A new family of plant antifungal proteins.

Plant seeds contain high concentrations of many antimicrobial proteins. These include chitinases, beta-1,3-glucanases, proteinase inhibitors, and ribosome-inactivating proteins. We recently reported the presence in corn seeds of zeamatin, a protein that has potent activity against a variety of fungi but has none of the above activities. Zeamatin is a 22-kDa protein that acts by causing membrane permeabilization Using a novel bioautography technique, we found similar antifungal proteins in the seeds of 6 of 12 plants examined. A polyclonal antiserum was raised against zeamatin and was used in immunoblots to confirm the presence of zeamatinlike proteins in these seeds. N-terminal amino acid sequencing was carried out on the antifungal proteins from corn, oats, sorghum, and wheat, and these sequences revealed considerable homology with each other. Interestingly, these N-terminal sequences are also similar to those of thaumatin, a pathogenesis-related protein from tobacco, and two salt stress-induced proteins. These results indicate that zeamatin is not unique but is a member of a previously unrecognized family of plant defense proteins that may include some species of pathogenesis-related proteins.     

Variants within the yeast Ty sequence family encode a class of structurally conserved proteins.

The Ty transposable elements of Saccharomyces cerevisiae form a heterogeneous family within which two broad structural classes (I and II) exist. The two classes differ by two large substitutions and many restriction sites. We show that, like class I elements a class II element, Tyl-17, also appears to contain at least two major protein coding regions, designated TYA and TYB, and the organisational relationship of these regions has been conserved. The TYA genes of both classes encode proteins, designated p1 proteins, with an approximate molecular weight of 50 Kd and, despite considerable variation between the TYA regions at the DNA level, the structures of these proteins are remarkably similar. These observations strongly suggest that the p1 proteins of Ty elements are functionally significant and that they have been subject to selection.     

A conserved family of immune effectors cleaves cellular ATP upon viral infection

1. Download : Download high-res image (156KB)
2. Download : Download full-size image

     

ORMDL proteins are a conserved new family of endoplasmic reticulum membrane proteins

Background  

Annotations of completely sequenced genomes reveal that nearly half of the genes identified are of unknown function, and that some belong to uncharacterized gene families. To help resolve such issues, information can be obtained from the comparative analysis of homologous genes in model organisms.     

The OXR domain defines a conserved family of eukaryotic oxidation resistance proteins

Background  

The NCOA7 gene product is an estrogen receptor associated protein that is highly similar to the human OXR1 gene product, which functions in oxidation resistance. OXR genes are conserved among all sequenced eukaryotes from yeast to humans. In this study we examine if NCOA7 has an oxidation resistance function similar to that demonstrated for OXR1. We also examine NCOA7 expression in response to oxidative stress and its subcellular localization in human cells, comparing these properties with those of OXR1.     

A new subfamily of structurally related human F-box proteins

F-box proteins, a critical component of the evolutionary conserved ubiquitin-protein ligase complex SCF (Skp1/Cdc53-Cullin1/F-box), recruit substrates for ubiquitination and consequent degradation through their specific protein-protein interaction domains. Here, we report the identification of full-length cDNAs encoding three novel human F-box proteins named FBG3, FBG4 and FBG5 which display similarity with previously identified NFB42 (FBX2) and FBG2 (FBX6) proteins. All five proteins are characterized by an approximately 180-amino-acid (aa) conserved C-terminal domain and thus constitute a third subfamily of mammalian F-box proteins. Analysis of genomic organization of the five FBG genes revealed that all of them consist of six exons and five introns. FBG1, FBG2 and FBG3 genes are located in tandem on chromosome 1p36, and FBG4 and FBG5 are mapped to chromosome 19q13. FBG genes are expressed in a limited number of human tissues including kidney, liver, brain and muscle tissues. Expression of rat FBG2 gene was found related to differentiation/proliferation status of hepatocytes. Specifically, FBG2 mRNA was expressed in foetal liver, decreased after birth and re-accumulated in adult liver. Expression of FBG2 was strongly inhibited in hepatoma cells by okadaic acid.     

Gene-for-gene interactions: bacterial avirulence proteins specify plant disease resistance

Resistance of plants to bacterial pathogens is often controlled by corresponding genes for resistance and avirulence in host and pathogen, respectively. Fifty years after discovery of the genetic basis of gene-for-gene interactions, several avirulence and plant resistance genes have been isolated and are being studied on the molecular level. Tremendous progress has been made due to a better understanding of type III secretion systems that are required for bacterial pathogenicity. We are beginning to grasp how the plant actually recognizes bacterial avirulence determinants. The current view is that the bacterium translocates avirulence proteins into the host cell by the Hrp type III secretion system and that recognition occurs in the plant cell.     

A family of plasmodesmal proteins with receptor-like properties for plant viral movement proteins

Plasmodesmata (PD) are essential but poorly understood structures in plant cell walls that provide symplastic continuity and intercellular communication pathways between adjacent cells and thus play fundamental roles in development and pathogenesis. Viruses encode movement proteins (MPs) that modify these tightly regulated pores to facilitate their spread from cell to cell. The most striking of these modifications is observed for groups of viruses whose MPs form tubules that assemble in PDs and through which virions are transported to neighbouring cells. The nature of the molecular interactions between viral MPs and PD components and their role in viral movement has remained essentially unknown. Here, we show that the family of PD-located proteins (PDLPs) promotes the movement of viruses that use tubule-guided movement by interacting redundantly with tubule-forming MPs within PDs. Genetic disruption of this interaction leads to reduced tubule formation, delayed infection and attenuated symptoms. Our results implicate PDLPs as PD proteins with receptor-like properties involved the assembly of viral MPs into tubules to promote viral movement.     

A family of clostridial and streptococcal ligand-binding proteins with conserved C-terminal repeat sequences

Analysis of the derived amino acid sequences of toxins A and B from Clostridium difficile has identified an extraordinarily large number of repeat amino acid units in the C-terminal regions of the proteins. Nearly one third of each of the proteins consist of repeating units which appear, at least in the case of toxin A, to be responsible for carbohydrate binding. Similar repeat units are also found in the C-terminal region of four glucosyltransferases from Streptococcus mutans and Streptococcus downei, and in four lytic enzymes from Streptococcus pneumoniae and its bacteriophages (HB-3, Cp-1 and Cp-9). In each case the repeats constitute the ligand-binding portion of the respective enzymes. A glucan-binding protein from S. mutans, which lacks enzymatic activity, has similar repeats spanning almost the entire molecule. This family of ligand-binding proteins appears to be of modular design, with one module consisting of a repetitive ligand-binding domain located in the C-terminal region and the other module(s) providing enzymatic functions.     

Haspin-like proteins: a new family of evolutionarily conserved putative eukaryotic protein kinases

Haspin (haploid germ cell-specific nuclear protein kinase) is reported to be a serine/threonine kinase that may play a role in cell-cycle cessation and differentiation of haploid germ cells. In addition, Haspin mRNA can be detected in diploid cell lines and tissues. Here, Haspin-like proteins are identified in several major eukaryotic phyla-including yeasts, plants, flies, fish, and mammals-and an extended group in Caenorhabditis elegans. The Haspin-like proteins have a complete but divergent eukaryotic protein kinase domain sequence. Although clearly related to one another and to other eukaryotic protein kinases, the Haspin-related proteins lack conservation of a subset of residues that are almost invariant in known kinases and possess distinctive inserted regions. In fact, phylogenetic analysis indicates that the Haspin-like proteins form a novel eukaryotic protein kinase family distinct from those previously defined. The identification of related proteins in model organisms provides some initial insight into their functional properties and will provide new experimental avenues by which to determine the function of the Haspin proteins in mammalian cells.     

Human ADP-ribosylation factors. A functionally conserved family of GTP-binding proteins

A new member, hARF4, of the ADP-ribosylation factor (ARF) family, a subset of the superfamily of regulatory GTP-binding proteins, has been cloned from a cDNA expression library. Two other human ARF cDNA sequences, designated human ARF1 and ARF3, have been reported previously and are 96% identical in amino acid sequence. A human ARF1 cDNA, significantly longer than previously described clones, was obtained, by cross-species hybridization using a bovine ARF1 cDNA probe. Bovine ARF1p and human ARF1p are 100% identical while each is only 80% identical to hARF4p. Thus, hARF4p is the most divergent of the mammalian ARF proteins identified. Northern blot analysis revealed the expression of at least three different ARF messages in human placenta and adrenal carcinoma cells. Both hARF1 and hARF4 encode GTP-binding proteins with predicted molecular masses of 20,000-21,000 Da. Biochemical analysis of the purified recombinant proteins revealed a high degree of conservation of nucleotide binding properties and in vitro ARF activities. ARF is an essential gene in the yeast, Saccharomyces cerevisiae, and is encoded by two genes. Expression of either hARF1p or hARF4p in yeast was found to rescue the lethal double mutant, arf1-arf2-, thus demonstrating the functional conservation of ARF functions between yeast and man. The combination of in vivo and in vitro assays for ARF function provides a specific and unambiguous means of determining bona fide ARF proteins from divergent species from among the rapidly increasing number of structurally related, small molecular weight GTP-binding proteins.     

Entry of oomycete and fungal effectors into plant and animal host cells

Fungal and oomycete pathogens cause many destructive diseases of plants and important diseases of humans and other animals. Fungal and oomycete plant pathogens secrete numerous effector proteins that can enter inside host cells to condition susceptibility. Until recently it has been unknown if these effectors enter via pathogen-encoded translocons or via pathogen-independent mechanisms. Here we review recent evidence that many fungal and oomycete effectors enter via receptor-mediated endocytosis, and can do so in the absence of the pathogen. Surprisingly, a large number of these effectors utilize cell surface phosphatidyinositol-3-phosphate (PI-3-P) as a receptor, a molecule previously known only inside cells. Binding of effectors to PI-3-P appears to be mediated by the cell entry motif RXLR in oomycetes, and by diverse RXLR-like variants in fungi. PI-3-P appears to be present on the surface of animal cells also, suggesting that it may mediate entry of effectors of fungal and oomycete animal pathogens, for example, RXLR effectors found in the oomycete fish pathogen, Saprolegnia parasitica. Reagents that can block PI-3-P-mediated entry have been identified, suggesting new therapeutic strategies.