Showing papers by "Natasha V. Raikhel published in 1995"

Small cysteine-rich antifungal proteins from radish: their role in host defense.

Abstract: Radish seeds have previously been shown to contain two homologous, 5-kD cysteine-rich proteins designated Raphanus sativus-antifungal protein 1 (Rs-AFP1) and Rs-AFP2, both of which exhibit potent antifungal activity in vitro. We now demonstrate that these proteins are located in the cell wall and occur predominantly in the outer cell layers lining different seed organs. Moreover, Rs-AFPs are preferentially released during seed germination after disruption of the seed coat. The amount of released proteins is sufficient to create a microenvironment around the seed in which fungal growth is suppressed. Both the cDNAs and the intron-containing genomic regions encoding the Rs-AFP preproteins were cloned. Transcripts (0.55 kb) hybridizing with an Rs-AFP1 cDNA-derived probe were present in near-mature and mature seeds. Such transcripts as well as the corresponding proteins were barely detectable in healthy uninfected leaves but accumulated systemically at high levels after localized fungal infection. The induced leaf proteins (designated Rs-AFP3 and Rs-AFP4) were purified and shown to be homologous to seed Rs-AFPs and to exert similar antifungal activity in vitro. A chimeric Rs-AFP2 gene under the control of the constitutive cauliflower mosaic virus 35S promoter conferred enhanced resistance to the foliar pathogen Alternaria longipes in transgenic tobacco. The term "plant defensins" is proposed to denote these defense-related proteins.

Protein Import into the Nucleus: An Integrated View

Abstract: The directed movement of macromolecules into and out of the nucleus is a fundamental process in eukaryotes and occurs through the nuclear pore complex (NPC). A diverse array of molecules are transported across the nuclear envelope including proteins, mRNAs, tRNAs, snRNP complexes, ribosomal subunits, and in specialized cases, DNA. The structural and functional differences between these molecules point to the mechanistic complexity of NPCs and other components of the nuclear transport apparatus. This machinery must not only recognize within transported molecules specific targeting signals that differ between proteins, RNA, and RNA/protein complexes, it must translocate these molecules across the nuclear envelope. Additional levels of complexity are necessary because molecules such as proteins may continually undergo bidirectional transport across the envelope. Beyond these basic functions, the nuclear transport apparatus is regulated at the level of individual substrates and at more global levels such as coupling to cell cycle regression.

Different sensitivity to wortmannin of two vacuolar sorting signals indicates the presence of distinct sorting machineries in tobacco cells.

Abstract: Vacuolar matrix proteins in plant cells are sorted from the secretory pathway to the vacuoles at the Golgi apparatus. Previously, we reported that the NH2-terminal propeptide (NTPP) of the sporamin precursor and the COOH-terminal propeptide (CTPP) of the barley lectin precursor contain information for vacuolar sorting. To analyze whether these propeptides are interchangeable, we expressed constructs consisting of wild-type or mutated NTPP with the mature part of barley lectin and sporamin with CTPP and mutated NTPP in tobacco BY-2 cells. The vacuolar localization of these constructs indicated that the signals were interchangeable. We next analyzed the effect of wortmannin, a specific inhibitor of mammalian phosphatidylinositol (PI) 3-kinase on vacuolar delivery by NTPP and CTPP in tobacco cells. Pulse-chase analysis indicated that 33 microM wortmannin caused almost complete inhibition of CTPP-mediated transport to the vacuoles, while NTPP-mediated transport displayed almost no sensitivity to wortmannin at this concentration. This indicates that there are at least two different mechanisms for vacuolar sorting in tobacco cells, and the CTPP-mediated pathway is sensitive to wortmannin. We compared the dose dependencies of wortmannin on the inhibition of CTPP-mediated vacuolar delivery of proteins and on the inhibition of the synthesis of phospholipids in tobacco cells. Wortmannin inhibited PI 3- and PI 4-kinase activities and phospholipid synthesis. Missorting caused by wortmannin displays a dose dependency that is similar to the dose dependency for the inhibition of synthesis of PI 4-phosphate and major phospholipids. This is different, however, than the inhibition of synthesis of PI 3-phosphate. Thus, the synthesis of phospholipids could be involved in CTPP-mediated vacuolar transport.

An Arabidopsis syntaxin homologue isolated by functional complementation of a yeast pep12 mutant

Abstract: The syntaxin family of integral membrane proteins are thought to function as receptors for transport vesicles, with different isoforms of this family localized to various membranes throughout the cell. The yeast Pep12 protein is a syntaxin homologue which may function in the trafficking of vesicles from the trans-Golgi network to the vacuole. We have isolated an Arabidopsis thaliana cDNA by functional complementation of a yeast pep12 mutant. The Arabidopsis cDNA (aPEP12) potentially encodes a 31-kDa protein which is homologous to yeast Pep12 and to other members of the syntaxin family, indicating that this protein may function in the docking or fusion of transport vesicles with the vacuolar membrane in plant cells. Northern blot analysis indicates that the mRNA is expressed in all tissues examined, although at a very low level in leaves. The mRNA is found in all cell types in roots and leaves, as shown by in situ hybridization experiments. The existence of plant homologues of proteins of the syntaxin family indicates that the basic vesicle docking and fusion machinery may be conserved in plants as it is in yeast and mammals.

Plant nuclear pore complex proteins are modified by novel oligosaccharides with terminal N-acetylglucosamine.

Abstract: Only a few nuclear pore complex (NPC) proteins, mainly in vertebrates and yeast but none in plants, have been well characterized. As an initial step to identify plant NPC proteins, we examined whether NPC proteins from tobacco are modified by N-acetylglucosamine (GlcNAc). Using wheat germ agglutinin, a lectin that binds specifically to GlcNAc in plants, specific labeling was often found associated with or adjacent to NPCs. Nuclear proteins containing GlcNAc can be partially extracted by 0.5 M salt, as shown by a wheat germ agglutinin blot assay, and at least eight extracted proteins were modified by terminal GlcNAc, as determined by in vitro galactosyltransferase assays. Sugar analysis indicated that the plant glycans with terminal GlcNAc differ from the single O-linked GlcNAc of vertebrate NPC proteins in that they consist of oligosaccharides that are larger in size than five GlcNAc residues. Most of these appear to be bound to proteins via a hydroxyl group. This novel oligosaccharide modification may convey properties to the plant NPC that are different from those of vertebrate NPCs.

Expression and Regulation of aERD2, a Gene Encoding the KDEL Receptor Homolog in Plants, and Other Genes Encoding Proteins Involved in ER-Golgi Vesicular Trafficking.

Abstract: aERD2 and aSAR1 of Arabidopsis are functional homologs of yeast genes encoding proteins essential for endoplasmic reticulum (ER)-to-Golgi transport. The regulation of these secretory pathway genes in yeast, mammals, and plants is not known. High levels of expression of aERD2 and aSAR1 were observed in roots, flowers, and inflorescence stems, with the highest levels being detected in roots. The aSAR1 transcript levels were highest in young leaves and declined during leaf maturation. Low levels of aERD2 were detected in both young and fully mature leaves when compared with roots. In situ hybridization showed that trichomes accumulate more aERD2 transcript as the leaf expands, whereas aSAR1 is expressed equally in all leaf cell types. Treating plants with tunicamycin, a drug that blocks N-glycosylation in the ER, or with cold shock, known to block secretory protein transport, led to a marked accumulation of aERD2 and aSAR1 transcripts. The Arabidopsis ARF gene, which encodes a GTPase probably involved in Golgi vesicle traffic, was not affected by these treatments. This study is an essential first step toward understanding the regulation of genes that encode proteins involved in vesicular trafficking.

Three Classes of Nuclear Import Signals Bind to Plant Nuclei

Abstract: Three nuclear localization signals (NLS), including an unusual Mat alpha 2-like NLS from maize (Zea mays) R, were found to compete for binding to plant nuclei. In addition, the authentic yeast Mat alpha 2 NLS, which does not function in mammals, was shown to function in plants in vivo. Our results indicate that plants possess a site at the nuclear pore complex that recognizes the three known classes of NLSs.

Isolation of a cDNA encoding a novel GTP-binding protein of Arabidopsis thaliana.

Abstract: A cDNA encoding for a 68 kDa GTP-binding protein was isolated from Arabidopsis thaliana (aG68). This clone is a member of a gene family that codes for a class of large GTP-binding proteins. This includes the mammalian dynamin, yeast Vps1p and the vertebrate Mx proteins. The predicted amino acid sequence was found to have high sequence conservation in the N-terminal GTP-binding domain sharing 54% identity to yeast Vps1p, 56% amino acid identity to rat dynamin and 38% identity to the murine Mx1 protein. The northern analysis shows expression in root, leaf, stem and flower tissues, but in mature leaves at lower levels. Southern analysis indicates that it may be a member of a small gene family or the gene may contain an intron.

Nuclear localization signal binding proteins in higher plant nuclei.

Abstract: The import of proteins into the nucleus is a vital process that is mediated by proteins which specifically recognize nuclear localization signals (NLSs). These factors have not been identified in plants. Previously, we demonstrated that higher plants possess a low-affinity binding site at the nuclear pore that specifically binds to several classes of functional NLSs. By the use of crosslinking reagents and a radiolabeled peptide to the bipartite NLS from the endogenous plant transcription factor Opaque2, two NLS binding proteins (NBPs) of 50-60 kDa and at least two NBPs of 30-40 kDa were identified. Competition studies indicated that labeling was specific for the functional NLS but not a mutant NLS impaired in vivo or a peptide unrelated to NLSs. Also, the apparent dissociation constant (100-300 microM) for labeling was similar to that of the binding site. Proteins of similar mass were labeled with two different crosslinking reagents, and concentration and time studies indicated that these NBPs were distinct proteins and not aggregates. Treatment with salt, detergent, or urea before or during NLS binding demonstrated that the properties of the binding site and the NBPs were identical. This tight correlation strongly indicates that some or all of the NBPs constitute the nuclear pore binding site. Overall, our results indicate that some components of NLS recognition are located at the nuclear pores in higher plants.

Targeting and Release of Phytohemagglutinin from the Roots of Bean Seedlings

Abstract: Phytohemagglutinin (PHA), an abundant vacuolar seed protein of the common bean (Phaseolus vulgaris), is a tetramer of two homologous polypeptides, PHA-E and PHA-L. The roots of bean seedlings release into the culture medium a cross-reacting lectin that is most closely related to PHA-E. Reverse-transcriptase polymerase chain reaction with root mRNA as template was used to identify PHA transcripts in the roots of bean seedlings. Roots were found to contain mRNA for PHA-E but not for PHA-L. Indirect immunocytochemical detection with colloidal gold and antibodies to deglycosylated PHA showed that in the meristem of the primary root, PHA accumulates in vacuoles. However, in elongated root cells PHA was found only in the cell walls, indicating targeting to an alternate location. These results are discussed in relation to the various mechanisms that may account for the release of a normally vacuolar protein by roots.