Exclusion of plastid nucleoids and ribosomes from stromules in tobacco and Arabidopsis

Stromules are stroma-filled tubules that extend from the surface of plastids and allow the transfer of proteins as large as 550 kDa between interconnected plastids. The aim of the present study was to determine if plastid DNA or plastid ribosomes are able to enter stromules, potentially permitting the transfer of genetic information between plastids. Plastid DNA and ribosomes were marked with green fluorescent protein (GFP) fusions to LacI, the lac repressor, which binds to lacO-related sequences in plastid DNA, and to plastid ribosomal proteins Rpl1 and Rps2, respectively. Fluorescence from GFP-LacI co-localised with plastid DNA in nucleoids in all tissues of transgenic tobacco (Nicotiana tabacum L.) examined and there was no indication of its presence in stromules, not even in hypocotyl epidermal cells, which contain abundant stromules. Fluorescence from Rpl1-GFP and Rps2-GFP was also observed in a punctate pattern in chloroplasts of tobacco and Arabidopsis [Arabidopsis thaliana (L.) Heynh.], and fluorescent stromules were not detected. Rpl1-GFP was shown to assemble into ribosomes and was co-localised with plastid DNA. In contrast, in hypocotyl epidermal cells of dark-grown Arabidopsis seedlings, fluorescence from Rpl1-GFP was more evenly distributed in plastids and was observed in stromules on a total of only four plastids (<0.02% of the plastids observed). These observations indicate that plastid DNA and plastid ribosomes do not routinely move into stromules in tobacco and Arabidopsis, and suggest that transfer of genetic information by this route is likely to be a very rare event, if it occurs at all.

Visualisation of stromules on Arabidopsis plastids

Stromules are thin stroma-filled tubules that extend from all plastid types in all multicellular plants examined. They are most easily visualised by epifluorescence or confocal microscopy of plastids containing green fluorescent protein (GFP) or other fluorescent proteins. Transient expression of gene constructs encoding plastid-targeted GFP following bombardment of whole plants or organs of Arabidopsis with gold or tungsten particles coated with plasmid DNA is a relatively rapid and simple means of producing material for observation of stromules.

A disulfide driven domain swap switches off the activity of Shigella IpaH9.8 E3 ligase

We show that the monomeric form of Shigella IpaH9.8 E3 ligase catalyses the ubiquitination of human U2AF35 in vitro, providing a molecular mechanism for the observed in vivo effect. We further discover that under non-reducing conditions IpaH9.8 undergoes a domain swap driven by the formation of a disulfide bridge involving the catalytic cysteine and that this dimer is unable to catalyse the ubiquitination of U2AF35. The crystal structure of the domain-swapped dimer is presented. The redox inactivation of IpaH9.8 could be a mechanism of regulating the activity of the IpaH9.8 E3 ligase in response to cell damage so that the host cell in which the bacteria resides is maintained in a benign state suitable for bacterial survival.

Myosin XI is required for actin-associated movement of plastid stromules

Stromules are highly dynamic stroma-filled tubules extending from the surface of plastids and occasionally interconnecting individual plastids, allowing the movement of complex biological molecules between the interconnected plastids. Experiments with inhibitors of cytoskeleton assembly have indicated the involvement of an actin-based system in stromule movement. However, the motor protein associated with the system had not been identified. Here, we present direct evidence that myosin XI is involved in the formation and movement of stromules in tobacco leaves. Application of 2,3-butanedione 2-monoxime, an inhibitor of myosin ATPase activity, resulted in the loss of stromules from tobacco leaf epidermal cells. Transient RNA interference of myosin XI in leaves of Nicotiana benthamiana also resulted in the loss of stromules from epidermal cells, without any effect on transcripts for actin or myosin VIII. Transient expression of a GFP-tagged myosin XI tail domain in tobacco leaf epidermal cells showed that the fusion protein localized to the chloroplast envelope, as well as to mitochondria and other organelles. Our findings identify myosin XI as a key protein involved in the formation and movement of stromules.

COP1 and ELF3 control circadian function and photoperiodic flowering by regulating GI stability

Seasonal changes in day length are perceived by plant photoreceptors and transmitted to the circadian clock to modulate developmental responses such as flowering time. Blue-light-sensing cryptochromes, the E3 ubiquitin-ligase COP1, and clock-associated proteins ELF3 and GI regulate this process, although the regulatory link between them is unclear. Here we present data showing that COP1 acts with ELF3 to mediate day length signaling from CRY2 to GI within the photoperiod flowering pathway. We found that COP1 and ELF3 interact in vivo and show that ELF3 allows COP1 to interact with GI in vivo, leading to GI degradation in planta. Accordingly, mutation of COP1 or ELF3 disturbs the pattern of GI cyclic accumulation. We propose a model in which ELF3 acts as a substrate adaptor, enabling COP1 to modulate light input signal to the circadian clock through targeted destabilization of GI.

Arrestin-like proteins mediate ubiquitination and endocytosis of the yeast metal transporter Smf1

Many plasma membrane proteins in yeast are ubiquitinated and endocytosed, but how they are recognized for modification has remained unknown. Here, we show that the manganese transporter Smf1 is endocytosed when cells are exposed to cadmium ions, that this endocytosis depends on Rsp5-dependent ubiquitination of specific lysines and that it also requires phosphorylation at nearby sites. This phosphorylation is, however, constitutive rather than stress-induced. Efficient ubiquitination requires Ecm21 or Csr2, two members of a family of arrestin-like yeast proteins that contain several PY motifs and bind to Rsp5. Ecm21 also binds to phosphorylated Smf1, providing a link between Rsp5 and its substrate. PY motif-containing arrestin-like proteins are found in many species, including humans, and might have a general role as ubiquitin ligase adaptors.

An Arabidopsis mutant able to green after extended dark periods shows decreased transcripts of seed protein genes and altered sensitivity to abscisic acid

An Arabidopsis mutant showing an altered ability to green on illumination after extended periods of darkness has been isolated in a screen for genomes uncoupled (gun) mutants. Following illumination for 24 h, 10-day-old dark-grown mutant seedlings accumulated five times more chlorophyll than wild-type seedlings and this was correlated with differences in plastid morphology observed by transmission electron microscopy. The mutant has been named greening after extended darkness 1 (ged1). Microarray analysis showed much lower amounts of transcripts of genes encoding seed storage proteins, oleosins, and late embryogenesis abundant (LEA) proteins in 7-day-old seedlings of ged1 compared with the wild type. RNA gel-blot analyses confirmed very low levels of transcripts of seed protein genes in ged1 seedlings grown for 2-10 d in the dark, and showed higher amounts of transcripts of photosynthesis-related genes in illuminated 10-day-old dark-grown ged1 seedlings compared with the wild type. Consensus elements similar to abscisic acid (ABA) response elements (ABREs) were detected in the upstream regions of all genes highly affected in ged1. Germination of ged1 seeds was hypersensitive to ABA, although no differences in ABA content were detected in 7-day-old seedlings. This suggests the mutant may have an altered responsiveness to ABA, affecting expression of ABA-responsive genes and plastid development during extended darkness.

The ancestral symbiont sensor kinase CSK links photosynthesis with gene expression in chloroplasts

We describe a novel, typically prokaryotic, sensor kinase in chloroplasts of green plants. The gene for this chloroplast sensor kinase (CSK) is found in cyanobacteria, prokaryotes from which chloroplasts evolved. The CSK gene has moved, during evolution, from the ancestral chloroplast to the nuclear genomes of eukaryotic algae and green plants. The CSK protein is now synthesised in the cytosol of photosynthetic eukaryotes and imported into their chloroplasts as a protein precursor. In the model higher plant Arabidopsis thaliana, CSK is autophosphorylated and required for control of transcription of chloroplast genes by the redox state of an electron carrier connecting photosystems I and II. CSK therefore provides a redox regulatory mechanism that couples photosynthesis to gene expression. This mechanism is inherited directly from the cyanobacterial ancestor of chloroplasts, is intrinsic to chloroplasts, and is targeted to chloroplast genes.

Preparation and characterization of 4-dimethylaminopyridine-stabilized palladium nanoparticles

4-Dimethylaminopyridine (DMAP)-stabilized palladium nanoparticles with a mean diameter of 3.4 +/- 0.5 nm are prepared from the aqueous phase reduction of Na2PdCl4 using NaBH4 in the presence of DMAP. TEM and UV-vis spectroscopy characterization of the nanoparticle dispersion shows no obvious change in the nanoparticles several months after preparation. 1H NMR spectroscopy of the nanoparticles shows that the nanoparticle dispersion also contains a boron/DMAP complex and two palladium/DMAP complexes. One of the palladium complexes crystallizes out of the dispersion and is identified as Pd(DMAP)4(OH)2 by X-ray crystallography. Following extensive analysis, it is believed that the palladium/DMAP complexes are formed following the oxidation of the palladium nanoparticles. The prepared nanoparticle dispersion promotes selective hydrogen/deuterium (H/D) exchange on the carbon atoms alpha to the endocyclic nitrogen atom on the DMAP-stabilizing ligands through reaction with D2O. This activity is attributed to the presence of the nanoparticles rather than to the presence of the oxidized palladium/DMAP complexes.

Multiple interactions drive adaptor-mediated recruitment of the ubiquitin ligase rsp5 to membrane proteins in vivo and in vitro

Recognition of membrane proteins by the Nedd4/Rsp5 ubiquitin ligase family is a critical step in their targeting to the multivesicular body pathway. Some substrates contain "PY" motifs (PPxY), which bind to WW domains in the ligase. Others lack PY motifs and instead rely on adaptors that recruit the ligase to them. To investigate the mechanism of adaptor-mediated ubiquitination, we have characterized the interactions between the adaptor Bsd2, the ubiquitin ligase Rsp5, and the membrane proteins Cps1, Tre1, and Smf1 from Saccharomyces cerevisiae. We have reconstituted adaptor-mediated modification of Cps1 and Tre1 in vitro, and we show that two PY motifs in Bsd2 and two WW domains (WW2 and WW3) in Rsp5 are crucial for this. The binding of a weak noncanonical DMAPSY motif in Bsd2 to WW3 is an absolute requirement for Bsd2 adaptor function. We show that sorting of the manganese transporter Smf1, which requires both Bsd2 and Tre1, depends upon two PY motifs in Bsd2 and one motif in Tre1 but only two WW domains in Rsp5. We suggest that sequential assembly of first a Bsd2/Rsp5 complex, then a Tre1/Bsd2/Rsp5 complex followed by a rearrangement of PY-WW interactions is required for the ubiquitination of Smf1.

Light and plastid signals regulate the expression of the pea plastocyanin gene through a common region at the 5' end of the coding region

Expression of the pea plastocyanin gene (PetE) is regulated by light and plastid signals. Previous work indicated that light and plastid regulation of pea PetE operates post-transcriptionally in transgenic tobacco, and requires the correct 5' terminus of the PetE transcript and the PetE-coding region. The post-transcriptional light and plastid regulation of pea PetE has now been demonstrated to operate in transgenic Arabidopsis, where in contrast the endogenous PETE gene is regulated transcriptionally. Transgenic tobacco seedlings containing constructs with progressive 3' deletions of the PetE-coding region fused to the luciferase (Luc) reporter gene demonstrate that the first 60 nucleotides of the coding region are sufficient for regulated accumulation of Luc transcripts by light and plastid signalling pathways affected by treatment with norflurazon and lincomycin. PetE constructs containing premature stop codons were generated to investigate whether translation has a role in light or plastid regulation. Insertion of a stop codon in place of the second codon of the PetE-coding region diminished both light and plastid regulation of PetE transcripts, whereas stop codons inserted later in the transcript had no effect on light or plastid regulation. These experiments indicate that the 5' end of the plastocyanin-coding region contains sequences important for regulation by light and plastid signals.

Entamoeba histolytica and Entamoeba dispar utilize externalized phosphatidylserine for recognition and phagocytosis of erythrocytes

Amebic erythrophagocytosis is characteristic of invasive amebiasis, and mutants deficient in erythrocyte ingestion are avirulent. We sought to understand the molecular mechanisms underlying erythrocyte phagocytosis by Entamoeba histolytica. Following adherence to amebae, erythrocytes became round and crenulated, and phosphatidylserine (PS) was exposed on their outer membrane leaflets. These changes were similar to the effects of calcium treatment on erythrocytes, which we utilized to separate ameba-induced exposure of erythrocyte PS from the process of phagocytosis. The adherence and phagocytosis of calcium-treated erythrocytes were less inhibited by galactose than were those of healthy erythrocytes, suggesting the existence of an amebic coreceptor specific for PS. To test whether PS was recognized by amebae, calcium-treated cells were incubated with annexin V prior to adherence to or ingestion by E. histolytica. Annexin V blocked both adherence (50% +/- 12% inhibition; P < 0.05) and phagocytosis (65% +/- 10%; P < 0.05), providing evidence that at least one galactose-independent coreceptor was involved in the adherence and ingestion of red blood cells. The coreceptor was inhibited by phospho-l-serine and to a lesser extent by phospho-d-serine but not by phospho-l-threonine, which is consistent with the coreceptor functioning in the adherence and ingestion of erythrocytes via recognition of PS. We expanded our investigations to the highly related but noninvasive parasite Entamoeba dispar and demonstrated that it was deficient in red-blood-cell adherence, induction of PS exposure, and phagocytosis. These findings establish phosphatidylserine involvement in erythrophagocytosis by amebae and suggest the existence of a PS receptor on the surfaces of both E. histolytica and E. dispar.

Stromules: a characteristic cell-specific feature of plastid morphology

Stromules (stroma-filled tubules) are highly dynamic structures extending from the surface of all plastid types examined so far, including proplastids, chloroplasts, etioplasts, leucoplasts, amyloplasts, and chromoplasts. Stromules are usually 0.35-0.85 microm in diameter and of variable length, from short beak-like projections to linear or branched structures up to 220 mum long. They are enclosed by the inner and outer plastid envelope membranes and enable the transfer of molecules as large as Rubisco (approximately 560 kDa) between interconnected plastids. Stromules occur in all cell types, but stromule morphology and the proportion of plastids with stromules vary from tissue to tissue and at different stages of plant development. In general, stromules are more abundant in tissues containing non-green plastids, and in cells containing smaller plastids. The primary function of stromules is still unresolved, although the presence of stromules markedly increases the plastid surface area, potentially increasing transport to and from the cytosol. Other functions of stromules, such as transfer of macromolecules between plastids and starch granule formation in cereal endosperm, may be restricted to particular tissues and cell types.

Modulation of F1 hybrid stature without altering parent plants through trans-activated expression of a mutated rice GAI homologue

Hybrid breeding, by taking advantage of heterosis, brings about many superior properties to the F1 progeny. However, some properties, such as increased plant height, are not desirable for agronomic purposes. To specifically counter the height increase associated with hybrid progeny, we employed an Arabidopsis model and tested a trans-activation system for specifically expressing a mutated GAI gene only in the F1 hybrid plants to reduce plant stature. A transcriptional activator, the Gal4 DNA-binding domain fused to the acidic activation domain of herpes simplex virus VP16 protein, driven by a maize ubiquitin promoter, was introduced in one parental line. A rice GAI homologue with an N-terminal deletion of the DELLA domain, driven by a promoter that is responsive to the transcriptional activator, was transferred into another parental line. After genetic crossing, trans-activation of the GAI mutant gene resulted in a dwarf phenotype. Over 50 pair-wise crosses between the parental lines were performed, and analyses suggested that the percentage of F1 progeny exhibiting dwarfism ranged from about 25% to 100%. Furthermore, the dwarfism trait introduced in F1 progeny did not seem to affect total seed yield. Our result suggests the feasibility of manipulating F1 hybrid progeny traits without affecting parent plants or the agronomic property of the progeny.

Arabidopsis COP10 forms a complex with DDB1 and DET1 in vivo and enhances the activity of ubiquitin conjugating enzymes

COP10 is a ubiquitin-conjugating enzyme variant (UEV), which is thought to act together with COP1, DET1, and the COP9 signalosome (CSN) in Arabidopsis to repress photomorphogenesis. Here, we demonstrate that COP10 interacts with ubiquitin-conjugating enzymes (E2s) in vivo, and can enhance their activity in vitro, an activity distinct from previous characterized UEVs such as MMS2 and UEV1. Furthermore, we show that COP10 forms a complex with UV-damaged DNA-binding protein 1a (DDB1a) and de-etiolated 1 (DET1), and physically interacts with COP1 and the CSN. Purified CDD (COP10, DDB1, DET1) complex also shows enhancement of E2 activity (UEA) similar to that observed with COP10 itself. Our data suggests that COP10, along with COP1 and the CSN, promotes the degradation of positive regulators of photomorphogenesis, such as the transcription factor HY5, via the ubiquitin/26S proteasome system. Thus, the CDD complex may act as a ubiquitylation-promoting factor to regulate photomorphogenesis.

Arabidopsis CAND1, an unmodified CUL1-interacting protein, is involved in multiple developmental pathways controlled by ubiquitin/proteasome-mediated protein Degradation

Ubiquitin/proteasome-mediated protein degradation controls various developmental pathways in eukaryotes. Cullin-containing complexes are both versatile and abundant groups of RING family ubiquitin E3 ligases, whose activities are subject to control by RUB/Nedd8 (for related to ubiquitin/neural precursor cell-expressed developmentally downregulated 8) modification of their cullin subunits. Here, we report the identification of an Arabidopsis thaliana counterpart of human CAND1 (cullin-associated and neddylation-dissociated) and demonstrate that it can preferentially interact with unmodified CUL1. The Arabidopsis cand1-1 null mutant displays distinct phenotypes, including late flowering, aerial rosettes, floral organ defects, low fertility, dwarfism, loss of apical dominance, and altered responses to multiple plant hormones. Molecular analyses show that many of these defects are because of compromised activity of CUL1-containing ubiquitin E3 ligases, indicating that CAND1 is required for their optimal activity. Furthermore, the cand1-1 mutant displays a partial constitutive photomorphogenic phenotype and has defects in HY5 degradation in the absence of light, a process mediated by a different RING family E3, COP1. Thus, our data provides genetic support for a critical role of CAND1 in regulating various ubiquitin E3 ligases and their targeted cellular and developmental pathways.

The diverse roles of ubiquitin and the 26S proteasome in the life of plants

A tightly regulated and highly specific system for the degradation of individual proteins is essential for the survival of all organisms. In eukaryotes, this is achieved by the tagging of proteins with ubiquitin and their subsequent recognition and degradation by the 26S proteasome. In plants, genetic analysis has identified many genes that regulate developmental pathways. Subsequent analysis of these genes has implicated ubiquitin and the 26S proteasome in the control of diverse developmental processes, and indicates that proteolysis is a crucial regulatory step throughout the life cycle of plants.

Composite arterial grafts versus conventional grafting for coronary artery bypass grafting

OBJECTIVES

Composite arterial grafts for coronary artery bypass grafting surgery allow complete arterial revascularization but are limited by the inflow of a single internal thoracic artery supplying all the grafted vessels. We reviewed the safety of composite arterial grafts using either bilateral internal thoracic arteries or a single internal thoracic artery and radial artery.

METHODS

From January 1999 to July 2002, 402 consecutive patients receiving composite grafts only were compared to a control group of patients (n = 542) undergoing coronary artery bypass grafting with internal thoracic artery and saphenous veins operated upon by the same surgeons. Two different statistical approaches were used to compare groups in this retrospective analysis. First, propensity score analysis with greedy matching technique was used to match patients from each group. Second, a multivariate analysis was performed looking at a combined patient outcome of death, intra-aortic balloon counterpulsation utilization, myocardial infarction, stroke, and prolonged ventilation on all patients in both groups.

RESULTS

After matching by propensity score, the major clinical outcomes in composite arterial (n = 249) and control (n = 249) groups were found to be similar. The in-hospital mortality in the composite group was 1.2% as compared with 0.4% in matched patients (P =.62). However, patients in the composite group were found to have a significantly longer pump time (P <.0001), longer clamp time (P <.0001), increased incidence of prolonged mechanical ventilation (12.8% vs 4.8%; P =.002), and higher incidence of combined morbidity outcome (13.6% vs 6.4%; P =.007) as compared with matched patients. Multivariable analysis showed that composite arterial grafting was an independent predictor of the combined morbidity outcome with an odds ratio of 2.1 (1.2-3.7).

CONCLUSIONS

These findings suggest that composite arterial grafting may be associated with an increase in risk-adjusted patient morbidity when compared with a conventional coronary artery bypass grafting group, although a mortality difference was not demonstrable.

Diagnostic and treatment challenges of neuronopathic Gaucher disease: two cases with an intermediate phenotype

Gaucher disease (GD) is a lysosomal storage disorder with a broad, overlapping clinical spectrum. The presented two case reports highlight the clinical evaluation required in neuronopathic GD to assist with medical management and genetic counselling.

The COP1-SPA1 interaction defines a critical step in phytochrome A-mediated regulation of HY5 activity

Arabidopsis COP1 is a constitutive repressor of photomorphogenesis that interacts with photomorphogenesis-promoting factors such as HY5 to promote their proteasome-mediated degradation. SPA1 is a repressor of phytochrome A-mediated responses to far-red light. Here we report that COP1 acts as part of a large protein complex and interacts with SPA1 in a light-dependent manner. We further demonstrate the E3 ubiquitin ligase activity of COP1 on HY5 in vitro and the alteration of that activity by SPA1. Thus, the COP1-SPA1 interaction defines a critical step in coordinating COP1-mediated ubiquitination and subsequent degradation of HY5 with PHYA signaling.