Giardia lamblia disrupts tight junctional ZO-1 and increases permeability in non-transformed human small intestinal epithelial monolayers: effects of epidermal growth factor.

In order to improve our understanding of the host cell-parasite interactions in giardiasis, this study assessed the effects of Giardia lamblia on epithelial permeability and tight junctional ZO-1, determined whether epidermal growth factor (EGF) may affect Giardia-induced epithelial injury, and evaluated if EGF modulates epithelial colonization by live G. lamblia trophozoites. Permeability was assessed in assays of trans-epithelial fluxes of FITC-dextran, and ZO-1 integrity was characterized by confocal laser immunofluorescence microscopy in confluent epithelial cell monolayers. G. lamblia significantly increased paracellular permeability and disrupted tight-junctional ZO-1 of a novel non-transformed human small intestinal epithelial cell line (SCBN). Pre-treatment with EGF prevented the development of these abnormalities and significantly inhibited attachment of live trophozoites to the enterocytes, independently of a direct microbiocidal action. These findings demonstrate that G. lamblia may cause intestinal pathophysiology by disrupting tight junctional ZO-1 and increasing epithelial permeability. Apical administration of EGF prevents these abnormalities, and reduces epithelial colonization by the live parasites.

Molecular characterization and expression analysis of a highly conserved rice mago nashil homolog.

Mago Nashi, a protein initially shown to be essential in the development of the Drosophila oocyte, is highly conserved among species and shows no homology to any other known cellular proteins. Here we report the nucleotide sequence of a cDNA and a partial gene that encode rice Mago Nashi protein homologs. In addition, we present the tissue-specific expression pattern of mago nashi at the level of RNA and protein. The rice Mago Nashi protein shares at least 73% amino acid identity with all known animal homologs. Genomic DNA gel blot analysis indicates that two copies of the mago nashi gene exist in the rice genome, one of which has identical intron positions to those found in an Arabidopsis homolog. mago nashi is expressed in root, leaf and developing seed tissue as determined by RNA and protein gel blot analysis. Evidence from Drosophila, Caenorhabditis elegans and human studies of Mago Nashi suggests that a major function of this protein is its involvement in RNA localization. The highly conserved amino acid sequence of all Mago Nashi protein homologs across kingdoms suggests that the plant version of this protein may similarly be involved in RNA localization.

Messenger RNA targeting of rice seed storage proteins to specific ER subdomains.

Rice seeds, a rich reserve of starch and protein, are a major food source in many countries. Unlike the seeds of other plants, which typically accumulate one major type of storage protein, rice seeds use two major classes, prolamines and globulin-like glutelins. Both storage proteins are synthesized on the endoplasmic reticulum (ER) and translocated to the ER lumen, but are then sorted into separate intracellular compartments. Prolamines are retained in the ER lumen as protein bodies whereas glutelins are transported and stored in protein storage vacuoles. Mechanisms responsible for the retention of prolamines within the ER lumen and their assembly into intracisternal inclusion granules are unknown, but the involvement of RNA localization has been suggested. Here we show that the storage protein RNAs are localized to distinct ER membranes and that prolamine RNAs are targeted to the prolamine protein bodies by a mechanism based on RNA signal(s), a process that also requires a translation initiation codon. Our results indicate that the ER may be composed of subdomains that specialize in the synthesis of proteins directed to different compartments of the plant endomembrane system.

Developing prolamine protein bodies are associated with the cortical cytoskeleton in rice endosperm cells.

The mRNAs that encode the prolamine storage proteins in rice (Oryza sativa L.) endosperm cells are enriched on the surface of the prolamine protein bodies (PBs), a subcellular structure consisting of a prolamine intracisternal granule surrounded by rough endoplasmic reticulum membrane. Previous biochemical studies (D.G. Muench et al., 1998, Plant Physiol. 116: 559-569) have shown that prolamine mRNAs may be anchored to the PB surface via the cytoskeleton. To better understand the mechanism and role of mRNA localization in rice endosperm cells, we studied the subcellular development of prolamine PBs and their relationship with the cytoskeleton in rice endosperm cells. Confocal microscopy of endosperm cells showed that, unlike the glutelin PBs, the developing prolamine PBs are not randomly distributed within the cell, but instead are often enriched in the cortical region of the cell only a few micrometers beneath the plasma membrane. In addition, the peripheral prolamine PBs are closely associated with the cortical microtubule and actin filament networks. The cortical enrichment of rice prolamine protein bodies represents a unique example of endoplasmic reticulum subdomain localization in plant cells. The interaction of this endoplasmic reticulum subdomain with the cytoskeleton provides new insights on the possible mechanism and role of mRNA localization in plants.

Identification of polypeptides associated with an enriched cytoskeleton-protein body fraction from developing rice endosperm.

Recent evidence has shown that the prolamine polysomes are attached not only to the endoplasmic reticulum membranes that bound the prolamine protein bodies (PBs) but also to cytoskeleton elements associated with this subcellular fraction. To learn more about the nature of the proteins that are associated with this supra-macromolecular complex, proteins extracted from an enriched cytoskeleton-PB fraction were resolved by two-dimensional polyacrylamide gel electrophoresis under non-equilibrium conditions and analyzed for their composition by immunological and biochemical methods. Immunoblot analysis indicated the presence of the cytoskeletal proteins, actin and tubulin, and the cytoskeletal-associated protein EF1 alpha in this fraction. Microsequencing of selected polypeptides revealed a diversity of protein sequences. In addition to contaminating storage proteins which are selectively solubilized by the isolation procedure, several ribosomal proteins and histone H3 were also identified. Some of the remaining polypeptides showed partial homology to protein sequences deposited in the database, several of which are cytoskeleton-associated proteins.

Molecular cloning, expression and subcellular localization of a BiP homolog from rice endosperm tissue.

The ER luminal binding protein, BiP, has been linked to prolamine protein body formation in rice. To obtain further information on the possible role of this chaperone in protein body formation we have cloned and sequenced a BiP cDNA homolog from rice endosperm. The rice sequence is very similar to the maize BiP exhibiting 92% nucleotide identity and 96% deduced amino acid sequence identity in the coding region. Substantial amino acid sequence homology exists between rice BiP and BiP homologs from several other plant and animal species including long stretches of conservation through the amino-terminal ATPase domain. Considerable variation, however, is observed within the putative carboxy-terminal peptide-binding domain between the plant and nonplant BiP sequences. A single hand of approximately 2.4 kb was visible when RNA gel blots of total RNA purified from seed tissue were probed with radiolabeled rice BiP cDNA. This band increased in intensity during seed development up to 10 days after flowering, and then decreased gradually until seed maturity. Protein gel blots indicated that BiP polypeptide accumulation parallels that of the prolamine polypeptides throughout seed development. Immunocytochemical analysis demonstrated that BiP is localized in a non-stochastic fashion in the endoplasmic reticulum membrane complex of developing endosperm cells. It is abundant on the periphery of the protein inclusion body but not in the central portion of the protein body or in the cisternal ER membranes connecting the protein bodies. These data support a model which proposes that BiP associates with the newly synthesized prolamine polypeptide to facilitate its folding and assembly into a protein inclusion body, and is then recycled.

Hypoxically inducible barley alanine aminotransferase: cDNA cloning and expression analysis.

A 1.75 kb cDNA containing the entire coding sequence of the hypoxically inducible alanine aminotransferase (AlaAT) from barley roots was isolated and sequenced. This clone has an open reading frame of 1446 bp, and a deduced amino acid sequence of 482 residues, giving an estimated protein molecular mass of 52,885 Da. RNA blot analysis of barley root tissue showed a 4-fold increase of a single AlaAT-2 mRNA band after 12-24 hours of hypoxic stress, followed by a decrease in message levels after 48 h of hypoxic conditions. AlaAT-2 protein concentration increased in a similar pattern to AlaAT activity in root tissue, to almost 6-fold the aerobic level after 96 h of hypoxic stress. AlaAT-2 activity increased more than 2-fold in roots of Panicum miliaceum exposed to hypoxia, and is the same isoform as the light inducible AlaAT in P. miliaceum leaves. The unique expression patterns of AlaAT-2 in root and leaf tissue upon exposure to different environmental stimuli is also discussed.

Long-Term Anaerobic Metabolism in Root Tissue (Metabolic Products of Pyruvate Metabolism).

The onset of anaerobiosis in barley root tissue (Hordeum vulgare L. cv Himalaya) results in the following metabolic responses. There are rapid increases in the levels of pyruvate, lactate, and ethanol. Malate and succinate concentrations increase over the first 12 h, after which they return to the levels found in oxygenated root tissue. Alanine concentration increases over the first 12 h, and this is matched by a corresponding decrease in aspartate. The initial stoichiometric decline in aspartate and increase in alanine suggests that the amino group of aspartate is conserved by transaminating pyruvate to alanine. Aspartate catabolism also probably provides the initial source of carbon for reduction to succinate under anoxic conditions. Under long-term anaerobiosis (>24 h), there is no further accumulation of any of the fermentative end products other than ethanol, which also represents the major metabolic end product during long-term anaerobiosis. Although a number of the enzymes involved in fermentative respiration have been found to be induced under anaerobic conditions, neither aspartate amino-transferase nor malate dehydrogenase is induced in barley root tissue. The observations suggest that the long-term adaptations to hypoxic conditions may be quite different than the more well-characterized short-term adaptations.

Purification and characterization of an anaerobically induced alanine aminotransferase from barley roots.

Alanine aminotransferase (AlaAT, EC 2.6.1.2) is an enzyme that is induced under anaerobic conditions in cereal roots. In barley (Hordeum vulgare L.) roots, there are a number of isoforms of AlaAT. We have identified the anaerobically induced isoform and have purified it to homogeneity. The isolation procedure involved a two-step ammonium sulfate precipitation, gel filtration, ion-exchange chromatography, and chromatofocusing. The enzyme was purified approximately 350-fold to a specific activity of 2231 units/milligram protein. The apparent molecular masses of the native and sodium dodecyl sulfate-denatured AlaAT proteins are 97 and 50 kilodaltons, respectively, indicating that the native enzyme is probably a homodimer. AlaAT has a number of interesting characteristics when compared with other plant aminotransferases. AlaAT does not require the presence of pyridoxyl-5-phosphate to retain its activity, and it appears to be very specific in the reactions that it will catalyze.