Localization of Expression of Three Cold-Induced Genes, blt101, blt4.9, and blt14, in Different Tissues of the Crown and Developing Leaves of Cold-Acclimated Cultivated Barley1

Tissues expressing mRNAs of three cold-induced genes, blt101, blt14, and blt4.9, and a control gene, elongation factor 1α, were identified in the crown and immature leaves of cultivated barley (Hordeum vulgare L. cv Igri). Hardiness and tissue damage were assessed. blt101 and blt4.9 mRNAs were not detected in control plants; blt14 was expressed in control plants but only in the inner layers of the crown cortex. blt101 was expressed in many tissues of cold-acclimated plants but most strongly in the vascular-transition zone of the crown; blt14 was expressed only in the inner layers of the cortex and in cell layers partly surrounding vascular bundles in the vascular-transition zone; expression of blt4.9, which codes for a nonspecific lipid-transfer protein, was confined to the epidermis of the leaf and to the epidermis of the older parts of the crown. None of the cold-induced genes was expressed in the tunica, although the control gene was most strongly expressed there. Thus, the molecular aspects of acclimation differed markedly between tissues. Damage in the vascular-transition zone of the crown correlated closely with plant survival. Therefore, the strong expression of blt101 and blt14 in this zone may indicate a direct role in freezing tolerance of the crown.

Actin Depolymerization Affects Stress-Induced Translational Activity of Potato Tuber Tissue1

Changes in polymerized actin during stress conditions were correlated with potato (Solanum tuberosum L.) tuber protein synthesis. Fluorescence microscopy and immunoblot analyses indicated that filamentous actin was nearly undetectable in mature, quiescent aerobic tubers. Mechanical wounding of postharvest tubers resulted in a localized increase of polymerized actin, and microfilament bundles were visible in cells of the wounded periderm within 12 h after wounding. During this same period translational activity increased 8-fold. By contrast, low-oxygen stress caused rapid reduction of polymerized actin coincident with acute inhibition of protein synthesis. Treatment of aerobic tubers with cytochalasin D, an agent that disrupts actin filaments, reduced wound-induced protein synthesis in vivo. This effect was not observed when colchicine, an agent that depolymerizes microtubules, was used. Neither of these drugs had a significant effect in vitro on run-off translation of isolated polysomes. However, cytochalasin D did reduce translational competence in vitro of a crude cellular fraction containing both polysomes and cytoskeletal elements. These results demonstrate the dependence of wound-induced protein synthesis on the integrity of microfilaments and suggest that the dynamics of the actin cytoskeleton may affect translational activity during stress conditions.

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.

Rearrangement of Actin Microfilaments in Plant Root Hairs Responding to Rhizobium etli Nodulation Signals1

The response of the actin cytoskeleton to nodulation (Nod) factors secreted by Rhizobium etli has been studied in living root hairs of bean (Phaseolus vulgaris) that were microinjected with fluorescein isothiocyanate-phalloidin. In untreated control cells or cells treated with the inactive chitin oligomer, the actin cytoskeleton was organized into long bundles that were oriented parallel to the long axis of the root hair and extended into the apical zone. Upon exposure to R. etli Nod factors, the filamentous actin became fragmented, as indicated by the appearance of prominent masses of diffuse fluorescence in the apical region of the root hair. These changes in the actin cytoskeleton were rapid, observed as soon as 5 to 10 min after application of the Nod factors. It was interesting that the filamentous actin partially recovered in the continued presence of the Nod factor: by 1 h, long bundles had reformed. However, these cells still contained a significant amount of diffuse fluorescence in the apical zone and in the nuclear area, presumably indicating the presence of short actin filaments. These results indicate that Nod factors alter the organization of actin microfilaments in root hair cells, and this could be a prelude for the formation of infection threads.

Essential roles for four cytoplasmic intermediate filament proteins in Caenorhabditis elegans development

The structural proteins of the cytoplasmic intermediate filaments (IFs) arise in the nematode Caenorhabditis elegans from eight reported genes and an additional three genes now identified in the complete genome. With the use of double-stranded RNA interference (RNAi) for all 11 C. elegans genes encoding cytoplasmic IF proteins, we observe phenotypes for the five genes A1, A2, A3, B1, and C2. These range from embryonic lethality (B1) and embryonic/larval lethality (A3) to larval lethality (A1 and A2) and a mild dumpy phenotype of adults (C2). Phenotypes A2 and A3 involve displaced body muscles and paralysis. They probably arise by reduction of hypodermal IFs that participate in the transmission of force from the muscle cells to the cuticle. The B1 phenotype has multiple morphogenetic defects, and the A1 phenotype is arrested at the L1 stage. Thus, at least four IF genes are essential for C. elegans development. Their RNAi phenotypes are lethal defects due to silencing of single IF genes. In contrast to C. elegans, no IF genes have been identified in the complete Drosophila genome, posing the question of how Drosophila can compensate for the lack of these proteins, which are essential in mammals and C. elegans. We speculate that the lack of IF proteins in Drosophila can be viewed as cytoskeletal alteration in which, for instance, stable microtubules, often arranged as bundles, substitute for cytoplasmic IFs.

Three SNARE complexes cooperate to mediate membrane fusion

Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins of the syntaxin, SNAP-25, and VAMP families mediate intracellular membrane fusion through the formation of helical bundles that span opposing membranes. Soluble SNARE domains that lack their integral membrane anchors inhibit membrane fusion by forming nonfunctional complexes with endogenous SNARE proteins. In this study we investigate the dependence of membrane fusion on the concentration of a soluble SNARE coil domain derived from VAMP2. The increase in the inhibition of fusion observed with increasing concentration of inhibitor is best fit to a function that suggests three SNARE complexes cooperate to mediate fusion of a single vesicle. These three complexes likely contribute part of a protein and lipidic fusion pore.

Rapid induction of intraneuronal neurofibrillary tangles in apolipoprotein E-deficient mice

Cultured hippocampal slices prepared from apolipoprotein E-deficient mice were exposed to an inhibitor of cathepsins B and L and then processed for immunocytochemistry using antibodies against human paired helical filaments. Dense, AT8-immunopositive deposits were found in the subiculum, stratum oriens of hippocampal field CA1, and the hilus of the dentate gyrus. This distribution agrees with that described for tangles in Alzheimer's disease. The appearance of the labeled structures fell into categories that correspond to previously proposed stages in the progression of intraneuronal neurofibrillary tangles in human hippocampus. Electron microscopic analyses confirmed that microtubule disruption and twisted bundles of filaments were present in neurons in the affected areas. These results support the hypothesis that partial lysosomal dysfunction is a contributor to Alzheimer's disease and suggest a simple model for studying an important component of the disease.

Forced expression of tropomyosin 2 or 3 in v-Ki-ras-transformed fibroblasts results in distinct phenotypic effects.

Transformation of cells in tissue culture results in a variety of cellular changes including alterations in cell growth, adhesiveness, motility, morphology, and organization of the cytoskeleton. Morphological and cytoskeletal changes are perhaps the most readily apparent features of transformed cells. Although a number of studies have documented a decrease in the expression of specific tropomyosin (TM) isoforms in transformed cells, it remains to be determined if the suppression of TM synthesis is essential in the establishment and maintenance of the transformed pheno-type. To address the roles of different TM isoforms in transformed cells we have examined the effects of expressing specific TM isoforms in transformed cells using a Kirsten virus-transformed cell line (ATCC NRK1569) as a model system. In contrast to normal fibroblasts, the NRK 1569 cells contain reduced levels of TM-1 and undetectable levels of TM-2 and TM-3. These cells have a rounded morphology and are devoid of stress fibers. Employing expression plasmids for TM-2 and TM-3, stable cell lines were established from the NRK 1569 cells that express these isoforms individually. We demonstrate that expression of TM-2 or TM-3 leads to increased cell spreading accompanied by the formation of identifiable microfilament bundles, as well as significant restoration of well-defined vinculin-containing focal adhesion plaques, although expression of each isoform exhibited distinct properties. In addition, cells expressing TM-2, but not TM-3, exhibited contact-inhibited cell growth and a requirement for serum.

The human CAS protein which is homologous to the CSE1 yeast chromosome segregation gene product is associated with microtubules and mitotic spindle.

Human CAS cDNA contains a 971-aa open reading frame that is homologous to the essential yeast gene CSE1. CSE1 is involved in chromosome segregation and is necessary for B-type cyclin degradation in mitosis. Using antibodies to CAS, it was shown that CAS levels are high in proliferating and low in nonproliferating cells. Here we describe the distribution of CAS in cells and tissues analyzed with antibodies against CAS. CAS is an approximately 100-kDa protein present in the cytoplasm of proliferating cells at levels between 2 x 10(5) and 1 x 10(6) molecules per cell. The intracellular distribution of CAS resembles that of tubulin. In interphase cells, anti-CAS antibody shows microtubule-like patterns and in mitotic cells it labels the mitotic spindle. CAS is removed from microtubules by mild detergent treatment (cytoskeleton preparations) and in vincristine- or taxol-treated cells. CAS is diffusely distributed in the cytoplasm with only traces present in tubulin paracrystals or bundles. Thus, CAS appears to be associated with but not to be an integral part of microtubules. Immunohistochemical staining of frozen tissues shows elevated amounts of CAS in proliferating cells such as testicular spermatogonia and cells in the basal layer cells of the colon. CAS was also concentrated in the respiratory epithelium of the trachea and in axons and Purkinje cells in the cerebellum. These cells contain many microtubules. The cellular location of CAS is consistent with an important role in cell division as well as in ciliary movement and vesicular transport.

X-ray structure of a vanadium-containing enzyme: chloroperoxidase from the fungus Curvularia inaequalis.

The chloroperoxidase (EC 1.11.1.-) from the fungus Curvularia inaequalis belongs to a class of vanadium enzymes that oxidize halides in the presence of hydrogen peroxide to the corresponding hypohalous acids. The 2.1 A crystal structure (R = 20%) of an azide chloroperoxidase complex reveals the geometry of the catalytic vanadium center. Azide coordinates directly to the metal center, resulting in a structure with azide, three nonprotein oxygens, and a histidine as ligands. In the native state vanadium will be bound as hydrogen vanadate(V) in a trigonal bipyramidal coordination with the metal coordinated to three oxygens in the equatorial plane, to the OH group at one apical position, and to the epsilon 2 nitrogen of a histidine at the other apical position. The protein fold is mainly alpha-helical with two four-helix bundles as main structural motifs and an overall structure different from other structures. The helices pack together to a compact molecule, which explains the high stability of the protein. An amino acid sequence comparison with vanadium-containing bromoperoxidase from the seaweed Ascophyllum nodosum shows high similarities in the regions of the metal binding site, with all hydrogen vanadate(V) interacting residues conserved except for lysine-353, which is an asparagine.

Plant histochemistry by correlation peak imaging.

Using a new NMR correlation-peak imaging technique, we were able to investigate noninvasively the spatial distribution of carbohydrates and amino acids in the hypocotyl of castor bean seedlings. In addition to the expected high sucrose concentration in the phloem area of the vascular bundles, we could also observe high levels of sucrose in the cortex parenchyma, but low levels in the pith parenchyma. In contrast, the glucose concentration was found to be lower in the cortex parenchyma than in the pith parenchyma. Glutamine and/or glutamate was detected in the cortex parenchyma and in the vascular bundles. Lysine and arginine were mainly visible in the vascular bundles, whereas valine was observed in the cortex parenchyma, but not in the vascular bundles. Although the physiological significance of these metabolite distribution patterns is not known, they demonstrate the potential of spectroscopic NMR imaging to study noninvasively the physiology and spatial metabolic heterogeneity of living plants.

Mutagenesis in the C-terminal region of human interleukin 5 reveals a central patch for receptor alpha chain recognition.

Cassette mutagenesis was used to identify side chains in human interleukin 5 (hIL-5) that mediate binding to hIL-5 receptor alpha chain (hIL-5R alpha). A series of single alanine substitutions was introduced into a stretch of residues in the C-terminal region, including helix D, which previously had been implicated in receptor alpha chain recognition and which is aligned on the IL-5 surface so as to allow the topography of receptor binding residues to be examined. hIL-5 and single site mutants were expressed in COS cells, their interactions with hIL-5R alpha were measured by a sandwich surface plasmon resonance biosensor method, and their biological activities were measured by an IL-5-dependent cell proliferation assay. A pattern of mutagenesis effects was observed, with greatest impact near the interface between the two four-helix bundles of IL-5, in particular at residues Glu-110 and Trp-111, and least at the distal ends of the D helices. This pattern suggests the possibility that residues near the interface of the two four-helix bundles in hIL-5 comprise a central patch or hot spot, which constitutes an energetically important alpha chain recognition site. This hypothesis suggests a structural explanation for the 1:1 stoichiometry observed for the complex of hIL-5 with hIL-5R alpha.

Location of the active site of allosteric chorismate mutase from Saccharomyces cerevisiae, and comments on the catalytic and regulatory mechanisms.

The active site of the allosteric chorismate mutase (chorismate pyruvatemutase, EC 5.4.99.5) from yeast Saccharomyces cerevisiae (YCM) was located by comparison with the mutase domain (ECM) of chorismate mutase/prephenate dehydratase [prephenate hydro-lyase (decarboxylating), EC 4.2.1.51] (the P protein) from Escherichia coli. Active site domains of these two enzymes show very similar four-helix bundles, each of 94 residues which superimpose with a rms deviation of 1.06 A. Of the seven active site residues, four are conserved: the two arginines, which bind to the inhibitor's two carboxylates; the lysine, which binds to the ether oxygen; and the glutamate, which binds to the inhibitor's hydroxyl group in ECM and presumably in YCM. The other three residues in YCM (ECM) are Thr-242 (Ser-84), Asn-194 (Asp-48), and Glu-246 (Gln-88). This Glu-246, modeled close to the ether oxygen of chorismate in YCM, may function as a polarizing or ionizable group, which provides another facet to the catalytic mechanism.

Alpha 1(E)-catenin is an actin-binding and -bundling protein mediating the attachment of F-actin to the membrane adhesion complex.

Calcium-dependent homotypic cell-cell adhesion, mediated by molecules such as E-cadherin, guides the establishment of classical epithelial cell polarity and contributes to the control of migration, growth, and differentiation. These actions involve additional proteins, including alpha- and beta-catenin (or plakoglobin) and p120, as well as linkage to the cortical actin cytoskeleton. The molecular basis for these interactions and their hierarchy of interaction remain controversial. We demonstrate a direct interaction between F-actin and alpha (E)-catenin, an activity not shared by either the cytoplasmic domain of E-cadherin or beta-catenin. Sedimentation assays and direct visualization by transmission electron microscopy reveal that alpha 1(E)-catenin binds and bundles F-actin in vitro with micromolar affinity at a catenin/G-actin monomer ratio of approximately 1:7 (mol/mol). Recombinant human beta-catenin can simultaneously bind to the alpha-catenin/actin complex but does not bind actin directly. Recombinant fragments encompassing the amino-terminal 228 residues of alpha 1(E)-catenin or the carboxyl-terminal 447 residues individually bind actin in cosedimentation assays with reduced affinity compared with the full-length protein, and neither fragment bundles actin. Except for similarities to vinculin, neither region contains sequences homologous to established actin-binding proteins. Collectively these data indicate that alpha 1 (E)-catenin is a novel actin-binding and -bundling protein and support a model in which alpha 1(E)-catenin is responsible for organizing and tethering actin filaments at the zones of E-cadherin-mediated cell-cell contact.

Repacking protein cores with backbone freedom: structure prediction for coiled coils.

Progress in homology modeling and protein design has generated considerable interest in methods for predicting side-chain packing in the hydrophobic cores of proteins. Present techniques are not practically useful, however, because they are unable to model protein main-chain flexibility. Parameterization of backbone motions may represent a general and efficient method to incorporate backbone relaxation into such fixed main-chain models. To test this notion, we introduce a method for treating explicitly the backbone motions of alpha-helical bundles based on an algebraic parameterization proposed by Francis Crick in 1953 [Crick, F. H. C. (1953) Acta Crystallogr. 6, 685-689]. Given only the core amino acid sequence, a simple calculation can rapidly reproduce the crystallographic main-chain and core side-chain structures of three coiled coils (one dimer, one trimer, and one tetramer) to within 0.6-A root-mean-square deviations. The speed of the predictive method [approximately 3 min per rotamer choice on a Silicon Graphics (Mountain View, CA) 4D/35 computer] permits it to be used as a design tool.