Actin crosslinking protein EF-1a of Dictyostelium discoideum has a unique bonding rule that allows square-packed bundles

Identification of a modulator of the actin cytoskeleton, mitochondria, nutrient metabolism and lifespan in yeast

In yeast, actin cables are F-actin bundles that are essential for cell division through their function as tracks for cargo movement from mother to daughter cell. Actin cables also affect yeast lifespan by promoting transport and inheritance of higher-functioning mitochondria to daughter cells. Here, we report that actin cable stability declines with age. Our genome-wide screen for genes that affect actin cable stability identified the open reading frame YKL075C. Deletion of YKL075C results in increases in actin cable stability and abundance, mitochondrial fitness, and replicative lifespan. Transcriptome analysis revealed a role for YKL075C in regulating branched-chain amino acid (BCAA) metabolism. Consistent with this, modulation of BCAA metabolism or decreasing leucine levels promotes actin cable stability and function in mitochondrial quality control. Our studies support a role for actin stability in yeast lifespan, and demonstrate that this process is controlled by BCAA and a previously uncharacterized ORF YKL075C, which we refer to as actin, aging and nutrient modulator protein 1 (AAN1).

Actin cables affect lifespan by supporting movement and inheritance of fitter mitochondria to daughter cells in yeast. Here the authors show that branched-chain amino acid (BCAA) levels affect actin cable stability and a role for YKL075C/AAN1 in control of BCAA metabolism and actin cable stability and function.

Mechanical Forces and Their Effect on the Ribosome and Protein Translation Machinery

Mechanical forces acting on biological systems, at both the macroscopic and microscopic levels, play an important part in shaping cellular phenotypes. There is a growing realization that biomolecules that respond to force directly applied to them, or via mechano-sensitive signalling pathways, can produce profound changes to not only transcriptional pathways, but also in protein translation. Forces naturally occurring at the molecular level can impact the rate at which the bacterial ribosome translates messenger RNA (mRNA) transcripts and influence processes such as co-translational folding of a nascent protein as it exits the ribosome. In eukaryotes, force can also be transduced at the cellular level by the cytoskeleton, the cell’s internal filamentous network. The cytoskeleton closely associates with components of the translational machinery such as ribosomes and elongation factors and, as such, is a crucial determinant of localized protein translation. In this review we will give (1) a brief overview of protein translation in bacteria and eukaryotes and then discuss (2) how mechanical forces are directly involved with ribosomes during active protein synthesis and (3) how eukaryotic ribosomes and other protein translation machinery intimately associates with the mechanosensitive cytoskeleton network.

Inappropriate expression of the translation elongation factor 1A disrupts genome stability and metabolism

The translation elongation factor eEF1A is one of the most abundant proteins found within cells, and its role within protein synthesis is well documented. Levels of eEF1A are tightly controlled, with inappropriate expression linked to oncogenesis. However, the mechanisms by which increased eEF1A expression alters cell behaviour are unknown. Our analyses in yeast suggest that elevation of eEF1A levels leads to stabilisation of the spindle pole body and changes in nuclear organisation. Elevation of the eEF1A2 isoform also leads to altered nuclear morphology in cultured human cells, suggesting a conserved role in maintaining genome stability. Gene expression and metabolomic analyses reveal that the level of eEF1A is crucial for the maintenance of metabolism and amino acid levels in yeast, most likely because of its role in the control of vacuole function. Increased eEF1A2 levels trigger lysosome biogenesis in cultured human cells, also suggesting a conserved role within metabolic control mechanisms. Taken together, our data suggest that the control of eEF1A levels is important for the maintenance of a number of cell functions beyond translation and that its de-regulation might contribute to its oncogenic properties.

Summary: The translation elongation factor eEF1A is elevated in some cancers. We use yeast and human cell models to show that eEF1A elevation leads to genome instability and metabolic alterations that might affect its oncogenic properties.

Yeast translation elongation factor-1A binds vacuole-localized Rho1p to facilitate membrane integrity through F-actin remodeling

Rho GTPases are molecular switches that modulate a variety of cellular processes, most notably those involving actin dynamics. We have previously shown that yeast vacuolar membrane fusion requires re-organization of actin filaments mediated by two Rho GTPases, Rho1p and Cdc42p. Cdc42p initiates actin polymerization to facilitate membrane tethering; Rho1p has a role in the late stages of vacuolar fusion, but its mode of action is unknown. Here, we identified eEF1A as a vacuolar Rho1p-interacting protein. eEF1A (encoded by the TEF1 and TEF2 genes in yeast) is an aminoacyl-tRNA transferase needed during protein translation. eEF1A also has a second function that is independent of translation; it binds and organizes actin filaments into ordered cable structures. Here, we report that eEF1A interacts with Rho1p via a C-terminal subdomain. This interaction occurs predominantly when both proteins are in the GDP-bound state. Therefore, eEF1A is an atypical downstream effector of Rho1p. eEF1A does not promote vacuolar fusion; however, overexpression of the Rho1p-interacting subdomain affects vacuolar morphology. Vacuoles were destabilized and prone to leakage when treated with the eEF1A inhibitor narciclasine. We propose a model whereby eEF1A binds to Rho1p-GDP on the vacuolar membrane; it is released upon Rho1p activation and then bundles actin filaments to stabilize fused vacuoles. Therefore, the Rho1p-eEF1A complex acts to spatially localize a pool of eEF1A to vacuoles where it can readily organize F-actin.

How and why does β-actin mRNA target?

beta-Actin mRNA is localized near the leading edge in several cell types where actin polymerization is actively promoting forward protrusion. The localization of the beta-actin mRNA near the leading edge is facilitated by a short sequence in the 3'UTR (untranslated region), the 'zipcode'. Localization of the mRNA at this region is important physiologically. Treatment of chicken embryo fibroblasts with antisense oligonucleotides complementary to the localization sequence (zipcode) in the 3'UTR leads to delocalization of beta-actin mRNA, alteration of cell phenotype and a decrease in cell motility. The dynamic image analysis system (DIAS) used to quantify movement of cells in the presence of sense and antisense oligonucleotides to the zipcode showed that net pathlength and average speed of antisense-treated cells were significantly lower than in sense-treated cells. This suggests that a decrease in persistence of direction of movement and not in velocity results from treatment of cells with zipcode-directed antisense oligonucleotides. We postulate that delocalization of beta-actin mRNA results in delocalization of nucleation sites and beta-actin protein from the leading edge followed by loss of cell polarity and directional movement. Hence the physiological consequences of beta-actin mRNA delocalization affect the stability of the cell phenotype.

Actin bundling and polymerisation properties of eukaryotic elongation factor 1 alpha (eEF1A), histone H2A-H2B and lysozyme in vitro

Elongation factor 1 alpha (eEF1A) is a positively charged protein which has been shown to interact with the actin cytoskeleton. However, to date, a specific actin binding site within the eEF1A sequence has not been identified and the mechanism by which eEF1A interacts with actin remains unresolved. Many protein-protein interactions occur as a consequence of their physicochemical properties and actin bundle formation has been shown to result from non-specific electrostatic interaction with basic proteins. This study investigated interactions between actin, eEF1A and two other positively charged proteins which are not regarded as classic actin binding proteins (namely lysozyme and H2A-H2B) in order to compare their actin organising effects in vitro. For the first time using atomic force microscopy (AFM) we have been able to image the interaction of eEF1A with actin and the subsequent bundling of actin in vitro. Interestingly, we found that eEF1A dramatically increases the rate of polymerisation (45-fold above control levels). We also show for the first time that H2A-H2B has remarkably similar effects upon actin bundling (relative bundle size/number) and polymerisation (35-fold increase above control levels) as eEF1a. The presence of lysozyme resulted in bundles which were distinct from those formed due to eEF1A and H2A-H2B. Lysozyme also increased the rate of actin polymerisation above the control level (by 10-fold). Given the striking similarities between the actin bundling and polymerisation properties of eEF1A and H2A-H2B, our results hint that dimerisation and electrostatic binding may provide clues to the mechanism through which eEF1A-actin bundling occurs.

A cytoplasm-specific activity encoded by the Trithorax-like ATX1 gene

Eukaryotes produce multiple products from a single gene locus by alternative splicing, translation or promoter usage as mechanisms expanding the complexity of their proteome. Trithorax proteins, including the Arabidopsis Trithorax-like protein ATX1, are histone modifiers regulating gene activity. Here, we report that a novel member of the Trithorax family has a role unrelated to chromatin. It is encoded from an internal promoter in the ATX1 locus as an isoform containing only the SET domain (soloSET). It is located exclusively in the cytoplasm and its substrate is the elongation factor 1A (EF1A). Loss of SET, but not of the histone modifying ATX1-SET activity, affects cytoskeletal actin bundling illustrating that the two isoforms have distinct functions in Arabidopsis cells.

Emerging role for the cytoskeleton as an organizer and regulator of translation

The cytoskeleton is an intricate and dynamic fibrous network that has an essential role in the generation and regulation of cell architecture and cellular mechanical properties. The cytoskeleton also evolved as a scaffold that supports diverse biochemical pathways. Recent evidence favours the hypothesis that the cytoskeleton participates in the spatial organization and regulation of translation, at both the global and local level, in a manner that is crucial for cellular growth, proliferation and function.

Elongation factors are involved in cytokinesis of sea urchin eggs

Cleavage furrows (CFs) have been isolated from dividing sea urchin eggs and the protein constituents have been analyzed by two-dimensional gel electrophoresis (Fujimoto & Mabuchi, J. Biochem. 122, 518-524, 1997). Two proteins of 51 and 32 kDa, respectively, have been found to be enriched in the CF preparation. Here, we show that these proteins are identical to the protein elongation factor 1alpha (EF-1alpha) and 1beta (EF-1beta), respectively. Furthermore, the CF 51-kDa protein is identical to the 51-kDa protein which had been isolated as a component of the microtubule organizing granules of mitotic sea urchin eggs. The 51-kDa protein bundles F-actin in vitro. This activity is suppressed by Ca(2+)/calmodulin or GTPgammaS. The 32-kDa protein binds EF-1alpha both in vitro and in cell extract, and is shown to suppress the F-actin-bundling activity of the 51-kDa protein. Microinjection of a monoclonal antibody against the 51-kDa protein or that of His-tagged 32-kDa protein into dividing sea urchin eggs at the onset of cleavage leads to failure of cytokinesis. These results strongly suggest that EF-1alpha is involved in maintenance of the structure of the contractile ring and EF-1beta regulates the F-actin-bundling activity of EF-1alpha.

EF1A1-actin interactions alter mRNA stability to determine differential osteopontin expression in HepG2 and Hep3B cells

Cancer progression depends on an accumulation of metastasis-supporting physiological changes which are regulated by cell signaling molecules. One such molecule, osteopontin (OPN), is a secreted phosphoprotein which mediates increased cellular migratory and invasive behavior, increased metastasis, protection from apoptosis, promotion of colony formation and 3D growth ability, induction of tumor-associated inflammatory cells, and induction of expression of angiogenic factors. Studies show that OPN expression is controlled by complex regulatory pathways at the transcriptional level in several cancers, but the molecular mechanisms which determine expression of OPN in HCC are largely unknown. In HepG2 and Hep3B tumor cell lines that differentially express OPN mRNA and protein, we identify elongation translation factor-1A1 (EF1A1) to be the trans-acting factor regulating differential OPN mRNA stability between HepG2 and Hep3B cell lines and characterize its interactions with G- and F-actin. EF1A1 binds to the OPN 5'-UTR to regulate OPN mRNA half-life. EF1A1 binds to actin in Hep3B cells. Pharmacologic manipulation to increase the G:F actin ratio in Hep3B increases OPN mRNA half-life and protein expression with simultaneous decrease in EF1A1 binding to OPN 5'-UTR. The converse findings were noted in HepG2 cells. Overall, our results suggest that EF1A1 regulation of OPN mRNA stability is actin dependent. EF1A1 has not been previously identified as a regulatory factor in OPN expression in cancer.

Proteomic analysis of anoxia tolerance in the developing zebrafish embryo

While some species and tissue types are injured by oxygen deprivation, anoxia tolerant organisms display a protective response that has not been fully elucidated and is well-suited to genomic and proteomic analysis. However, such methodologies have focused on transcriptional responses, prolonged anoxia, or have used cultured cells or isolated tissues. In this study of intact zebrafish embryos, a species capable of >24 h survival in anoxia, we have utilized 2D difference in gel electrophoresis to identify changes in the proteomic profile caused by near-lethal anoxic durations as well as acute anoxia (1 h), a timeframe relevant to ischemic events in human disease when response mechanisms are largely limited to post-transcriptional and post-translational processes. We observed a general stabilization of the proteome in anoxia. Proteins involved in oxidative phosphorylation, antioxidant defense, transcription, and translation changed over this time period. Among the largest proteomic alterations was that of muscle cofilin 2, implicating the regulation of the cytoskeleton and actin assembly in the adaptation to acute anoxia. These studies in an intact embryo highlight proteomic components of an adaptive response to anoxia in a model organism amenable to genetic analysis to permit further mechanistic insight into the phenomenon of anoxia tolerance.

Tumor necrosis factor-alpha downregulates endothelial nitric oxide synthase mRNA stability via translation elongation factor 1-alpha 1

Endothelium-derived nitric oxide (NO) is an important regulator of vascular function. NO is produced by endothelial NO synthase (eNOS), whose expression is downregulated by tumor necrosis factor (TNF)-alpha at the posttranscriptional level. To elucidate the molecular basis of TNF-alpha-mediated eNOS mRNA instability, eNOS 3' untranslated region (3'-UTR) binding proteins were purified by RNA affinity chromatography from cytosolic fractions of TNF-alpha-stimulated human umbilical vein endothelial cells (HUVECs). The formation of 3'-UTR ribonucleoprotein complexes, with molecular weight of 52 and 57 kDa, was increased by TNF-alpha. Matrix-assisted laser desorption ionization time-of-flight mass spectrometric analysis of the 52-kDa protein identified 3 peptides that comprise the peptide sequence of translation elongation factor 1-alpha 1 (eEF1A1). In HUVECs, TNF-alpha rapidly increased eEF1A1 expression, which is maximal after 1 hour and persists for up to 48 hours. RNA gel mobility-shift and UV cross-linking assays indicated that recombinant glutathione S-transferase-eEF1A1 fusion protein specifically binds to a UC-rich sequence in the 3'-UTR of eNOS mRNA. In addition, the domain III of eEF1A1 mediates the binding of eNOS 3'-UTR in eEF1A1. Overexpression of eEF1A1 markedly attenuated the expression of eNOS and luciferase gene fused with eNOS 3'-UTR in both COS-7 cells and bovine aortic endothelial cells (BAECs). Furthermore, adenovirus-mediated overexpression of eEF1A1 increased eNOS mRNA instability, whereas knockdown of eEF1A1 substantially attenuated TNF-alpha-induced destabilization of eNOS mRNA and downregulation of eNOS expression in HUVECs. These results indicate that eEF1A1 is a novel eNOS 3'-UTR binding protein that plays a critical role in mediating TNF-alpha-induced decrease in eNOS mRNA stability.

Arabidopsis phosphatidylinositol phosphate kinase 1 binds F-actin and recruits phosphatidylinositol 4-kinase beta1 to the actin cytoskeleton

The actin cytoskeleton can be influenced by phospholipids and lipid-modifying enzymes. In animals the phosphatidylinositol phosphate kinases (PIPKs) are associated with the cytoskeleton through a scaffold of proteins; however, in plants such an interaction was not clear. Our approach was to determine which of the plant PIPKs interact with actin and determine whether the PIPK-actin interaction is direct. Our results indicate that AtPIPK1 interacts directly with actin and that the binding is mediated through a predicted linker region in the lipid kinase. AtPIPK1 also recruits AtPI4Kbeta1 to the cytoskeleton. Recruitment of AtPI4Kbeta1 to F-actin was dependent on the C-terminal catalytic domain of phosphatidylinositol-4-phosphate 5-kinase but did not require the presence of the N-terminal 251 amino acids, which includes 7 putative membrane occupation and recognition nexus motifs. In vivo studies confirm the interaction of plant lipid kinases with the cytoskeleton and suggest a role for actin in targeting PIPKs to the membrane.

Improper organization of the actin cytoskeleton affects protein synthesis at initiation

Although the actin cytoskeleton and the translation machinery are considered to be separate cellular complexes, growing evidence supports overlapping regulation of the two systems. Because of its interaction with actin, the eukaryotic translation elongation factor 1A (eEF1A) is proposed to be a regulator or link between these processes. Using a genetic approach with the yeast Saccharomyces cerevisiae, specific regions of eEF1A responsible for actin interactions and bundling were identified. Five new mutations were identified along one face of eEF1A. Dramatic changes in cell growth, cell morphology, and actin cable and patch formation as well as a unique effect on total translation in strains expressing the F308L or S405P eEF1A mutant form were observed. The translation effects do not correlate with reduced translation elongation but instead include an initiation defect. Biochemical analysis of the eEF1A mutant forms demonstrated reduced actin-bundling activity in vitro. Reduced total translation and/or the accumulation of 80S ribosomes in strains with either a mutation or a null allele of genes encoding actin itself or actin-regulating proteins Tpm1p, Mdm20p, and Bnirp/Bni1p was observed. Our data demonstrate that eEF1A, other actin binding proteins, and actin mutants affect translation initiation through the actin cytoskeleton.

eEF1A2 activates Akt and stimulates Akt-dependent actin remodeling, invasion and migration

eEF1A2 (eukaryotic protein elongation factor 1 alpha 2) is a protein translation factor that is likely a human oncogene by virtue of its capacity to transform mammalian cells and its high expression in tumors of the ovary, breast and lung. Here, we show that expression of eEF1A2 is sufficient to stimulate the formation of filopodia in BT549 human breast cancer cells and non-transformed Rat2 cells. Filopodia formation in eEF1A2-expressing cells is dependent on the activity of phosphatidylinositol-3 kinase (PI3K), and the ROCK and Akt kinases. Furthermore, eEF1A2 expression is sufficient to activate Akt in a PI3K-dependent fashion and inactivation of eEF1A2 by short interfering RNA reduces Akt activity. Using breast cancer cell line BT 549, we show that eEF1A2 expression stimulates cell migration and invasion in a largely PI3K- and Akt-dependent manner. These results suggest that eEF1A2 regulates oncogenesis through Akt and PI3K-dependent cytoskeletal remodeling.

The yeast actin cytoskeleton: from cellular function to biochemical mechanism

All cells undergo rapid remodeling of their actin networks to regulate such critical processes as endocytosis, cytokinesis, cell polarity, and cell morphogenesis. These events are driven by the coordinated activities of a set of 20 to 30 highly conserved actin-associated proteins, in addition to many cell-specific actin-associated proteins and numerous upstream signaling molecules. The combined activities of these factors control with exquisite precision the spatial and temporal assembly of actin structures and ensure dynamic turnover of actin structures such that cells can rapidly alter their cytoskeletons in response to internal and external cues. One of the most exciting principles to emerge from the last decade of research on actin is that the assembly of architecturally diverse actin structures is governed by highly conserved machinery and mechanisms. With this realization, it has become apparent that pioneering efforts in budding yeast have contributed substantially to defining the universal mechanisms regulating actin dynamics in eukaryotes. In this review, we first describe the filamentous actin structures found in Saccharomyces cerevisiae (patches, cables, and rings) and their physiological functions, and then we discuss in detail the specific roles of actin-associated proteins and their biochemical mechanisms of action.

Translation elongation factor 1A is essential for regulation of the actin cytoskeleton and cell morphology

The binding of eukaryotic translation elongation factor 1A (eEF1A) to actin is a noncanonical function that may link two distinct cellular processes, cytoskeleton organization and gene expression. Using the yeast Saccharomyces cerevisiae, we have established an in vivo assay that directly identifies specific regions and residues of eEF1A responsible for actin interactions and bundling. Using a unique genetic screen, we isolated a series of eEF1A mutants with reduced actin bundling activity. These mutations alter actin cytoskeleton organization but not translation, indicating that these are separate functions of eEF1A. This demonstrates for the first time a direct consequence of eEF1A on cytoskeletal organization in vivo and the physiological significance of this interaction.

pH, EF-1alpha and the cytoskeleton

One of the unexpected cellular components found interacting with the cytoskeleton is elongation factor 1 alpha (EF-1alpha). How this interaction is regulated is not clear, but pH may be a potent regulator. Interestingly, pH also regulates the amount of protein translation occurring in many cell systems. In this paper, the authors suggest that sequestration of EF-1alpha in the cytoskeleton may play a key role in regulating the spatial distribution of macromolecular assembly in a way that is dependent on cytoplasmic pH.

Formation of Hirano bodies in Dictyostelium and mammalian cells induced by expression of a modified form of an actin-crosslinking protein

We report the serendipitous development of the first cultured cell models of Hirano bodies. Myc-epitope-tagged forms of the 34 kDa actin bundling protein (amino acids 1-295) and the CT fragment (amino acids 124-295) of the 34 kDa protein that exhibits activated actin binding and calcium-insensitive actin filament crosslinking activity were expressed in Dictyosteliumand mammalian cells to assess the behavior of these modified forms in vivo. Dictyostelium cells expressing the CT-myc fragment: (1) form ellipsoidal regions that contain ordered assemblies of F-actin, CT-myc, myosin II, cofilin and α-actinin; (2) grow and develop more slowly than wildtype, but produce normal morphogenetic structures; (3) perform pinocytosis and phagocytosis normally; and (4) produce a level of total actin equivalent to wildtype, but a higher level of F-actin. The paracrystalline inclusions bear a striking resemblance to Hirano bodies, which are associated with a number of pathological conditions. Furthermore, expression of the CT fragment in murine L cells results in F-actin rearrangements characterized by loss of stress fibers, accumulation of numerous punctate foci, and large perinuclear aggregates, the Hirano bodies. Thus, failure to regulate the activity and/or affinity of an actin crosslinking protein can provide a signal for formation of Hirano bodies. More generally, formation of Hirano bodies is a cellular response to or a consequence of aberrant function of the actin cytoskeleton. The results reveal that formation of Hirano bodies is not necessarily related to cell death. These cultured cell models should facilitate studies of the biochemistry, genetics and physiological effects of Hirano bodies.

Interactions of elongation factor 1alpha with F-actin and beta-actin mRNA: implications for anchoring mRNA in cell protrusions

The targeting of mRNA and local protein synthesis is important for the generation and maintenance of cell polarity. As part of the translational machinery as well as an actin/microtubule-binding protein, elongation factor 1alpha (EF1alpha) is a candidate linker between the protein translation apparatus and the cytoskeleton. We demonstrate in this work that EF1alpha colocalizes with beta-actin mRNA and F-actin in protrusions of chicken embryo fibroblasts and binds directly to F-actin and beta-actin mRNA simultaneously in vitro in actin cosedimentation and enzyme-linked immunosorbent assays. To investigate the role of EF1alpha in mRNA targeting, we mapped the two actin-binding sites on EF1alpha at high resolution and defined one site at the N-terminal 49 residues of domain I and the other at the C-terminal 54 residues of domain III. In vitro actin-binding assays and localization in vivo of recombinant full-length EF1alpha and its various truncates demonstrated that the C terminus of domain III was the dominant actin-binding site both in vitro and in vivo. We propose that the EF1alpha-F-actin complex is the scaffold that is important for beta-actin mRNA anchoring. Disruption of this complex would lead to delocalization of the mRNA. This hypothesis was tested by using two dominant negative polypeptides: the actin-binding domain III of EF1alpha and the EF1alpha-binding site of yeast Bni1p, a protein that inhibits EF1alpha binding to F-actin and also is required for yeast mRNA localization. We demonstrate that either domain III of EF1alpha or the EF1alpha-binding site of Bni1p inhibits EF1alpha binding to beta-actin mRNA in vitro and causes delocalization of beta-actin mRNA in chicken embryo fibroblasts. Taken together, these results implicate EF1alpha in the anchoring of beta-actin mRNA to the protrusion in crawling cells.

Elongation factor 1beta is an actin-binding protein

A 17 kDa polypeptide found in association with actin in cellular extracts of Dictyostelium discoideum was identified as a proteolytic fragment of eEF1beta. Antibody elicited against the 17 kDa protein reacted with a single 29 kDa polypeptide in Dictyostelium, indicating that the 17 kDa peptide arises from degradation of a larger precursor. The cDNA isolated from a Dictyostelium library using this antibody as a probe encodes Dictyostelium elongation factor 1beta. Amino acid degradation of the 17 kDa protein fragment confirmed the identity of the protein as eEF1beta. Direct interaction of eEF1beta with actin in vitro was further demonstrated in mixtures of actin with the 17 kDa protein fragment of Dictyostelium eEF1beta, recombinant preparations of Dictyostelium eEF1beta expressed in Escherichia coli, and the intact eEF1betagamma complex purified from wheat germ. Localization of eEF1beta in Dictyostelium by immunofluorescence microscopy reveals both diffuse cytoplasmic staining, and some concentration in the cortical and hyaline cytoplasm. The results support the existence of physical and functional interactions of the translation apparatus with the cytoskeleton, and suggest that eEF1beta may function in a dual role both to promote the elongation phase of protein synthesis, and to interact with cytoplasmic actin.

Overexpression of translation elongation factor 1A affects the organization and function of the actin cytoskeleton in yeast

The translation elongation factor 1 complex (eEF1) plays a central role in protein synthesis, delivering aminoacyl-tRNAs to the elongating ribosome. The eEF1A subunit, a classic G-protein, also performs roles aside from protein synthesis. The overexpression of either eEF1A or eEF1B alpha, the catalytic subunit of the guanine nucleotide exchange factor, in Saccharomyces cerevisiae results in effects on cell growth. Here we demonstrate that overexpression of either factor does not affect the levels of the other subunit or the rate or accuracy of protein synthesis. Instead, the major effects in vivo appear to be at the level of cell morphology and budding. eEF1A overexpression results in dosage-dependent reduced budding and altered actin distribution and cellular morphology. In addition, the effects of excess eEF1A in actin mutant strains show synthetic growth defects, establishing a genetic connection between the two proteins. As the ability of eEF1A to bind and bundle actin is conserved in yeast, these results link the established ability of eEF1A to bind and bundle actin in vitro with nontranslational roles for the protein in vivo.

Elongation factor-1 alpha is a novel substrate of rho-associated kinase

Rho-associated kinase (Rho-kinase), which is activated by the Rho small GTPase, phosphorylates the myosin-binding subunit (MBS) of myosin phosphatase, myosin light chain (MLC), the ERM family proteins, and adducin, thereby regulating the formation of stress fibers, focal adhesions, microvillus formation, and cell motility. Here, to further understand the role of Rho-kinase in the regulation of the numerous cellular processes by Rho, we purified a novel substrate of Rho-kinase having a molecular mass of 48 kDa (p48) from a rat liver cytosol extract. Mass spectral analysis revealed p48 to be elongation factor-1 alpha (EF-1 alpha), which is known as an actin-binding protein besides a cofactor of polypeptide elongation. Rho-kinase directly phosphorylated recombinant EF-1alpha in vitro. A high- speed cosedimentation assay revealed that phosphorylation of EF-1 alpha by Rho-kinase decreased the binding activity of EF-1 alpha to filamentous actin (F-actin). A low-speed sedimentation assay revealed that phosphorylation of EF-1 alpha by Rho-kinase decreased the F-actin-bundling activity. In addition, EF-1 alpha bound to MBS of myosin phosphatase, suggesting that both Rho-kinase and myosin phosphatase regulate the phosphorylation state of EF-1 alpha downstream of Rho as other substrates of Rho-kinase, i.e., MLC, adducin, and the ERM family. These results suggest that the Rho/Rho-kinase pathway regulates the organization of actin cytoskeleton via the phosphorylation of EF-1 alpha.

Location of the binding site of the mannose-specific lectin comitin on F-actin

We have used electron microscopy and computer image processing to produce a three-dimensional reconstruction of F-actin filaments decorated with the putative lectin and actin-binding protein comitin. These reconstructions show that comitin binds to F-actin at high radius primarily to actin subdomain 1. This location is distinctly different from the binding site on F-actin for other actin bundling proteins, such as members of the alpha-actinin family, and may result from the positively charged comitin interacting with negatively charged sites near the actin N terminus in subdomain 1. The location of the comitin binding site and its restriction to subdomain 1 on a single actin monomer is consistent with comitin's having a function distinct from other actin-binding proteins and, for example, would enable comitin to link bundled actin filaments to the Golgi.

The effect of F-actin on the binding and hydrolysis of guanine nucleotide by Dictyostelium elongation factor 1A

Indirect evidence implicates actin as a cofactor in eukaryotic protein synthesis. The present study directly examines the effects of F-actin on the biochemical properties of eukaryotic elongation factor 1A (eEF1A, formerly EF1alpha), a major actin-binding protein. The basal mechanism of eEF1A alone is determined under physiological conditions with the critical finding that glycerol and guanine nucleotide are required to prevent protein aggregation and loss of enzymatic activity. The dissociation constants (Kd) for GDP and GTP are 2.5 microM and 0.6 microM, respectively, and the kcat of GTP hydrolysis is 1.0 x 10(-3) s-1. When eEF1A binds to F-actin, there is a 7-fold decrease in the affinity for guanine nucleotide and an increase of 35% in the rate of GTP hydrolysis. Based upon our results and the relevant cellular concentrations, the predominant form of cellular eEF1A is calculated to be GTP.eEF1A.F-actin. We conclude that F-actin does not significantly modulate the basal enzymatic properties of eEF1A; however, actin may still influence protein synthesis by sequestering GTP.eEF1A away from interactions with its known translational ligands, e.g. aminoacyl-tRNA and ribosomes.

Actin-bundling protein isolated from pollen tubes of lily. Biochemical and immunocytochemical characterization

A 135-kD actin-bundling protein was purified from pollen tubes of lily (Lilium longiflorum) using its affinity to F-actin. From a crude extract of the pollen tubes, this protein was coprecipitated with exogenously added F-actin and then dissociated from F-actin by treating it with high-ionic-strength solution. The protein was further purified sequentially by chromatography on a hydroxylapatite column, a gel-filtration column, and a diethylaminoethyl-cellulose ion-exchange column. In the present study, this protein is tentatively referred to as P-135-ABP (Plant 135-kD Actin-Bundling Protein). By the elution position from a gel-filtration column, we estimated the native molecular mass of purified P-135-ABP to be 260 kD, indicating that it existed in a dimeric form under physiological conditions. This protein bound to and bundled F-actin prepared from chicken breast muscle in a Ca2+-independent manner. The binding of 135-P-ABP to actin was saturated at an approximate stoichiometry of 26 actin monomers to 1 dimer of P-135-ABP. By transmission electron microscopy of thin sections, we observed cross-bridges between F-actins with a longitudinal periodicity of 31 nm. Immunofluorescence microscopy using rhodamine-phalloidin and antibodies against the 135-kD polypeptide showed that P-135-ABP was colocalized with bundles of actin filaments in lily pollen tubes, leading us to conclude that it is the factor responsible for bundling the filaments.

Morphogenesis of Escherichia coli

The shape of Escherichia coli is strikingly simple compared to those of higher eukaryotes. In fact, the end result of E. coli morphogenesis is a cylindrical tube with hemispherical caps. It is argued that physical principles affect biological forms. In this view, genes code for products that contribute to the production of suitable structures for physical factors to act upon. After introduction of a physical model, the discussion is focused on the shape-maintaining (peptidoglycan) layer of E. coli. This is followed by a detailed analysis of the structural relationship of the cellular interior to the cytoplasmic membrane. A basic theme of this review is that the transcriptionally active nucleoid and the cytoplasmic translation machinery form a structural continuity with the growing cellular envelope. An attempt has been made to show how this dynamic relationship during the cell cycle affects cell polarity and how it leads to cell division.

Site-directed mutants of post-translationally modified sites of yeast eEF1A using a shuttle vector containing a chromogenic switch

Eukaryotic elongation factor 1A (eEF1A, formerly eEF-1 alpha) carries aminoacyl-tRNAs into the A-site of the ribosome in a GTP-dependent manner. In order to probe the structure/function relationships of eEF1A, we have generated site-directed mutants using a modification of a highly versatile yeast shuttle vector, which consists of the insertion of a 66 base long synthetic DNA fragment in the vector's polylinker. Via oligonucleotide-directed mutagenesis, the modification permits the identification of mutant clones based on a chromogenic screen of beta-galactosidase activity. Mutagenesis reactions are performed with two or more oligonucleotides, one introducing the chromogenic shift, and the other(s) introducing the mutation(s) of interest in eEF1A. Several rounds of chromogenic shifts and additional mutations can be performed in succession on the same vector. To address the possible function of the methylated lysines in yeast eEF1A, we have changed the post-translationally modified lysines (residue 30, 79, 316 and 390) to arginines using the above methodology. Yeast with eEF1A mutants that substitute arginine in all four sites do not show any phenotypic change. There is also an apparent equivalency of wild-type and mutant yeast eEF1A in in vitro assays. It is concluded that the post-translational modifications of eEF1A are not of major importance for eEF1A's role in translation.

The structure, function, and assembly of actin filament bundles

The cellular organization, function, and molecular composition of selected biological systems with prominent actin filament bundles are reviewed. An overall picture of the great variety of functions served by actin bundles emerges from this overview. A unifying theme is that the actin cross-linking proteins are conserved throughout the eukaryotic kingdom and yet assembled in a variety of combinations to produce actin bundles of differing functions. Mechanisms of actin bundle formation in vitro are considered illustrating the variety of physical and chemical driving forces in this exceedingly complex process. Our limited knowledge regarding the formation of actin filament bundles in vivo is contrasted with the elegant biophysical studies performed in vitro but nonetheless reveals that interactions with membranes, nucleation sites, and other organizational components must contribute to formation of actin bundles in vivo.

F-actin sequesters elongation factor 1alpha from interaction with aminoacyl-tRNA in a pH-dependent reaction

The machinery of eukaryotic protein synthesis is found in association with the actin cytoskeleton. A major component of this translational apparatus, which is involved in the shuttling of aa-tRNA, is the actin-binding protein elongation factor 1alpha (EF-1alpha). To investigate the consequences for translation of the interaction of EF-1alpha with F-actin, we have studied the effect of F-actin on the ability of EF-1alpha to bind to aa-tRNA. We demonstrate that binding of EF-1alpha:GTP to aa-tRNA is not pH sensitive with a constant binding affinity of approximately 0.2 microM over the physiological range of pH. However, the sharp pH dependence of binding of EF-1alpha to F-actin is sufficient to shift the binding of EF-1alpha from F-actin to aa-tRNA as pH increases. The ability of EF-1alpha to bind either F-actin or aa-tRNA in competition binding experiments is also consistent with the observation that EF-1alpha's binding to F-actin and aa-tRNA is mutually exclusive. Two pH-sensitive actin-binding sequences in EF-1alpha are identified and are predicted to overlap with the aa-tRNA-binding sites. Our results suggest that pH-regulated recruitment and release of EF-1alpha from actin filaments in vivo will supply a high local concentration of EF-1alpha to facilitate polypeptide elongation by the F-actin-associated translational apparatus.

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.

Immunofluorescence studies of human fibroblasts demonstrate the presence of the complex of elongation factor-1 beta gamma delta in the endoplasmic reticulum

The eukaryotic elongation factor-1 (EF-1) consists of four subunits, EF-1 alpha, EF-1 beta, EF-1 gamma and EF-1 delta which induce efficient transfer of aminoacyl-tRNA to the ribosome. In this process EF-1 alpha.GTP acts as the carrier of the aminoacyl-tRNA on its way to the ribosome. After release of aminoacyl-tRNA to the ribosome under concomitant hydrolysis of GTP, the inactive EF-1 alpha.GDP form is recycled to EF-1 alpha.GTP by EF-1 beta gamma delta. In eukaryotic cells the concentration of EF-1 alpha exceeds that of the complex beta gamma delta by a factor of 5–10. In order to delineate the intracellular localization of the different subunits of EF-1, antibodies against the EF-1 subunits have been elicited and indirect immunofluorescence microscopy experiments were performed. In human fibroblasts, the guanine nucleotide exchange part of EF-1, EF-1 beta gamma delta, was found to co-localize with the endoplasmic reticulum (ER), displaying a distinct fine-structure in its staining pattern. The guanine nucleotide-binding subunit of EF-1, EF-1 alpha, shows a more diffuse distribution throughout the cytoplasm and is, in addition, associated with the nucleus.

The polyelectrolyte nature of F-actin and the mechanism of actin bundle formation

Polymerized (F-)actin is induced to form bundles by a number of polycations including divalent metal ions, Co(NH3)63+, and basic polypeptides. The general features of bundle formation are largely independent of the specific structure of the bundling agent used. A threshold concentration of polycation is required to form lateral aggregates of actin filaments. The threshold concentration varies strongly with the valence of the cation and increases with the ionic strength of the solution. Polyanions such as nucleoside phosphates or oligomers of acidic amino acids disaggregate actin bundles into single filaments. These features are similar to the phenomenon of DNA condensation and can be explained analogously by polyelectrolyte theories. Similar results were found when F-actin was bundled by the peptide corresponding to the actin binding site of myristoylated alanine-rich protein kinase C substrate protein (MARCKS) or by smooth muscle calponin, suggesting that a broad class of actin bundling factors may function in a common manner. Physiologic concentrations of both small ions and large proteins can induce actin interfilament association independent of a requirement for specific binding sites.

The Dictyostelium cytoskeleton

New avenues of cytoskeleton research in Dictyostelium discoideum have opened up with the cloning of the alpha- and beta-tubulin genes and the characterization of kinesins and cytoplasmic dynein. Much research, however, continues to focus on the actin cytoskeleton and its dynamics during chemotaxis, morphogenesis, and other motile processes. New actin-associated proteins are being identified and characterized by biochemical means and through isolation of mutants lacking individual components. This work is shedding light on the roles of specific actin assemblies in various biological processes.

pH regulation of the F-actin binding properties of Dictyostelium elongation factor 1 alpha

ABP50, an F-actin bundling protein from Dictyostelium, is also the protein synthesis co-factor, elongation factor 1 alpha (EF1 alpha). Concomitant with cAMP stimulation in Dictyostelium is a cytoplasmic alkalinization (Aerts, R. J., DeWit, R. J. W., and Van Lookeren Campagne, M. M. (1987) FEBS Lett. 220, 366-370) and a redistribution of EF1 alpha (Dharmawardhane, S., Demma, M., Yang, F., and Condeelis, J. (1991) Cell Motil. Cytoskel. 20, 279-288). In addition, others have shown a correlation between intracellular pH and the level of protein synthesis in Dictyostelium (Aerts, R. J., Durston, A. J., and Moolenaar, W. H. (1985) Cell 43, 653-657). The present study investigates the relationship between pH and the F-actin binding properties of EF1 alpha. We found that increasing pH over the physiological range 6.2-7.8 causes a loss of EF1 alpha-mediated F-actin bundling and single filament binding, with corresponding increases in the amount of free EF1 alpha in vitro. Similar results also were obtained by cell fractionation and confocal immunofluorescence microscopy. The EF1 alpha binding constant (Kd) for F-actin is increased from 0.2 microM to > 2.2 microM over the same pH range. In addition, EF1 alpha-induced actin bundle formation is freely reversible by changes in pH. Thus, pH may be a potent modulator of cytoarchitecture in Dictyostelium and may also influence mRNA translation rates by modifying the interactions between the protein synthetic machinery and the actin cytoskeleton.

A 13-A map of the actin-scruin filament from the limulus acrosomal process

We have determined the structure of the actin-scruin filament to 13-A resolution using a combination of low-dose EM and image analysis. The three-dimensional map reveals four actin-actin contacts: two within each strand and two between strands. The conformation of the actin subunit is different from that in the Holmes et al. (1990) model as refined by Lorenz et al. (1993). In particular, subdomain II is tilted in a similar way to that seen by Orlova and Egelman (1993) in F-Mg2(+)-ADP actin filaments in the absence of Ca2+. Scruin appears to consist of two domains of approximately equal volume. Each scruin subunit cross-links the two strands in the actin filament. Domain I of scruin contacts subdomain I of actin and makes a second contact at the junction of subdomains III and IV. Domain II of scruin contacts actin at subdomains I and II of a neighboring actin subunit. The two scruin domains thus bind differently to actin.

ABP50: an actin-binding elongation factor 1 alpha from Dictyostelium discoideum

ABP50 is a polypeptide elongation factor 1 alpha from Dictyostelium that is associated with the actin cytoskeleton. Upon chemotactic stimulation, ABP50 undergoes a dramatic cytoplasmic redistribution into newly formed surface projections and in vitro binds to and bundles actin filaments. Many questions are raised by this interaction pertaining to the spatiotemporal regulation of protein synthesis and cytoskeletal organization by extracellular signals.