Isolation of a novel SUMO protein from tomato that suppresses EIX-induced cell death

SUMO protein modification

SUMO (small ubiquitin-related modifier) family proteins are not only structurally but also mechanistically related to ubiquitin in that they are posttranslationally attached to other proteins. As ubiquitin, SUMO is covalently linked to its substrates via amide (isopeptide) bonds formed between its C-terminal glycine residue and the epsilon-amino group of internal lysine residues. The enzymes involved in the reversible conjugation of SUMO are similar to those mediating the ubiquitin conjugation. Since its discovery in 1996, SUMO has received a high degree of attention because of its intriguing and essential functions, and because its substrates include a variety of biomedically important proteins such as tumor suppressor p53, c-jun, PML and huntingtin. SUMO modification appears to play important roles in diverse processes such as chromosome segregation and cell division, DNA replication and repair, nuclear protein import, protein targeting to and formation of certain subnuclear structures, and the regulation of a variety of processes including the inflammatory response in mammals and the regulation of flowering time in plants.

New insights to the function of phytopathogenic bacterial type III effectors in plants

Phytopathogenic bacteria use the type III secretion system (TTSS) to inject effector proteins into plant cells. This system is essential for bacteria to multiply in plant tissue and to promote the development of disease symptoms. Until recently, little was known about the function of TTSS effectors in bacterial-plant interactions. New studies dissecting the molecular and biochemical action of TTSS effectors show that these proteins contribute to bacterial pathogenicity by interfering with plant defense signal transduction. These investigations provide us with a fresh view of how bacteria manipulate plant physiology to colonize their hosts.

Receptor-like proteins involved in plant disease resistance

SUMMARY Race-specific resistance in plants against microbial pathogens is governed by several distinct classes of resistance (R) genes. This review focuses on the class that consists of the plasma membrane-bound leucine-rich repeat proteins known as receptor-like proteins (RLPs). The first isolated resistance genes of the RLP class are the tomato Cf genes, which confer resistance to the fungal pathogen Cladosporium fulvum. To date, several other RLP genes are known to be implicated in resistance in other plant-pathogen interactions. These include HcrVf2 from apple, Ve1 and Ve2 from tomato, and RPP27 from Arabidopsis, which are involved in resistance to Venturia, Verticillium and Peronospora, respectively. Furthermore, the tomato RLP gene LeEix initiates defence responses upon elicitation with a fungal ethylene-inducing xylanase (EIX) of non-pathogenic Trichoderma. The tomato Cf genes, which are the most intensively studied RLP resistance genes, are usually found in clusters of several homologues. Whereas some of these homologues are functional Cf resistance genes, others have no known function in resistance. Different evolutionary processes contribute to variation in functional Cf genes, and functional as well as non-functional homologues may provide a source for the generation of novel Cf resistance genes. To date, little is known of the proteins that interact with Cf proteins to initiate defence responses. In contrast to the LeEix protein and the corresponding EIX elicitor, for which a direct interaction was found, no direct interaction between Cf proteins and the corresponding C. fulvum elicitors has been demonstrated. Analogous to the CLAVATA signalling complex, which comprises an RLP, a receptor-like kinase (RLK) and a small proteineous ligand, Cf proteins may form a complex with RLKs and thus initiate signalling upon recognition of the corresponding elicitors. The presence of RLP resistance genes in diverse plant species suggests that these genes play an important role in the extracellular recognition of plant pathogens.

SUMO conjugation in plants

Covalent attachment of small proteins to substrates can regulate protein activity in eukaryotes. SUMO, the small ubiquitin-related modifier, can be covalently linked to a broad spectrum of substrates. An understanding of SUMO's role in plant biology is still in its infancy. In this review, we briefly summarize the enzymology of SUMO conjugation (sumoylation), and the current knowledge of SUMO modification in Arabidopsis thaliana (L.) Heynh. and other plants, in comparison to animals and fungi. Furthermore, we assemble a list of potential pathway components in the genome of A. thaliana that have either been functionally defined, or are suggested by similarity to pathway components from other organisms.

Proteases in pathogenesis and plant defence

Plant pathogens deliver a variety of virulence factors to host cells to suppress basal defence responses and create suitable environments for their propagation. Plants have in turn evolved disease resistance genes whose products detect the virulence factors as a signal of invasion and activate effective defence responses. Understanding how a virulence effector contributes to virulence on susceptible hosts but becomes an avirulence factor that triggers defence responses on resistance hosts has been a major focus in plant research. Recent studies have shown that a growing list of pathogen-encoded effectors functions as proteases that are secreted into plant cells to modify host proteins. In addition, several plant proteases have been found to function in activation of the defence mechanism. These findings reveal that post-translational modification of host proteins through proteolytic processing is a widely used mechanism in regulating the plant defence response.

Xanthomonas type III effector XopD targets SUMO-conjugated proteins in planta

Xanthomonas campestris pathovar vesicatoria (Xcv) uses the type III secretion system (TTSS) to inject effector proteins into cells of Solanaceous plants during pathogenesis. A number of Xcv TTSS effectors have been identified; however, their function in planta remains elusive. Here, we provide direct evidence for a functional role for a phytopathogenic bacterial TTSS effector in planta by demonstrating that the Xcv effector XopD encodes an active cysteine protease with plant-specific SUMO substrate specificity. XopD is injected into plant cells by the TTSS during Xcv pathogenesis, translocated to subnuclear foci and hydrolyses SUMO-conjugated proteins in vivo. Our studies suggest that XopD mimics endogenous plant SUMO isopeptidases to interfere with the regulation of host proteins during Xcv infection.

Cysteine proteases in phytopathogenic bacteria: identification of plant targets and activation of innate immunity

Phytopathogenic bacteria use the type-III secretion system (TTSS) to inject effector proteins into plant cells, presumably to colonize their hosts. The function of these proteins inside plant cells has remained a mystery for years. The recent discovery that the effectors XopD, AvrXv4, AvrPphB, and AvrRpt2 have cysteine protease functions reveals that the proteolysis of host substrates is an important strategy employed by pathogens to alter plant physiology. Moreover, the characterization of these proteases and their targets provides new insight to mechanisms of bacterial virulence and the activation of plant immunity.

Characterization of the Xanthomonas AvrXv4 effector, a SUMO protease translocated into plant cells

Homologs of the Yersinia virulence factor YopJ are found in both animal and plant bacterial pathogens, as well as in plant symbionts. The conservation of this effector family indicates that several pathogens may use YopJ-like proteins to regulate bacteria-host interactions during infection. YopJ and YopJ-like proteins share structural homology with cysteine proteases and are hypothesized to functionally mimic small ubiquitin-like modifier (SUMO) proteases in eukaryotic cells. Strains of the phytopathogenic bacterium Xanthomonas campestris pv. vesicatoria are known to possess four YopJ-like proteins, AvrXv4, AvrBsT, AvrRxv, and XopJ. In this work, we have characterized AvrXv4 to determine if AvrXv4 functions like a SUMO protease in planta during Xanthomonas-plant interactions. We provide evidence that X. campestris pv. vesicatoria secretes and translocates the AvrXv4 protein into plant cells during infection in a type III-dependent manner. Once inside the plant cell, AvrXv4 is localized to the plant cytoplasm. By performing AvrXv4 deletion and mutational analysis, we have identified amino acids required for type III delivery and for host recognition. We show that AvrXv4 recognition by resistant plants requires a functional protease catalytic core, the domain that is conserved in all of the putative YopJ-like cysteine proteases. We also show that AvrXv4 expression in planta leads to a reduction in SUMO-modified proteins, demonstrating that AvrXv4 possesses SUMO isopeptidase activity. Overall, our studies reveal that the YopJ-like effector AvrXv4 encodes a type III SUMO protease effector that is active in the cytoplasmic compartment of plant cells.

Interaction between a geminivirus replication protein and the plant sumoylation system

Geminiviruses are small DNA viruses that replicate in nuclei of infected plant cells after accumulation of host replication machinery. Tomato golden mosaic virus (TGMV) and Tomato yellow leaf curl Sardinia virus (TYLCSV) encode a protein, RepAC1 (or Rep), that is essential for viral replication. Rep/RepAC1 is an oligomeric protein that binds to double-stranded DNA, catalyzes cleavage and ligation of single-stranded DNA, and is sufficient for host induction. It also interacts with several host proteins, including the cell cycle regulator, retinoblastoma, and essential components of the cell DNA replication machinery, like proliferating nuclear cell antigen (PCNA) and RFC-1. To identify other cellular proteins that interact with Rep/RepAC1 protein, a Nicotiana benthamiana cDNA library was screened with a yeast two-hybrid assay. The host cell sumoylation enzyme, NbSCE1 (N. benthamiana SUMO-conjugating enzyme, homolog to Saccharomyces cerevisiae UBC9), was found to interact specifically with RepAC1. Mapping studies localized the interaction to the N-terminal half of RepAC1. Effects on geminivirus replication were observed in transgenic plants with altered levels of SUMO, the substrate for UBC9.

At the crossroads of SUMO and NF-kappaB

BACKGROUND

Recognition of pathogens by immune receptors leads to activation of macrophages, dendritic cells, and lymphocytes. Signals are communicated to enhance expression of target molecules such as cytokines and adhesion molecules, depending on activation of various inducible transcription factors, among which the family NF-kappaB transcription factors plays an evolutionarily conserved and critical role. Classical activation of NF-kappaB involves phosphorylation, polyubiquitination and subsequent degradation of the inhibitor molecules of NF-kappaB, referred to as IkappaB. Modification of IkappaBalpha, one of the mammalian IkappaB isoforms, with the small ubiquitin-like modifier (SUMO) results its protection from degradation.

PRESENTATION OF THE HYPOTHESIS

SUMO-IkappaBalpha localizes in the nucleus. The nuclear SUMO-IkappaBalpha pool may be dynamic. SUMO-IkappaBalpha functions as synergy control factor.

TESTING THE HYPOTHESIS

Immunoprecipitation from cellular fractions, 35S methionine pulse-chase, and FRET assays should reveal the localization of SUMO-IkappaBalpha and the dynamics of the pool. Expression of SUMOylation defective IkappaBalpha in an IkappaBalpha -/- background should yield insights into the function of SUMO-IkappaBalpha.

IMPLICATION OF THE HYPOTHESIS

IkappaBalpha contains the required SUMOylation motif but IkappaBbeta does not. The suggested study would provide evidence whether or not IkappaBalpha and IkappaBbeta can substitute each other. In addition, the suggested assays would reveal a possible redundancy in controlling transcriptional activity of NF-kappaB.

The small ubiquitin-like modifier (SUMO) protein modification system in Arabidopsis. Accumulation of SUMO1 and -2 conjugates is increased by stress

Small ubiquitin-like modifier (SUMO) is a member of the superfamily of ubiquitin-like polypeptides that become covalently attached to various intracellular target proteins as a way to alter their function, location, and/or half-life. Here we show that the SUMO conjugation system operates in plants through a characterization of the Arabidopsis SUMO pathway. An eight-gene family encoding the SUMO tag was discovered as were genes encoding the various enzymes required for SUMO processing, ligation, and release. A diverse array of conjugates could be detected, some of which appear to be SUMO isoform-specific. The levels of SUMO1 and -2 conjugates but not SUMO3 conjugates increased substantially following exposure of seedlings to stress conditions, including heat shock, H(2)O(2), ethanol, and the amino acid analog canavanine. The heat-induced accumulation could be detected within 2 min from the start of a temperature upshift, suggesting that SUMO1/2 conjugation is one of the early plant responses to heat stress. Overexpression of SUMO2 enhanced both the steady state levels of SUMO2 conjugates under normal growth conditions and the subsequent heat shock-induced accumulation. This accumulation was dampened in an Arabidopsis line engineered for increased thermotolerance by overexpressing the cytosolic isoform of the HSP70 chaperonin. Taken together, the SUMO conjugation system appears to be a complex and functionally heterogeneous pathway for protein modification in plants with initial data indicating that one important function may be in stress protection and/or repair.

Abiotic stress signal transduction in plants: Molecular and genetic perspectives

Low temperature, drought and salinity are major adverse environmental factors that limit plant productivity. Understanding the mechanisms by which plants perceive and transduce these stress signals to initiate adaptive responses is essential for engineering stress-tolerant crop plants. Molecular and biochemical studies suggest that abiotic stress signaling in plants involves receptor-coupled phosphorelay, phosphoinositol-induced Ca2+ changes, mitogen-activated protein kinase cascades and transcriptional activation of stress-responsive genes. In addition, protein posttranslational modifications and adapter or scaffold-mediated protein-protein interactions are also important in abiotic stress signal transduction. Most of these signaling modules, however, have not been genetically established to function in plant abiotic stress signal transduction. To overcome the scarcity of abiotic stress-specific phenotypes for conventional genetic screens, molecular genetic analysis using stress-responsive promoter-driven reporter is suggested as an alternative approach to genetically dissect abiotic stress signaling networks in plants.

SUMO--nonclassical ubiquitin

SUMO (small ubiquitin-related modifier) is the best-characterized member of a growing family of ubiquitin-related proteins. It resembles ubiquitin in its structure, its ability to be ligated to other proteins, as well as in the mechanism of ligation. However, in contrast to ubiquitination-often the first step on a one-way road to protein degradation-SUMOlation does not seem to mark proteins for degradation. In fact, SUMO may even function as an antagonist of ubiquitin in the degradation of selected proteins. While most SUMO targets are still at large, available data provide compelling evidence for a role of SUMO in the regulation of protein-protein interactions and/or subcellular localization.

SUMO, ubiquitin's mysterious cousin

Covalent modification of cellular proteins by the ubiquitin-like modifier SUMO regulates various cellular processes, such as nuclear transport, signal transduction, stress response and cell-cycle progression. But, in contrast to ubiquitylation, sumoylation does not tag proteins for degradation, but seems to enhance their stability or modulate their subcellular compartmentalization.