Purification of a trypsin inhibitor (PFTI) from pumpkin fruit phloem exudate and isolation of putative trypsin and chymotrypsin inhibitor cDNA clones

Calcium powered phloem protein of SEO gene family "Forisome" functions in wound sealing and act as biomimetic smart materials

Forisomes protein belongs to SEO gene family and is unique to Fabaceae family. These proteins are located in sieve tubes of phloem and function to prevent loss of nutrient-rich photoassimilates, upon mechanical injury/wounding. Forisome protein is also known as ATP independent, mechanically active proteins. Despite the wealth of information role of forisome in plants are not yet fully understood. Recent reports suggest that forisomes protein can act as ideal model to study self assembly mechanism for development of nanotechnological devices like microfluidic system application in space exploration mission. Improvement in micro instrument is highly demanding and has been a key technology by NASA in future space exploration missions. Based on its physical parameters, forisome are found to be ideal biomimetic materials for micro fluidic system because the conformational shifts can be replicated in vitro and are fully reversible over large number of cycles. By the use of protein engineering forisome recombinant protein can be tailored. Due to its unique ability to convert chemical energy into mechanical energy forisome has received much attention. For nanotechnological application and handling biomolecules such as DNA, RNA, protein and cell as a whole microfluidic system will be the most powerful technology. The discovery of new biomimetic smart materials has been a key factor in development of space science and its requirements in such a challenging environment. The field of microfludic, particularly in terms of development of its components along with identification of new biomimetic smart materials, deserves more attention. More biophysical investigation is required to characterize it to make it more suitable under parameters of performance.

Calcium-energized motor protein forisome controls damage in phloem: potential applications as biomimetic "smart" material

Forisomes are ATP independent, mechanically active proteins from the Fabaceae family (also called Leguminosae). These proteins are located in sieve tubes of phloem and function to prevent loss of nutrient-rich photoassimilates, upon mechanical injury/wounding. Forisomes are SEO (sieve element occlusion) gene family proteins that have recently been shown to be involved in wound sealing mechanism. Recent findings suggest that forisomes could act as an ideal model to study self assembly mechanism for the development of nanotechnological devices like microinstruments, the microfluidic system frequently used in space exploration missions. Technology enabling improvement in micro instruments has been identified as a key technology by NASA in future space exploration missions. Forisomes are designated as biomimetic smart materials which are calcium-energized motor proteins. Since forisomes are biomolecules from plant systems it can be doctored through genetic engineering. In contrast, "smart" materials which are not derived from plants are difficult to modify in their properties. Current levels of understanding about forisomes conformational shifts with respect to calcium ions and pH changes requires supplement of future advances with relation to its 3D structure to understand self assembly processes. In plant systems it forms blood clots in the form of occlusions to prevent nutrient fluid leakage and thus proves to be a unique damage control system of phloem tissue.

Serpins in plants and green algae

Control of proteolysis is important for plant growth, development, responses to stress, and defence against insects and pathogens. Members of the serpin protein family are likely to play a critical role in this control through irreversible inhibition of endogenous and exogenous target proteinases. Serpins have been found in diverse species of the plant kingdom and represent a distinct clade among serpins in multicellular organisms. Serpins are also found in green algae, but the evolutionary relationship between these serpins and those of plants remains unknown. Plant serpins are potent inhibitors of mammalian serine proteinases of the chymotrypsin family in vitro but, intriguingly, plants and green algae lack endogenous members of this proteinase family, the most common targets for animal serpins. An Arabidopsis serpin with a conserved reactive centre is now known to be capable of inhibiting an endogenous cysteine proteinase. Here, knowledge of plant serpins in terms of sequence diversity, inhibitory specificity, gene expression and function is reviewed. This was advanced through a phylogenetic analysis of amino acid sequences of expressed plant serpins, delineation of plant serpin gene structures and prediction of inhibitory specificities based on identification of reactive centres. The review is intended to encourage elucidation of plant serpin functions.

Purification and characterization of native and recombinant SaPIN2a, a plant sieve element-localized proteinase inhibitor

SaPIN2a encodes a proteinase inhibitor in nightshade (Solanum americanum), which is specifically localized to the enucleate sieve elements. It has been proposed to play an important role in phloem development by regulating proteolysis in sieve elements. In this study, we purified and characterized native SaPIN2a from nightshade stems and recombinant SaPIN2a expressed in Escherichia coli. Purified native SaPIN2a was found as a charge isomer family of homodimers, and was weakly glycosylated. Native SaPIN2a significantly inhibited serine proteinases such as trypsin, chymotrypsin, and subtilisin, with the most potent inhibitory activity on subtilisin. It did not inhibit cysteine proteinase papain and aspartic proteinase cathepsin D. Recombinant SaPIN2a had a strong inhibitory effect on chymotrypsin, but its inhibitory activities toward trypsin and especially toward subtilisin were greatly reduced. In addition, native SaPIN2a can effectively inhibit midgut trypsin-like activities from Trichoplusia ni and Spodoptera litura larvae, suggesting a potential for the production of insect-resistant transgenic plants.

The squash aspartic proteinase inhibitor SQAPI is widely present in the cucurbitales, comprises a small multigene family, and is a member of the phytocystatin family

The squash (Cucurbita maxima) phloem exudate-expressed aspartic proteinase inhibitor (SQAPI) is a novel aspartic acid proteinase inhibitor, constituting a fifth family of aspartic proteinase inhibitors. However, a comparison of the SQAPI sequence to the phytocystatin (a cysteine proteinase inhibitor) family sequences showed approximately 30% identity. Modeling SQAPI onto the structure of oryzacystatin gave an excellent fit; regions identified as proteinase binding loops in cystatin coincided with regions of SQAPI identified as hypervariable, and tryptophan fluorescence changes were also consistent with a cystatin structure. We show that SQAPI exists as a small gene family. Characterization of mRNA and clone walking of genomic DNA (gDNA) produced 10 different but highly homologous SQAPI genes from Cucurbita maxima and the small family size was confirmed by Southern blotting, where evidence for at least five loci was obtained. Using primers designed from squash sequences, PCR of gDNA showed the presence of SQAPI genes in other members of the Cucurbitaceae and in representative members of Coriariaceae, Corynocarpaceae, and Begoniaceae. Thus, at least four of seven families of the order Cucurbitales possess member species with SQAPI genes, covering approximately 99% of the species in this order. A phylogenetic analysis of these Cucurbitales SQAPI genes indicated not only that SQAPI was present in the Cucurbitales ancestor but also that gene duplication has occurred during evolution of the order. Phytocystatins are widespread throughout the plant kingdom, suggesting that SQAPI has evolved recently from a phytocystatin ancestor. This appears to be the first instance of a cystatin being recruited as a proteinase inhibitor of another proteinase family.

Physical and chemical interactions between aphids and plants

Aphids feed from sieve tubes deep inside the host plant. Therefore, aphids must be able to recognize their host plant(s) and to direct their stylets which must be long and thin enough to reach and puncture the sieve tubes at a particular site. Sieve tubes in angiosperms are longitudinal arrays of sieve element/companion cell modules which are highly sensitive to disturbance of any kind. The sieve tubes dispose of elaborate sealing mechanisms such as protein plugging and callose sealing which are triggered by a rise in calcium in the sieve tubes. Aphids seem to have developed a range of physical and chemical measures to limit the amount of calcium influx in response to stylet puncturing. Loss of sieve-element turgor pressure induced by stylet insertion is minimized by the minute stylet volume. Turgor-dependent Ca(2+) influx, possibly mediated by mechano sensitive Ca(2+) channels, must therefore be limited. The components of the sheath and watery saliva play a pivotal role in establishing the physical and chemical constraints on the rise of calcium. Most likely, sheath saliva prevents the influx of calcium from the apoplast by sealing the stylet puncture site while watery saliva may prevent plugging and sealing of sieve plates by potential interaction with SE sap ingredients.

Phloem sap proteins: their identities and potential roles in the interaction between plants and phloem-feeding insects

The phloem is a well-known target of sucking and piercing insects that utilize the transported fluid as their major nutrient source. In addition to small molecules like sugars and amino acids, phloem sap of higher land plants contains proteins that can accumulate up to high concentrations. Although the knowledge about the identities of these phloem sap proteins is increasing, the functions of most of them are still poorly understood. Since many phloem sap proteins have predicted roles in wound and defence responses, they constitute a class of compounds that can potentially influence plant-insect interactions. However, there are as yet no studies published that have examined direct effects of phloem sap proteins on insect feeding or vice versa. This review summarizes the current knowledge about the identities of phloem sap proteins, focused on polypeptides with probable functions in wound and defence reactions, and their potential impact on plant-insect interactions is discussed.

Cucurbit phloem serpins are graft-transmissible and appear to be resistant to turnover in the sieve element-companion cell complex

Serpins are unique inhibitors of serine proteases that are located in various plant tissues and organs. An orthologue of the pumpkin (Cucurbita maxima) phloem serpin CmPS-1 was amplified from cucumber (Cucumis sativus) RNA by RT-PCR, cloned, and designated as CsPS-1 (GenBank accession no. AJ866989). Alternative amino acid sequences in the reactive centre loop suggest distinct inhibitory specificity between CmPS-1 and CsPS-1. A difference in the electrophoretic mobility of these serpins was used in heterografts to establish that serpins are phloem-mobile. Immuno light microscopy revealed that the phloem serpins are localized exclusively to sieve elements (SE), while the phloem filament protein CmPP1, used as a reference, is localized to both SEs and companion cells (CCs). Similar to CmPS-1, CsPS-1 accumulates over time in phloem exudates, indicating that serpins differ from other phloem-mobile proteins whose concentrations appear to be stable in phloem exudates. These differences could reflect alternative mechanisms regulating protein turnover and/or inaccessibility of protein degradation. The functionality of the pore/plasmodesma units connecting SEs and CCs was tested with graft-transmitted CmPP1 as a transport marker. The occurrence of CmPP1 in the CCs of the Cucumis graft partner shows that translocated 88 kDa phloem filament protein monomers can symplasmically exit the SE and accumulate in the CC. By contrast, serial sections probed with the serpin antibody demonstrate that the 43 kDa serpin does not enter CCs. Collectively, these data indicate that CCs play a decisive role in homeostasis of exudate proteins; proteins not accessing the CCs accumulate in SEs and display a time-dependent increase in concentration.

Evolutionary mechanisms acting on proteinase inhibitor variability

The interaction of proteinase inhibitors produced, in most cases, by host organisms and the invasive proteinases of pathogens or parasites or the dietary proteinases of predators, results in an evolutionary 'arms race' of rapid and ongoing change in both interacting proteins. The importance of these interactions in pathogenicity and predation is indicated by the high level and diversity of observable evolutionary activity that has been found. At the initial level of evolutionary change, recruitment of other functional protein-folding families has occurred, with the more recent evolution of one class of proteinase inhibitor from another, using the same mechanism and proteinase contact residues. The combination of different inhibitor domains into a single molecule is also observed. The basis from which variation is possible is shown by the high rate of retention of gene duplication events and by the associated process of inhibitory domain multiplication. At this level of reorganization, mutually exclusive splicing is also observed. Finally, the major mechanism by which variation is achieved rapidly is hypervariation of contact residues, an almost ubiquitous feature of proteinase inhibitors. The diversity of evolutionary mechanisms in a single class of proteins is unlikely to be common, because few systems are under similar pressure to create variation. Proteinase inhibitors are therefore a potential model system in which to study basic evolutionary process such as functional diversification.

Proteomics of curcurbit phloem exudate reveals a network of defence proteins

Many different proteins can be separated from the sap of mature sieve tubes of different plant species. To date, only a limited number of those have been identified and functionally characterised. Due to sieve tubes inability of transcription and translation, the proteins are most probably synthesised in the intimately connected companion cells and transported into the sieve elements through plasmodesmata. The specific protein composition of phloem sap suggests an important role of these proteins not only for sieve tube maintenance, but also for whole plant physiology and development. Here we describe a comprehensive analysis of the phloem protein composition employing one- and high-resolution two-dimensional gel electrophoresis and partial sequencing by mass spectrometry. In this study more than 300 partial sequences generated by hybrid mass spectrometry were used to identify a total of 45 different proteins from the phloem exudates of cucumber (Cucumis sativus L. cv. Hoffmanns Giganta) and pumpkin (Cucurbita maxima Duch. cv. Gelber Zentner) plants. In addition to previously described phloem proteins, it was possible to localise proteins with high similarity to an acyl-CoA binding protein, a glyoxalase, a malate dehydrogenase, a rhodanese-like protein, a drought-induced protein, and a beta-glucosidase. The results indicate that the majority of the so far identified proteins are involved in stress and defence reactions.

Transgenic tobacco and apple plants expressing biotin-binding proteins are resistant to two cosmopolitan insect pests, potato tuber moth and lightbrown apple moth, respectively

Tobacco (Nicotiana tabacum cv. Samsun) and apple (Malus x domestica cv. Royal Gala) plants expressing avidin or strepavidin were produced using Agrobacterium tumefaciens-mediated transformation. ELISA assays showed that avidin expression ranged from 3.1 to 4.6 microM in tobacco and from 1.9 to 11.2 microM in apple and streptavidin expression ranged from 11.4 to 24.5 microM in tobacco and from 0.4 to 14.6 microM in apple. Expressed at these levels, both biotin-binding proteins conferred a high level of insect resistance on transformed tobacco plants to larval potato tuber moth (PTM), Phthorimaea operculella (Zeller) (fam. Gelechiidae) and on apple plants to larvae of the lightbrown apple moth (LBAM) Epiphyas postvittana (Walker) (fam. Tortricidae). More than 90% of PTM larvae died on tobacco plants expressing either avidin or streptavidin genes within 9 days of inoculation. Mortality of LBAM larvae was significantly higher (P < 0.05) on three avidin-expressing (89.6, 84.9 and 80.1%) and two streptavidin-expressing (90 and 82.5%) apple plant lines than on non-transformed control plants (14.1%) after 21 days. Weight of LBAM larvae was also significantly reduced by feeding on all apple shoots expressing avidin and on apple shoots expressing streptavidin at levels of 3.8 microM and above.

Toxicity to the pea aphid Acyrthosiphon pisum of anti-chymotrypsin isoforms and fragments of Bowman-Birk protease inhibitors from pea seeds

Aphids feed on a protein-poor diet and are insensitive to several serine protease inhibitors. However, among the Bowman-Birk family of plant trypsin inhibitors (BBI), some members display significant toxicity to the pea aphid Acyrthosiphon pisum. A BBI isoform purified from pea seeds (PsTI-2) displays an IC50 of 41 microM and a LC50 of 48 microM at 7 days. Our data show that the chymotrypsin-directed active site from these bifunctional inhibitors is responsible for this activity, and that artificial cyclic peptides bearing the Bowman-Birk anti-chymotrypsin head induce much greater toxicity and growth inhibition than their anti-trypsin counterparts. The toxic syndrome included a rapid behavioural response of aphids on diets containing the toxic peptides, with induced restlessness after only 1 h of exposure to the chymotrypsin inhibitor. Nevertheless, chymotrypsin activity was not detected in aphid guts, using two chromogenic chymotrypsin substrates, and the physiological target of the chymotrypsin inhibitor remains unknown. These data show for the first time that plant chymotrypsin inhibitors, still widely unexplored, may act as paradoxical toxicants to aphids and serve as defensive metabolites for phloem-feeding insects.

Putative protein digestion in a sap-sucking homopteran plant pest (rice brown plant hopper; Nilaparvata lugens: Delphacidae)--identification of trypsin-like and cathepsin B-like proteases

Sap-sucking phytophagous insect species of the order Hemiptera have been assumed not to carry out digestive proteolysis, but instead to rely on free amino acids in the phloem and xylem saps for their nutritional requirements. Extracts prepared from isolated guts of rice brown planthopper (Nilaparvata lugens), a homopteran crop pest, were shown to contain protease activity, with hydrolysis of both protein and synthetic peptide substrates being observed. Assays with specific inhibitors suggested that a trypsin-like serine protease was responsible for most of hydrolytic activity against synthetic substrates. A cDNA library was prepared from RNA extracted from N. lugens gut tissue, and screened for protease-encoding sequences. cDNAs for a cathepsin B-like protease and a trypsin-like protease were isolated and fully characterised; the latter exhibits a novel C-terminal region and an unusual activation mechanism, and represents a small gene family. Soya bean Kunitz trypsin inhibitor (SKTI) is an effective inhibitor of protein hydrolysis by N. lugens gut extracts in vitro, explaining why transgenic rice plants expressing this protein are partially resistant to the insect (Mol. Breed. 5 (1999) 1). It is suggested that digestive proteolysis may be widespread in sap-sucking homoptera, and can make a significant contribution to nutrition.

Expression of biotin-binding proteins, avidin and streptavidin, in plant tissues using plant vacuolar targeting sequences

Tobacco plants have been developed which constitutively express high levels of the biotin-binding proteins, avidin and streptavidin. These plants were phenotypically normal and produced fertile pollen and seeds. The transgene was expressed and its product located in the vacuoles of most cell types in the plants. Targeting was achieved by use of N-terminal vacuolar targeting sequences derived from potato proteinase inhibitors which are known to target constitutively to vacuoles in potato tubers and, under wound-induction, in tomato leaves. Avidin was located in protein body-like structures within the vacuole and transgene protein levels remained relatively constant throughout the lifetime of the leaf. We describe two chimeric constructs with similar levels of expression. One comprised a potato proteinase inhibitor I signal peptide cDNA sequence attached to an avidin cDNA and the second a potato proteinase inhibitor II signal peptide genomic sequence (including an intron) attached to a core streptavidin synthetic sequence. We were unable to regenerate plants when transformation used constructs lacking the targeting sequences. The highest levels observed (up to 1.5% of total leaf protein) confirm the vacuole as the organelle of choice for stable storage of plant-toxic transgene products. The efficient targeting of these proteins did not result in any measured changes in plant biotin metabolism.

Avidin expressed in transgenic tobacco leaves confers resistance to two noctuid pests, Helicoverpa armigera and Spodoptera litura

Fertile transgenic tobacco plants with leaves expressing avidin in the vacuole have been produced and shown to halt growth and cause mortality in larvae of two noctuid lepidopterans, Helicoverpa armigera and Spodoptera litura. Late first instar H. armigera larvae and neonate (< 12-h-old) S. litura larvae placed on leaves excised from T0 tobacco expressing avidin at 3.1-4.6 microM (micromoles/kg of fresh leaf tissue) had very poor growth over their first 8 days on the leaves, significant numbers had died by days 11 or 12 and all were dead by day 22 (H. armigera) or day 25 (S. litura). Similar results were obtained when late first instar H. armigera larvae were placed on leaves from T1 plants expressing avidin at six different average concentrations, ranging from 3.7 to 17.3 microM. Two larvae on the lowest expressing leaves survived to pupation, but there was total mortality among the other groups and no relationship between avidin concentration and the effects on the larvae. Synergistic effects between avidin-expressing tobacco plants and a purified Bt toxin, Cry1Ba, were demonstrated. Late instar H. armigera larvae fed with leaves from T2 plants expressing avidin at average concentrations of either <5.3 or > 12.9 microM, and painted with Cry1Ba protein at a rate equivalent to an expression level of 0.5% of total leaf protein, died significantly faster than larvae given either of the two treatments alone. Larvae fed with avidin-expressing leaves painted with the protease inhibitor, aprotinin, at a rate equivalent to 1% of total leaf protein had mortality similar to those given avidin-leaves alone. There was no evidence of antagonism between these two proteins.

Serine proteinase inhibitors in the Compositae: distribution, polymorphism and properties

Multiple molecular forms of inhibitors of trypsin (TI) and chymotrypsin (CI), which are typical digestive enzymes of insects, mammals and micro-organisms, and subtilisin (SI), a proteinase of many bacteria and phytopathogenic fungi, were identified in seeds and vegetative organs of the majority of 128 wild and cultivated species representing 65 genera of three of the subfamilies of the Compositae. Inhibitors with M(r) ranging from 7450 to 7800 and combining activities towards subtilisin and trypsin and/or chymotrypsin (T/C/SI) had the widest distribution and may be involved in plant defense mechanisms. They were found in many species of the subfamilies Carduoideae (genera Carthamus, Centaurea, Cirsium), Cichorioideae (Lactuca, Taraxacum) and Asteroideae (Helianthus, Cosmos, Bidens). Partial amino acid sequencing showed that the safflower (Carthamus tinctorius) T/C/SI and Cosmos bipinnatus T/C/SI, T/SI and C/SI belonged to the potato I inhibitor family. The most active, variable and heterogeneous inhibitors were found in species of the tribe Heliantheae, which is placed in the evolutionary advanced subfamily Asteroideae. Seeds of Helianthus species, Eclipta prostrata, Gailardia aristata, Zinnia elegans and Silphium perfoliatum contained various TI with M(r) ranging from 1500 to 14,750, with some also containing SI. H. annuus seeds contain a unique cyclic TI of M(r) 1514 and similar TI were also present in other Helianthus spp. and the related species Tithonia diversifolia. Zinnia elegans contained a TI with M(r) 11,350 which appeared to represent a novel type of inhibitor distantly related to the cereal subgroup of Bowman-Birk inhibitors. TI and T/SI varied widely in H. annuus lines and wild Helianthus species in their presence or absence and composition. Similar T/SI components were found in the cultivated diploid H. annuus and annual diploid species with the B genome but not in perennials with the A genome. Some T/SI, SI and TI were detected in vegetative organs of sunflower and other Compositae. Studies of the polymorphism and distribution of proteinase inhibitors are relevant to the evolution of protective protein systems and the mechanisms of resistance to pathogenic organisms in the Compositae and other plants.

Primary phloem-specific expression of a Zinnia elegans homeobox gene

Some plant homeobox genes are expressed specifically in vascular cells and are assumed to function in the differentiation of specific types of vascular cells. However, homeobox genes exhibiting primary phloem-specific expression have not been reported. To elucidate the molecular mechanisms of vascular development, we undertook to isolate from Zinnia elegans primary phloem-specific homeobox genes that may function in phloem development. An HD-Zip type homeobox gene, ZeHB3, was isolated. This gene encodes a class I HD-Zip protein, and constitutes a gene subfamily with the Daucus carota gene CHB6, and Arabidopsis thaliana genes Athb-5, Athb-6, and Athb-16. In situ hybridization of 1-, 14- and 50-day-old plants demonstrated that ZeHB3 mRNA accumulation is restricted to a few cells destined to differentiate into phloem cells and to the immature phloem cells surrounding the sieve elements and companion cells. ZeHB3 protein was also localized to immature phloem cells. These findings clearly indicate that ZeHB3 is a novel homeobox gene that marks, and may function in, the early stages of phloem differentiation.

Isolation and amino acid sequence of a serine proteinase inhibitor from common flax (Linum usitatissimum) seeds

LUTI (Linum usitatissimum trypsin inhibitor), a member of the potato inhibitor I family, has been isolated from seeds of flax by ethanol fractionation, ion exchange chromatography on CM-Sephadex C-25, affinity purification on immobilized methylchymotrypsin (alpha-chymotrypsin in which His57 has been converted to 3-methylhistidine) in the presence of 5M NaCl, and finally by reversed-phase HPLC. The 7655 Da inhibitor consists of a single polypeptide chain of 69 residues with one disulfide bridge. The molecule is acetylated at the N terminus. Its primary structure has been determined after limited proteolysis of the native molecule with trypsin at the reactive site, cleavage with cyanogen bromide or arginyl endopeptidase (Arg-gingipain), and alcoholytic deacetylation of the N-terminally blocked serine. The association constants (K(a)) of LUTI with bovine beta-trypsin and alpha-chymotrypsin are 3.58x10(10) M(-1) and 5.02x10(5) M(-1), respectively. High NaCl concentration (3M) increased the association constant of LUTI with alpha-chymotrypsin to 6.64x10(7) M(-1). To our knowledge, LUTI is the first serine-proteinase-type inhibitor isolated from a plant of the Linaceae family.

Characterization of cucurbita maxima phloem serpin-1 (CmPS-1). A developmentally regulated elastase inhibitor

We report on the molecular, biochemical, and functional characterization of Cucurbita maxima phloem serpin-1 (CmPS-1), a novel 42-kDa serine proteinase inhibitor that is developmentally regulated and has anti-elastase properties. CmPS-1 was purified to near homogeneity from C. maxima (pumpkin) phloem exudate and, based on microsequence analysis, the cDNA encoding CmPS-1 was cloned. The association rate constant (k(a)) of phloem-purified and recombinant His(6)-tagged CmPS-1 for elastase was 3.5 +/- 1.6 x 10(5) and 2.7 +/- 0.4 x 10(5) m(-)(1) s(-)(1), respectively. The fraction of complex-forming CmPS-1, X(inh), was estimated at 79%. CmPS-1 displayed no detectable inhibitory properties against chymotrypsin, trypsin, or thrombin. The elastase cleavage sites within the reactive center loop of CmPS-1 were determined to be Val(347)-Gly(348) and Val(350)-Ser(351) with a 3:2 molar ratio. In vivo feeding assays conducted with the piercing-sucking aphid, Myzus persicae, established a close correlation between the developmentally regulated increase in CmPS-1 within the phloem sap and the reduced ability of these insects to survive and reproduce on C. maxima. However, in vitro feeding experiments, using purified phloem CmPS-1, failed to demonstrate a direct effect on aphid survival. Likely roles of this novel phloem serpin in defense against insects/pathogens are discussed.

Determination of a high precision structure of a novel protein, Linum usitatissimum trypsin inhibitor (LUTI), using computer-aided assignment of NOESY cross-peaks

The solution structure of a novel 69 residue proteinase inhibitor, Linum usitatissimum trypsin inhibitor (LUTI), was determined using a method based on computer aided assignment of nuclear Overhauser enhancement spectroscopy (NOESY) data. The approach applied uses the program NOAH/DYANA for automatic assignment of NOESY cross-peaks. Calculations were carried out using two unassigned NOESY peak lists and a set of determined dihedral angle restraints. In addition, hydrogen bonds involving amide protons were identified during calculations using geometrical criteria and values of HN temperature coefficients. Stereospecific assignment of beta-methylene protons was carried out using a standard procedure based on nuclear Overhauser enhancement intensities and 3J(alpha)(beta) coupling constants. Further stereospecific assignment of methylene protons and diastereotopic methyl groups were established upon structure-based method available in the program GLOMSA and chemical shift calculations. The applied algorithm allowed us to assign 1968 out of 2164 peaks (91%) derived from NOESY spectra recorded in H2O and 2H2O. The final experimental data input consisted of 1609 interproton distance restraints, 88 restraints for 44 hydrogen bonds, 63 torsion angle restraints and 32 stereospecifically assigned methylene proton pairs and methyl groups. The algorithm allowed the calculation of a high precision protein structure without the laborious manual assignment of NOESY cross-peaks. For the 20 best conformers selected out of 40 refined ones in the program CNS, the calculated average pairwise rmsd values for residues 3 to 69 were 0.38 A (backbone atoms) and 1.02 A (all heavy atoms). The three-dimensional LUTI structure consists of a mixed parallel and antiparallel beta-sheet, a single alpha-helix and shows the fold of the potato 1 family of proteinase inhibitors. Compared to known structures of the family, LUTI contains Arg and Trp residues at positions P6' and P8', respectively, instead of two Arg residues, involved in the proteinase binding loop stabilization. A consequence of the ArgTrp substitution at P8' is a slightly more compact conformation of the loop relative to the protein core.

Macromolecular trafficking in the phloem

Prior to the early 1990s, the functional aspects of long-distance movement in the phloem were viewed primarily in terms of the transport of sugars and other photoassimilates. The soluble proteins in phloem exudates were often considered to be artifacts arising from the cytoplasmic degeneration of the conducting elements or a confounding anomaly of the sugar transport system. Recent work indicates a much more complex system where proteins and other macromolecules play significant roles in phloem function and long-distance signaling throughout the plant.

Diet, reflux and the development of squamous cell carcinoma of the oesophagus in Africa

BACKGROUND

Squamous cancer of the oesophagus has reached epidemic proportions in Africa in the past few decades. There are many known associations but as yet no well established theory of causation.

METHODS

Relevant literature was found by manual review of appropriate journals and literature, Medline searches and cross-referencing. Published theories of causation of endemic cancer of the oesophagus were assessed in the light of available evidence. A hypothesis based on that evidence was formulated.

RESULTS AND CONCLUSION

A predominantly maize-based diet is high in linoleic acid, a precursor for gastric prostaglandin synthesis. In combination with low intake of other fatty acids and riboflavin, high levels of prostaglandin E2 are produced in gastric mucosa, leading to reduced gastric acid secretion, relaxation of the pylorus and a reduction in lower oesophageal sphincter pressure. These events result in combined reflux of duodenal and gastric juices low in acidity into the oesophagus. Resulting dysplasia strongly predisposes to local squamous carcinogenesis. This is now a fertile area for research, which may open the way for preventive action.

A gene encoding a major Kunitz proteinase inhibitor of storage organs of winged bean is also expressed in the phloem of stems

Winged bean Kunitz chymotrypsin inhibitor (WCI) accumulates abundantly in seeds and tuberous roots, and small amounts of the WCI protein and mRNA can also be detected in stems. In this study, we analyzed the localization of the WCI protein in stems of winged bean. The results demonstrated that the WCI protein was localized in sieve tubes. Furthermore, we showed that the 5' region of the WCI-3b gene, which exhibited strong transcriptional activity in developing seeds, also promoted transcription of a reporter gene in the phloem of stems of transgenic tobacco.