Elongation factor 1 alpha is a component of the subcortical actin bundles of characean algae

LreEF1A4, a Translation Elongation Factor from Lilium regale, Is Pivotal for Cucumber Mosaic Virus and Tobacco Rattle Virus Infections and Tolerance to Salt and Drought

Eukaryotic translation elongation factors are implicated in protein synthesis across different living organisms, but their biological functions in the pathogenesis of cucumber mosaic virus (CMV) and tobacco rattle virus (TRV) infections are poorly understood. Here, we isolated and characterized a cDNA clone, LreEF1A4, encoding the alpha subunit of elongation factor 1, from a CMV-elicited suppression subtractive hybridization library of Lilium regale. The infection tests using CMV remarkably increased transcript abundance of LreEF1A4; however, it also led to inconsistent expression profiles of three other LreEF1A homologs (LreEF1A1-3). Protein modelling analysis revealed that the amino acid substitutions among four LreEF1As may not affect their enzymatic functions. LreEF1A4 was ectopically overexpressed in petunia (Petunia hybrida), and transgenic plants exhibited delayed leaf and flower senescence, concomitant with increased transcription of photosynthesis-related genes and reduced expression of senescence-associated genes, respectively. A compromised resistance to CMV and TRV infections was found in transgenic petunia plants overexpressing LreEF1A4, whereas its overexpression resulted in an enhanced tolerance to salt and drought stresses. Taken together, our data demonstrate that LreEF1A4 functions as a positive regulator in viral multiplication and plant adaption to high salinity and dehydration.

Holophytochrome-Interacting Proteins in Physcomitrella: Putative Actors in Phytochrome Cytoplasmic Signaling

Phytochromes are the principle photoreceptors in light-regulated plant development, primarily acting via translocation of the light-activated photoreceptor into the nucleus and subsequent gene regulation. However, several independent lines of evidence indicate unambiguously that an additional cytoplasmic signaling mechanism must exist. Directional responses in filament tip cells of the moss Physcomitrella patens are steered by phy4 which has been shown to interact physically with the blue light receptor phototropin at the plasma membrane. This complex might perceive and transduce vectorial information leading to cytoskeleton reorganization and finally a directional growth response. We developed yeast two-hybrid procedures using photochemically functional, full-length phy4 as bait in Physcomitrella cDNA library screens and growth assays under different light conditions, revealing Pfr-dependent interactions possibly associated with phytochrome cytoplasmic signaling. Candidate proteins were then expressed in planta with fluorescent protein tags to determine their intracellular localization in darkness and red light. Of 14 candidates, 12 were confirmed to interact with phy4 in planta using bimolecular fluorescence complementation. We also used database information to study their expression patterns relative to those of phy4. We discuss the likely functional characteristics of these holophytochrome-interacting proteins (HIP's) and their possible roles in signaling.

Cytoplasmic protein methylation is essential for neural crest migration

Post-translational methylation of the non-histone, actin-binding protein EF1α1 is essential for neural crest migration.

As they initiate migration in vertebrate embryos, neural crest cells are enriched for methylation cycle enzymes, including S-adenosylhomocysteine hydrolase (SAHH), the only known enzyme to hydrolyze the feedback inhibitor of trans-methylation reactions. The importance of methylation in neural crest migration is unknown. Here, we show that SAHH is required for emigration of polarized neural crest cells, indicating that methylation is essential for neural crest migration. Although nuclear histone methylation regulates neural crest gene expression, SAHH and lysine-methylated proteins are abundant in the cytoplasm of migratory neural crest cells. Proteomic profiling of cytoplasmic, lysine-methylated proteins from migratory neural crest cells identified 182 proteins, several of which are cytoskeleton related. A methylation-resistant form of one of these proteins, the actin-binding protein elongation factor 1 alpha 1 (EF1α1), blocks neural crest migration. Altogether, these data reveal a novel and essential role for post-translational nonhistone protein methylation during neural crest migration and define a previously unknown requirement for EF1α1 methylation in migration.

Arabidopsis VILLIN2 and VILLIN3 are required for the generation of thick actin filament bundles and for directional organ growth

In plant cells, actin filament bundles serve as tracks for myosin-dependent organelle movement and play a role in the organization of the cytoplasm. Although virtually all plant cells contain actin filament bundles, the role of the different actin-bundling proteins remains largely unknown. In this study, we investigated the role of the actin-bundling protein villin in Arabidopsis (Arabidopsis thaliana). We used Arabidopsis T-DNA insertion lines to generate a double mutant in which VILLIN2 (VLN2) and VLN3 transcripts are truncated. Leaves, stems, siliques, and roots of vln2 vln3 double mutant plants are twisted, which is caused by local differences in cell length. Microscopy analysis of the actin cytoskeleton showed that in these double mutant plants, thin actin filament bundles are more abundant while thick actin filament bundles are virtually absent. In contrast to full-length VLN3, truncated VLN3 lacking the headpiece region does not rescue the phenotype of the vln2 vln3 double mutant. Our results show that villin is involved in the generation of thick actin filament bundles in several cell types and suggest that these bundles are involved in the regulation of coordinated cell expansion.

eEF1A phosphorylation in the nucleus of insulin-stimulated C2C12 myoblasts: Ser⁵³ is a novel substrate for protein kinase C βI

Recent data indicate that some PKC isoforms are translocated to the nucleus, in response to certain stimuli, where they play an important role in nuclear signaling events. To identify novel interacting proteins of conventional PKC (cPKC) at the nuclear level during myogenesis and to find new PKC isozyme-specific phosphosubstrates, we performed a proteomics analysis of immunoprecipitated nuclear samples from mouse myoblast C2C12 cells following insulin administration. Using a phospho(Ser)-PKC substrate antibody, specific interacting proteins were identified by LC-MS/MS spectrometry. A total of 16 proteins with the exact and complete motif recognized by the phospho-cPKC substrate antibody were identified; among these, particular interest was given to eukaryotic elongation factor 1α (eEF1A). Nuclear eEF1A was focalized in the nucleoli, and its expression was observed to increase following insulin treatment. Of the cPKC isoforms, only PKCβI was demonstrated to be expressed in the nucleus of C2C12 myocytes and to co-immunoprecipitate with eEF1A. In-depth analysis using site-directed mutagenesis revealed that PKCβI could phosphorylate Ser⁵³ of the eEF1A2 isoform and that the association between eEF1A2 and PKCβI was dependent on the phosphorylation status of eEF1A2.

Plant 115-kDa actin-filament bundling protein, P-115-ABP, is a homologue of plant villin and is widely distributed in cells

In many cases, actin filaments are arranged into bundles and serve as tracks for cytoplasmic streaming in plant cells. We have isolated an actin-filament bundling protein, which is composed of 115-kDa polypeptide (P-115-ABP), from the germinating pollen of lily, Lilium longiflorum [Nakayasu et al. (1998) BIOCHEM: Biophys. Res. Commun. 249: 61]. P-115-ABP shared similar antigenicity with a plant 135-kDa actin-filament bundling protein (P-135-ABP), a plant homologue of villin. A full-length cDNA clone (ABP115; accession no. AB097407) was isolated from an expression cDNA library of lily pollen by immuno-screening using antisera against P-115-ABP and P-135-ABP. The amino acid sequence of P-115-ABP deduced from this clone showed high homology with those of P-135-ABP and four villin isoforms of Arabidopsis thaliana (AtVLN1, AtVLN2, AtVLN3 and AtVLN4), especially AtVLN4, indicating that P-115-ABP can also be classified as a plant villin. The P-115-ABP isolated biochemically from the germinating lily pollen was able to arrange F-actin filaments with uniform polarity into bundles and this bundling activity was suppressed by Ca2+-calmodulin (CaM), similar to the actin-filament bundling properties of P-135-ABP. The P-115-ABP type of plant villin was widely distributed in plant cells, from algae to land plants. In root hair cells of Hydrocharis dubia, this type of plant villin was co-localized with actin-filament bundles in the transvacuolar strands and the sub-cortical regions. Microinjection of the antiserum against P-115-ABP into living root hair cells caused the disappearance of transvaculor strands and alteration of the route of cytoplasmic streaming. In internodal cells of Chara corallina in which the P-135-ABP type of plant villin is lacking, the P-115-ABP type showed co-localization with actin-filament cables anchored on the intracellular surface of chloroplasts. These results indicated that plant villins are widely distributed and involved in the organization of actin filaments into bundles throughout the plant kingdom.

Remodeling the cytoskeleton for growth and form: an overview with some new views

The cytoskeleton coordinates all aspects of growth in plant cells, including exocytosis of membrane and wall components during cell expansion. This review seeks to integrate current information about cytoskeletal components in plants and the role they play in generating cell form. Advances in genome analysis have fundamentally changed the nature of research strategies and generated an explosion of new information on the cytoskeleton-associated proteins, their regulation, and their role in signaling to the cytoskeleton. Some of these proteins appear novel to plants, but many have close homologues in other eukaryotic systems. It is becoming clear that the mechanisms behind cell growth are essentially similar across the growth continuum, which ranges from tip growth to diffuse expansion. Remodeling of the actin cytoskeleton at sites of exocytosis is an especially critical feature of polarized and may also contribute to axial growth. We evaluate the most recent work on the signaling mechanisms that continually remodel the actin cytoskeleton via the activation of actin-binding proteins (ABPs) and consider the role the microtubule cytoskeleton plays in this process.

Interaction of elongation factor 1alpha from Zea mays (ZmEF-1alpha) with F-actin and interplay with the maize actin severing protein, ZmADF3

EF-1alpha is an abundant eukaryotic protein whose principle function appears to be to bind aminoacyl-tRNA to the ribosome. However, it is also known that EF-1alpha from other sources binds both microtubules and microfilaments. We report the expression of Zea mays EF-1alpha (ZmEF-1alpha) in bacteria and that this protein has similar actin-binding properties as other EF-1alpha members. ZmEF-1alpha bundles actin filaments at low pH (6.5) and inhibits the addition of monomer at both filament ends, possibly as a consequence. ZmEF-1alpha binds actin filaments at all pH values tested (pH 6.0-8.0), indicating that one actin binding site is not pH sensitive. One of the actin-binding sites was determined to reside within domain I (1-223) of ZmEF-1alpha, but this domain did not affect the kinetics of polymerisation. We show that the bundling activity of ZmEF-1alpha is modulated by ZmADF3 a (a Zea mays ADF/cofilin), an actin filament severing protein, in vitro. Bundling of actin filaments caused by ZmEF-1alpha was enhanced in the presence of ZmADF3. The pH-dependent activities of both proteins in vitro suggests that they may work together to respond to temporal and spatial intracellular pH changes to regulate the pattern of the growth of plant cells.

Calcium-regulated proteolysis of eEF1A

Eukaryotic elongation factor 1alpha (eEF1A) can be post-translationally modified by the addition of phosphorylglycerylethanolamine (PGE). [(14)C]Ethanolamine was incorporated into the PGE modification, and with carrot (Daucus carota L.) suspension culture cells, eEF1A was the only protein that incorporated detectable quantities of [(14)C]ethanolamine (Ransom et al., 1998). When 1 mM CaCl(2) was added to microsomes containing [(14)C]ethanolamine-labeled eEF1A ([(14)C]et-eEF1A), there was a 60% decrease in the amount of [(14)C]et-eEF1A recovered after 10 min. The loss of endogenous [(14)C]et-eEF1A was prevented by adding EGTA. Recombinant eEF1A, which did not contain the PGE modification, also was degraded by microsomes in a Ca(2+)-regulated manner, indicating that PGE modification was not necessary for proteolysis; however, it enabled us to quantify enodgenous eEF1A. By monitoring [(14)C]et-eEF1A, we found that treatment with phospholipase D or C, but not phospholipase A(2), resulted in a decrease in [(14)C]et-eEF1A from carrot microsomes. The fact that there was no loss of [(14)C]et-eEF1A with phospholipase A(2) treatment even in the presence of 1 mM Ca(2+) suggested that the loss of membrane lipids was not essential for eEF1A proteolysis and that lysolipids or fatty acids decreased proteolysis. At micromolar Ca(2+) concentrations, proteolysis of eEF1A was pH sensitive. When 1 microM CaCl(2) was added at pH 7.2, 35% of [(14)C]et-eEF1A was lost; while at pH 6.8, 10 microM CaCl(2) was required to give a similar loss of protein. These data suggest that eEF1A may be an important downstream target for Ca(2+) and lipid-mediated signal transduction cascades.

Interaction of ZPR1 with translation elongation factor-1alpha in proliferating cells

The zinc finger protein ZPR1 is present in the cytoplasm of quiescent mammalian cells and translocates to the nucleus upon treatment with mitogens, including epidermal growth factor (EGF). Homologues of ZPR1 were identified in yeast and mammals. These ZPR1 proteins bind to eukaryotic translation elongation factor-1alpha (eEF-1alpha). Studies of mammalian cells demonstrated that EGF treatment induces the interaction of ZPR1 with eEF-1alpha and the redistribution of both proteins to the nucleus. In the yeast Saccharomyces cerevisiae, genetic analysis demonstrated that ZPR1 is an essential gene. Deletion analysis demonstrated that the NH2-terminal region of ZPR1 is required for normal growth and that the COOH-terminal region was essential for viability in S. cerevisiae. The yeast ZPR1 protein redistributes from the cytoplasm to the nucleus in response to nutrient stimulation. Disruption of the binding of ZPR1 to eEF-1alpha by mutational analysis resulted in an accumulation of cells in the G2/M phase of cell cycle and defective growth. Reconstitution of the ZPR1 interaction with eEF-1alpha restored normal growth. We conclude that ZPR1 is essential for cell viability and that its interaction with eEF-1alpha contributes to normal cellular proliferation.

Purification of an actin-binding protein composed of 115-kDa polypeptide from pollen tubes of lily

From lily pollen tubes, an actin-binding protein composed of 115-kDa polypeptide was purified sequentially by co-precipitation method with F-actin, hydroxylapatite column, gel filtration column and DE-52 ion exchange column chromatography. This component displayed a tendency to aggregate in solutions of low ionic strength, indicating a hydrophilic characteristic. Under physiological ionic conditions, this component bound to F-actin in an actin-concentration-dependent was saturable manner. Binding of this component to F-actin was independent of ATP and Ca(2+)-concentrations. Fluorescent microscopy revealed that F-actin labeled with rhodamine-phalloidin showed bundling in the presence of this component. Judging from the lack of antibody cross-reactivity, this component does not seem to be related to alpha-actinin of skeletal muscle and plant 135-kDa actin-bundling protein. Therefore, this component is the F-actin binding protein, which has not been identified thus far in plant cells.

Phosphoglycerylethanolamine posttranslational modification of plant eukaryotic elongation factor 1alpha

Eukaryotic elongation factor 1alpha (eEF-1A) is a multifunctional protein. There are three known posttranslational modifications of eEF-1A that could potentially affect its function. Except for phosphorylation, the other posttranslational modifications have not been demonstrated in plants. Using matrix-assisted laser desorption/ionization-mass spectrometry and peptide mass mapping, we show that carrot (Daucus carota L.) eEF-1A contains a phosphoglycerylethanolamine (PGE) posttranslational modification. eEF-1A was the only protein labeled with [14C]ethanolamine in carrot cells and was the predominant ethanolamine-labeled protein in Arabidopsis seedlings and tobacco (Nicotiana tabacum L.) cell cultures. In vivo-labeling studies using [3H]glycerol, [32P]Pi, [14C]myristic acid, and [14C]linoleic acid indicated that the entire phospholipid phosphatidylethanolamine is covalently attached to the protein. The PGE lipid modification did not affect the partitioning of eEF-1A in Triton X-114 or its actin-binding activity in in vitro assays. Our in vitro data indicate that this newly characterized posttranslational modification alone does not affect the function of eEF-1A. Therefore, the PGE lipid modification may work in combination with other posttranslational modifications to affect the distribution and the function of eEF-1A within the cell.

Microtubule-organizing centers and nucleating sites in land plants

Microtubule-organizing centers (MTOCs) are morphologically diverse cellular sites involved in the nucleation and organization of microtubules (MTs). These structures are synonymous with the centrosome in mammalian cells. In most land plant cells, however, no such structures are observed and some have argued that plant cells may not have MTOCs. This review summarizes a number of experimental approaches toward the elucidation of those subcellular sites involved in microtubule nucleation and organization. In lower land plants, structurally well-defined MTOCs are present, such as the blepharoplast, multilayered structure, and polar organizer. In higher plants, much of the nucleation and organization of MTs occurs on the nuclear envelope or other endomembranes, such as the plasmalemma and smooth (tubular) endoplasmic reticulum. In some instances, one endomembrane may serve as a site of nucleation whereas others serve as the site of organization. Structural and motor microtubule-associated proteins also appear to be involved in MT nucleation and organization. Immunochemical evidence indicates that at least several of the proteins found in mammalian centrosomes, gamma-tubulin, centrin, pericentrin, and polypeptides recognized by the monoclonal antibodies MPM-2, 6C6, and C9 also recognize putative lower land plant MTOCs, indicating shared mechanisms of nucleation/organization in plants and animals. The most recent data from tubulin incorporation in vivo, mutants with altered MT organization, and molecular studies indicate the potential of these research tools in investigation of MTOCs in plants.

Barley elongation factor 1 alpha: genomic organization, DNA sequence, and phylogenetic implications

A full length cDNA clone encoding the 447 amino acid long barley (Hordeum vulgare cv. Bomi) endosperm elongation factor 1 alpha (eF-1 alpha) was isolated by a differential screening procedure. RFLP mapping of eF-1 alpha showed that the barley genome contains a small eF-1 alpha gene family of 4 copies, with 1 copy of the gene being located on each of chromosomes 2, 4, 6, and 7. Analysis of barley endosperm total proteins by Western blot with antibodies directed towards wheat eF-1 alpha and the sea urchin 51 kDa proteins gave a single band of the expected molecular weight. Amino acid sequence comparison with other plant eF-1 alpha sequences showed that the isolated barley endosperm eF-1 alpha is more similar to the published wheat eF-1 alpha sequence than to eF-1 alpha sequences previously published for the barley cultivars Igri and Dicktoo. The phylogenetic analysis suggests that the barley eF-1 alpha gene family can be divided into two subfamilies and that two ancestral genes existed before the divergence of monocotyledonous and dicotyledonous plants.

FLUORESCENCE MICROSCOPY OF LIVING PLANT CELLS

Since its inception, light microscopy has shown the elegance and subtlety with which function is expressed in the form of the cells, tissues, and organs of the plant. Recently, light microscopy has seen a resurgence in use fueled by advances in microscope design and computer-based image analysis. The structural resolution afforded by static, fixed samples is being increasingly supplemented by approaches using fluorescent analogs and selective fluorescent indicators, which visualize the dynamic processes in living, functioning cells. This review describes some of these approaches and discusses how they are taking us a step closer to viewing the intricate complexity with which plants organize and regulate their functions down to the subcellular level.

Bundling of actin filaments by elongation factor 1 alpha inhibits polymerization at filament ends

Elongation factor 1 alpha (EF1 alpha) is an abundant protein that binds aminoacyl-tRNA and ribosomes in a GTP-dependent manner. EF1 alpha also interacts with the cytoskeleton by binding and bundling actin filaments and microtubules. In this report, the effect of purified EF1 alpha on actin polymerization and depolymerization is examined. At molar ratios present in the cytosol, EF1 alpha significantly blocks both polymerization and depolymerization of actin filaments and increases the final extent of actin polymer, while at high molar ratios to actin, EF1 alpha nucleates actin polymerization. Although EF1 alpha binds actin monomer, this monomer-binding activity does not explain the effects of EF1 alpha on actin polymerization at physiological molar ratios. The mechanism for the inhibition of polymerization is related to the actin-bundling activity of EF1 alpha. Both ends of the actin filament are inhibited for polymerization and both bundling and the inhibition of actin polymerization are affected by pH within the same physiological range; at high pH both bundling and the inhibition of actin polymerization are reduced. Additionally, it is seen that the binding of aminoacyl-tRNA to EF1 alpha releases EF1 alpha's inhibiting effect on actin polymerization. These data demonstrate that EF1 alpha can alter the assembly of F-actin, a filamentous scaffold on which non-membrane-associated protein translation may be occurring in vivo.

Elongation factor-1 alpha is an overexpressed actin binding protein in metastatic rat mammary adenocarcinoma

Overexpression of elongation factor-1 alpha (EF1 alpha) mRNA has been correlated with increased metastatic potential in mammary adenocarcinoma; however, this relationship was not explored at the level of protein expression. As EF1 alpha has been shown in other cell types to be a component of the actin cytoskeleton, a likely effector in metastasis, the actin binding activity of EF1 alpha from metastatic and nonmetastatic rat breast tumors and cell lines was investigated. We have shown that EF1 alpha protein is overexpressed in metastatic compared to nonmetastatic cells and whole tumors. Similarly to other EF1 alpha s, both types of tumor EF1 alpha bind to F-actin, but EF1 alpha from metastatic cells has a reduced affinity for actin. In addition, there is a high correlation between the intracellular distribution of filamentous actin and EF1 alpha in those cytoskeletal structures thought to be important for supporting the cellular motility required for metastasis. Following stimulation with EGF, there is a parallel increase in the amount of F-actin and EF1 alpha associated with the cytoskeleton. The response to EGF can be blocked with cytochalasin D indicating that the binding of EF1 alpha to the cytoskeleton is mediated by F-actin. We propose that a weakened association of EF1 alpha with actin may be related to the metastatic process via an altered organization of the actin cytoskeleton and the differential translation of mRNAs associated with the cytoskeleton.

Trypanosoma cruzi elongation factor 1-alpha: nuclear localization in parasites undergoing apoptosis

The cloning and sequencing of the gene coding for Trypanosoma cruzi elongation factor 1 alpha (TcEF-1 alpha) was performed by screening a T. cruzi genomic library with a probe obtained through the polymerase chain reaction (PCR) amplification of T. cruzi DNA using two oligonucleotides deduced from the sequence of T. brucei EF-1 alpha. Southern blot analysis of T. cruzi digested genomic DNA and Northern blot hybridized with the labeled probe revealed that one copy of TcEF-1 alpha exist in the genome of the parasite. Indirect immunofluorescence technique using anti-EF-1 alpha antibodies and epimastigotes harvested after different days of in vitro culture showed that EF-1 alpha is localised in the cytoplasm of the parasites from the exponential growth phase. Surprisingly, during the stationary phase (ageing parasites), EF-1 alpha was found in the nucleus. Furthermore, treatment of parasites with the antibiotic drug geneticin (G418) which induces the death of epimastigotes by apoptosis showed selective localization of EF-1 alpha in the nucleus of dying parasites. This observation supports the notion already reported in the case of mammalian cells that EF-1 alpha could participate in the transcription processes and possibly in the case of T. cruzi, in the expression regulation of genes involved in the control of cell death. The possible transfection and genomic manipulation of T. cruzi may provide a model to study the role of TcEF-1 alpha in this phenomenon.