Molecular and biochemical characterization of tomato farnesyl-protein transferase

A raison d'être for two distinct pathways in the early steps of plant isoprenoid biosynthesis?

When compared to other organisms, plants are atypical with respect to isoprenoid biosynthesis: they utilize two distinct and separately compartmentalized pathways to build up isoprene units. The co-existence of these pathways in the cytosol and in plastids might permit the synthesis of many vital compounds, being essential for a sessile organism. While substrate exchange across membranes has been shown for a variety of plant species, lack of complementation of strong phenotypes, resulting from inactivation of either the cytosolic pathway (growth and development defects) or the plastidial pathway (pigment bleaching), seems to be surprising at first sight. Hundreds of isoprenoids have been analyzed to determine their biosynthetic origins. It can be concluded that in angiosperms, under standard growth conditions, C₂₀-phytyl moieties, C₃₀-triterpenes and C₄₀-carotenoids are made nearly exclusively within compartmentalized pathways, while mixed origins are widespread for other types of isoprenoid-derived molecules. It seems likely that this coexistence is essential for the interaction of plants with their environment. A major purpose of this review is to summarize such observations, especially within an ecological and functional context and with some emphasis on regulation. This latter aspect still requires more work and present conclusions are preliminary, although some general features seem to exist.

Andrastin A and barceloneic acid metabolites, protein farnesyl transferase inhibitors from Penicillium albocoremium: chemotaxonomic significance and pathological implications

A survey of Penicillium albocoremium was undertaken to identify potential taxonomic metabolite markers. One major and four minor metabolites were consistently produced by the 19 strains surveyed on three different media. Following purification and spectral studies, the metabolites were identified as the known protein farnesyl transferase inhibitors andrastin A (1) and barceloneic acid A (2) along with barceloneic acid B (3), barceloneic lactone (4), and methyl barceloneate (5). These compounds are significant taxonomic markers for P. albocoremium; moreover this is the first report of a methyl ester of a barceloneic acid being produced as a secondary metabolite. Tissue extracts created following pathogenicity trials involving P. albocoremium and Allium cepa confirmed the production of these five metabolites in planta. Barceloneic acid B was found to be biologically active against a P388 murine leukemia cell line.

Hypersensitivity of abscisic acid-induced cytosolic calcium increases in the Arabidopsis farnesyltransferase mutant era1-2

Cytosolic calcium increases were analyzed in guard cells of the Arabidopsis farnesyltransferase deletion mutant era1-2 (enhanced response to abscisic acid). At low abscisic acid (ABA) concentrations (0.1 microM), increases of guard cell cytosolic calcium and stomatal closure were activated to a greater extent in the era1-2 mutant compared with the wild type. Patch clamping of era1-2 guard cells showed enhanced ABA sensitivity of plasma membrane calcium channel currents. These data indicate that the ERA1 farnesyltransferase targets a negative regulator of ABA signaling that acts between the points of ABA perception and the activation of plasma membrane calcium influx channels. Experimental increases of cytosolic calcium showed that the activation of S-type anion currents downstream of cytosolic calcium and extracellular calcium-induced stomatal closure were unaffected in era1-2, further supporting the positioning of era1-2 upstream of cytosolic calcium in the guard cell ABA signaling cascade. Moreover, the suppression of ABA-induced calcium increases in guard cells by the dominant protein phosphatase 2C mutant abi2-1 was rescued partially in era1-2 abi2-1 double mutant guard cells, further reinforcing the notion that ERA1 functions upstream of cytosolic calcium and indicating the genetic interaction of these two mutations upstream of ABA-induced calcium increases.

Efficient prenylation by a plant geranylgeranyltransferase-I requires a functional CaaL box motif and a proximal polybasic domain

Geranylgeranyltransferase-I (GGT-I) is a heterodimeric enzyme that shares a common alpha-subunit with farnesyltransferase (FTase) and has a distinct beta-subunit. GGT-I preferentially modifies proteins, which terminate in a CaaL box sequence motif. Cloning of Arabidopsis GGT-I beta-subunit (AtGGT-IB) was achieved by a yeast (Saccharomyces cerevisiae) two-hybrid screen, using the tomato (Lycopersicon esculentum) FTase alpha-subunit (FTA) as bait. Sequence and structure analysis revealed that the core active site of GGT-I and FTase are very similar. AtGGT-IA/FTA and AtGGT-IB were co-expressed in baculovirus-infected insect cells to obtain recombinant protein that was used for biochemical and molecular analysis. The recombinant AtGGT-I prenylated efficiently CaaL box fusion proteins in which the a(2) position was occupied by an aliphatic residue, whereas charged or polar residues at the same position greatly reduced the efficiency of prenylation. A polybasic domain proximal to the CaaL box motif induced a 5-fold increase in the maximal reaction rate, and increased the affinity of the enzyme to the protein substrate by an order of magnitude. GGT-I retained high activity in a temperature range between 24 degrees C and 42 degrees C, and showed increased activity rate at relatively basic pH values of 7.9 and 8.5. Reverse transcriptase-polymerase chain reaction, protein immuno-blots, and transient expression assays of green fluorescent protein fusion proteins show that GGT-IB is ubiquitously expressed in a number of tissues, and that expression levels and protein activity were not changed in mutant plants lacking FTase beta-subunit.

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.

Prenylation of the floral transcription factor APETALA1 modulates its function

The Arabidopsis MADS box transcription factor APETALA1 (AP1) was identified as a substrate for farnesyltransferase and shown to be farnesylated efficiently both in vitro and in vivo. AP1 regulates the transition from inflorescence shoot to floral meristems and the development of sepals and petals. AP1 fused to green fluorescent protein (GFP) retained transcription factor activity and directed the expected terminal flower phenotype when ectopically expressed in transgenic Arabidopsis. However, ap1mS, a farnesyl cysteine-acceptor mutant of AP1, as well as the GFP-ap1mS fusion protein failed to direct the development of compound terminal flowers but instead induced novel phenotypes when ectopically expressed in Arabidopsis. Similarly, compound terminal flowers did not develop in era1-2 transformants that ectopically expressed AP1. Together, the results demonstrate that AP1 is a target of farnesyltransferase and suggest that farnesylation alters the function and perhaps specificity of the transcription factor.

Prenylation of the floral transcription factor APETALA1 modulates its function

The Arabidopsis MADS box transcription factor APETALA1 (AP1) was identified as a substrate for farnesyltransferase and shown to be farnesylated efficiently both in vitro and in vivo. AP1 regulates the transition from inflorescence shoot to floral meristems and the development of sepals and petals. AP1 fused to green fluorescent protein (GFP) retained transcription factor activity and directed the expected terminal flower phenotype when ectopically expressed in transgenic Arabidopsis. However, ap1mS, a farnesyl cysteine-acceptor mutant of AP1, as well as the GFP-ap1mS fusion protein failed to direct the development of compound terminal flowers but instead induced novel phenotypes when ectopically expressed in Arabidopsis. Similarly, compound terminal flowers did not develop in era1-2 transformants that ectopically expressed AP1. Together, the results demonstrate that AP1 is a target of farnesyltransferase and suggest that farnesylation alters the function and perhaps specificity of the transcription factor.

Cloning of the Arabidopsis WIGGUM gene identifies a role for farnesylation in meristem development

Control of cellular proliferation in plant meristems is important for maintaining the correct number and position of developing organs. One of the genes identified in the control of floral and apical meristem size and floral organ number in Arabidopsis thaliana is WIGGUM. In wiggum mutants, one of the most striking phenotypes is an increase in floral organ number, particularly in the sepals and petals, correlating with an increase in the width of young floral meristems. Additional phenotypes include reduced and delayed germination, delayed flowering, maturation, and senescence, decreased internode elongation, shortened roots, aberrant phyllotaxy of flowers, aberrant sepal development, floral buds that open precociously, and occasional apical meristem fasciation. As a first step in determining a molecular function for WIGGUM, we used positional cloning to identify the gene. DNA sequencing revealed that WIGGUM is identical to ERA1 (enhanced response to abscisic acid), a previously identified farnesyltransferase beta-subunit gene of Arabidopsis. This finding provides a link between protein modification by farnesylation and the control of meristem size. Using in situ hybridization, we examined the expression of ERA1 throughout development and found it to be nearly ubiquitous. This extensive expression domain is consistent with the pleiotropic nature of wiggum mutants and highlights a broad utility for farnesylation in plant growth and development.

Lipid-linked proteins of plants

Increasing numbers of plant proteins are being shown to have posttranslationally-attached lipids. The modifications include N-myristoylation, S-palmitoylation, prenylation by farnesyl or geranylgeranyl moieties, or attachment of glycosylphosphatidylinositol anchors. This report summarizes recent findings regarding the structure, metabolism and physiological functions of these important protein-linked lipids.

Farnesol is utilized for isoprenoid biosynthesis in plant cells via farnesyl pyrophosphate formed by successive monophosphorylation reactions

The ability of Nicotiana tabacum cell cultures to utilize farnesol (F-OH) for sterol and sesquiterpene biosynthesis was investigated. [(3)H]F-OH was readily incorporated into sterols by rapidly growing cell cultures. However, the incorporation rate into sterols was reduced by greater than 70% in elicitor-treated cell cultures whereas a substantial proportion of the radioactivity was redirected into capsidiol, an extracellular sesquiterpene phytoalexin. The incorporation of [(3)H]F-OH into sterols was inhibited by squalestatin 1, suggesting that [(3)H]F-OH was incorporated via farnesyl pyrophosphate (F-P-P). Consistent with this possibility, N. tabacum proteins were metabolically labeled with [(3)H]F-OH or [(3)H]geranylgeraniol ([(3)H]GG-OH). Kinase activities converting F-OH to farnesyl monophosphate (F-P) and, subsequently, F-P-P were demonstrated directly by in vitro enzymatic studies. [(3)H]F-P and [(3)H]F-P-P were synthesized when exogenous [(3)H]F-OH was incubated with microsomal fractions and CTP. The kinetics of formation suggested a precursor-product relationship between [(3)H]F-P and [(3)H]F-P-P. In agreement with this kinetic pattern of labeling, [(32)P]F-P and [(32)P]F-P-P were synthesized when microsomal fractions were incubated with F-OH and F-P, respectively, with [gamma-(32)P]CTP serving as the phosphoryl donor. Under similar conditions, the microsomal fractions catalyzed the enzymatic conversion of [(3)H]GG-OH to [(3)H]geranylgeranyl monophosphate and [(3)H]geranylgeranyl pyrophosphate ([(3)H]GG-P-P) in CTP-dependent reactions. A novel biosynthetic mechanism involving two successive monophosphorylation reactions was supported by the observation that [(3)H]CTP was formed when microsomes were incubated with [(3)H]CDP and either F-P-P or GG-P-P, but not F-P. These results document the presence of at least two CTP-mediated kinases that provide a mechanism for the utilization of F-OH and GG-OH for the biosynthesis of isoprenoid lipids and protein isoprenylation.

Protein farnesylation in plants: a greasy tale

Although farnesylation is required for a number of abscisic acid mediated responses in plants, knowledge of how this lipid modification of proteins regulates specific developmental and physiological processes remains unclear. Recent information from the Arabidopsis genome-sequencing project in combination with mutants deficient in farnesylation should unravel the role(s) of this process in plant signaling.

Prenylcysteine alpha-carboxyl methyltransferase in suspension-cultured tobacco cells

Isoprenylation is a posttranslational modification that is believed to be necessary, but not sufficient, for the efficient association of numerous eukaryotic cell proteins with membranes. Additional modifications have been shown to be required for proper intracellular targeting and function of certain isoprenylated proteins in mammalian and yeast cells. Although protein isoprenylation has been demonstrated in plants, postisoprenylation processing of plant proteins has not been described. Here we demonstrate that cultured tobacco (Nicotiana tabacum cv Bright Yellow-2) cells contain farnesylcysteine and geranylgeranylcysteine alpha-carboxyl methyltransferase activities with apparent Michaelis constants of 73 and 21 &mgr;M for N-acetyl-S-trans, trans-farnesyl-L-cysteine and N-acetyl-S-all-trans-geranylgeranyl-L-cysteine, respectively. Furthermore, competition analysis indicates that the same enzyme is responsible for both activities. These results suggest that alpha-carboxyl methylation is a step in the maturation of isoprenylated proteins in plants.

Chromatographic assay and peptide substrate characterization of partially purified farnesyl- and geranylgeranyltransferases from rat brain cytosol

A simple method for partially purifying both farnesyltransferase and geranylgeranyltransferase from rat brain cytosol is presented. Each of the final protein preparations contains one single transferase activity. A common method of measurement of both activities is described. The assay, which follows substrate prenylation, is also convenient for the measurement of the concomitant decrease in cosubstrates during the two transfer reactions. The quantitative HPLC detection of the prenylated substrates and of the cosubstrate consumption is used here to follow the purification processes. The same method is also used for substrate-specificity studies of the two enzymes performed on 18 synthetic hexapeptides derived from the C-terminus of proteins known to be prenylated in vivo. These studies partially confirm the reported differences in the substrate specificities of the two prenyltransferases. However, the observed recognition of overlapping sequences by the two enzymes might have important consequences for the inhibition of either of the enzymes in vivo and for the design of specific inhibitors.

Molecular characterization of a cDNA encoding a novel small GTP-binding protein from Arabidopsis thaliana

A full-length cDNA clone encoding a novel Rab protein AtRab alpha of the monomeric small GTP-binding protein family has been isolated from Arabidopsis thaliana. AtRab alpha has 210 amino acids with a calculated molecular mass of 23.3 kDa. The highest homology was found to Rab1x and Rab1y from Lotus japonicus. Southern blot analysis of genomic DNA indicated that AtRab alpha was encoded by a single copy gene. Northern blot analysis showed that expression of the AtRab alpha mRNA was rich in stems and roots, but poor in leaves and flowers, which is different from the expression pattern of other Arabidopsis Rab genes.

Mutational analysis of conserved residues of the beta-subunit of human farnesyl:protein transferase

The roles of 11 conserved amino acids of the beta-subunit of human farnesyl:protein transferase (FTase) were examined by performing kinetic and biochemical analyses of site-directed mutants. This biochemical information along with the x-ray crystal structure of rat FTase indicates that residues His-248, Arg-291, Lys-294, and Trp-303 are involved with binding and utilization of the substrate farnesyl diphosphate. Our data confirm structural evidence that amino acids Cys-299, Asp-297, and His-362 are ligands for the essential Zn2+ ion and suggest that Asp-359 may also play a role in Zn2+ binding. Additionally, we demonstrate that Arg-202 is important for binding the essential C-terminal carboxylate of the protein substrate.

Plant farnesyltransferase can restore yeast Ras signaling and mating

Farnesyltransferase (FTase) is a heterodimeric enzyme that modifies a group of proteins, including Ras, in mammals and yeasts. Plant FTase alpha and beta subunits were cloned from tomato and expressed in the yeast Saccharomyces cerevisiae to assess their functional conservation in farnesylating Ras and a-factor proteins, which are important for cell growth and mating. The tomato FTase beta subunit (LeFTB) alone was unable to complement the growth defect of ram1 delta mutant yeast strains in which the chromosomal FTase beta subunit gene was deleted, but coexpression of LeFTB with the plant alpha subunit gene (LeFTA) restored normal growth, Ras membrane association, and mating. LeFTB contains a novel 66-amino-acid sequence domain whose deletion reduces the efficiency of tomato FTase to restore normal growth to yeast ram1 delta strains. Coexpression of LeFTA and LeFTB in either yeast or insect cells yielded a functional enzyme that correctly farnesylated CaaX-motif-containing peptides. Despite their low degree of sequence homology, yeast and plant FTases shared similar in vivo and in vitro substrate specificities, demonstrating that this enzymatic modification of proteins with intermediates from the isoprenoid biosynthesis pathway is conserved in evolutionarily divergent eukaryotes.