transcripts for BiP, PDI and PPI are highest at earlier stages of development. The levels of transcripts for two small GTP binding proteins involved in the secretory pathway (Rab1 and Rab5) are also highest early in development, which is consistent with the retention of most of the gluten proteins within the ER to form protein bodies.

Genes encoding small GTP-binding proteins analogous to mammalian rac are preferentially expressed in developing cotton fibers

In animals, the small GTP-binding proteins, Rac and Rho, of the ras superfamily participate in the signal transduction pathway that regulates the organization of the actin cytoskeleton. We report here on the characterization of two distinct cDNA clones isolated from a cotton fiber cDNA library that code for homologs of animal Rac proteins. Using gene-specific probes, we have determined that amphidiploid cotton contains two genes that code for each of the two Rac proteins, designated Rac13 and Rac9, respectively. The gene for Rac13 shows highly enhanced expression in developing cotton fibers, with maximal expression occurring at the time of transition between primary and secondary wall synthesis. This is also the time at which reorganization of the cytoskeleton occurs, and thus the pattern of expression of Rac13 is consistent with its possible role, analogous to animal Rac, in the signal transduction pathway that controls cytoskeletal organization.