A gene encoding a vicilin-like protein is specifically expressed in fern spores. Evolutionary pathway of seed storage globulins

The B3 domain-containing transcription factor ZmABI19 coordinates expression of key factors required for maize seed development and grain filling

Grain filling in maize (Zea mays) is regulated by a group of spatiotemporally synchronized transcription factors (TFs), but the factors that coordinate their expression remain unknown. We used the promoter of the grain filling-specific TF gene Opaque2 (O2) to screen upstream regulatory factors and identified a B3 domain TF, ZmABI19, that directly binds to the O2 promoter for transactivation. zmabi19 mutants displayed developmental defects in the endosperm and embryo, and mature kernels were opaque and reduced in size. The accumulation of zeins, starch and lipids dramatically decreased in zmabi19 mutants. RNA sequencing revealed an alteration of the nutrient reservoir activity and starch and sucrose metabolism in zmabi19 endosperms, and plant phytohormone signal transduction and lipid metabolism in zmabi19 embryos. Chromatin immunoprecipitation followed by sequencing coupled with differential expression analysis identified 106 high-confidence direct ZmABI19 targets. ZmABI19 directly regulates multiple key grain filling TFs including O2, Prolamine-box binding factor 1, ZmbZIP22, NAC130, and Opaque11 in the endosperm and Viviparous1 in the embryo. A number of phytohormone-related genes were also bound and regulated by ZmABI19. Our results demonstrate that ZmABI19 functions as a grain filling initiation regulator. ZmABI19 roles in coupling early endosperm and embryo development are also discussed.

Cloning, expression and immunological characterisation of Coc n 1, the first major allergen from Coconut pollen

Coconut pollen has been documented to be a major contributor to the aeroallergen load in India, causing respiratory allergy in a large cohort of susceptible individuals. Here, we report the identification of the first major allergen from Coconut pollen, Coc n 1. The full-length sequence of the allergen was determined from previously identified peptides and overexpressed in E. coli. Recombinant Coc n 1 folded into a trimer and was found to possess allergenicity equivalent to its natural counterpart. Proteolytic processing of Coc n 1 led to the formation of an immunodominant ∼20 kDa C-terminal subunit and the site of cleavage was determined by amino acid microsequencing. Five linear IgE binding epitopes were predicted and mapped on the homology modelled structure of Coc n 1. Amongst three immunodominant epitopes, two were present towards the C-terminal end. Coc n 1 was found to belong to the highly diverse cupin superfamily and mimics its structure with known 7S globulin or vicilin allergens but lacks sequence similarity. Using sequence similarity networks, Coc n 1 clustered as a separate group containing unannotated cupin domain proteins and did not include known vicilin allergens except Gly m Bd 28 kDa, a Soybean major allergen. 7S globulins are major storage proteins and food allergens, but presence of such protein in pollen grains is reported for the first time. Further study on Coc n 1 may provide insights into its function in pollen grains and also in the development of immunotherapy to Coconut pollen allergy.

Where do Protein Bodies of Cereal Seeds Come From?

Protein bodies of cereal seeds consist of ordered, largely insoluble heteropolymers formed by prolamin storage proteins within the endoplasmic reticulum (ER) of developing endosperm cells. Often these structures are permanently unable to traffic along the secretory pathway, thus representing a unique example for the use of the ER as a protein storage compartment. In recent years, marked progress has been made in understanding what is needed to make a protein body and in formulating hypotheses on how protein body formation might have evolved as an efficient mechanism to store large amounts of protein during seed development, as opposed to the much more common system of seed storage protein accumulation in vacuoles. The major key evolutionary events that have generated prolamins appear to have been insertions or deletions that have disrupted the conformation of the eight-cysteine motif, a protein folding motif common to many proteins with different functions and locations along the secretory pathway, and, alternatively, the fusion between the eight-cysteine motif and domains containing additional cysteine residues.

Seed Storage Globulins: Origin and Evolution of Primary and Higher Order Structures

Legumin and vicilin are two-domain seed storage globulins similar in primary and higher order structures of their domains to single-domain plant germins as well as to the domains of two-domain and single-domain bacterial oxalate decarboxylases. Independent evolutionary pathways have been shown for the descent of the storage globulins and germins from two-domain and single-domain bacterial oxalate decarboxylases, respectively. As compared to vicilins, the primary and tertiary structures of legumins were found to most closely reflect the ancient features characteristic of a common precursor of storage globulins. During the evolution of the storage globulins, a mechanism specifically controlling their degradation has been formed. We found that limited proteolysis of soybean legumin and kidney bean vicilin in germinating seeds and in vitro leads to their regular changes, which initiate an extensive cleavage of storage globulin molecules by the one-by-one mechanism. As also shown, limited proteolysis of soybean legumin loosens the intersubunit interactions in its oligomeric molecule. Based on these data, we hypothesize that the deep one-by-one degradation of soybean legumin is triggered by its dissociation, which bares peptide bonds potentially susceptible to proteolytic attack but are masked in the oligomer.

Cytological and Proteomic Analyses of Osmunda cinnamomea Germinating Spores Reveal Characteristics of Fern Spore Germination and Rhizoid Tip Growth

Fern spore is a good single-cell model for studying the sophisticated molecular networks in asymmetric cell division, differentiation, and polar growth. Osmunda cinnamomea L. var. asiatica is one of the oldest fern species with typical separate-growing trophophyll and sporophyll. The chlorophyllous spores generated from sporophyll can germinate without dormancy. In this study, the spore ultrastructure, antioxidant enzyme activities, as well as protein and gene expression patterns were analyzed in the course of spore germination at five typical stages (i.e. mature spores, rehydrated spores, double-celled spores, germinated spores, and spores with protonemal cells). Proteomic analysis revealed 113 differentially expressed proteins, which were mainly involved in photosynthesis, reserve mobilization, energy supplying, protein synthesis and turnover, reactive oxygen species scavenging, signaling, and cell structure modulation. The presence of multiple proteoforms of 25 differentially expressed proteins implies that post-translational modification may play important roles in spore germination. The dynamic patterns of proteins and their encoding genes exhibited specific characteristics in the processes of cell division and rhizoid tip growth, which include heterotrophic and autotrophic metabolisms, de novo protein synthesis and active protein turnover, reactive oxygen species and hormone (brassinosteroid and ethylene) signaling, and vesicle trafficking and cytoskeleton dynamic. In addition, the function skew of proteins in fern spores highlights the unique and common mechanisms when compared with evolutionarily divergent spermatophyte pollen. These findings provide an improved understanding of the typical single-celled asymmetric division and polar growth during fern spore germination.

Variant vicilins from a resistant Vigna unguiculata lineage (IT81D-1053) accumulate inside Callosobruchus maculatus larval midgut epithelium

It has been demonstrated that variant vicilins are the main resistance factor of cowpea seeds (Vigna unguiculata) against attack by the cowpea beetle Callosobruchus maculatus. There is evidence that the toxic properties of these storage proteins may be related to their interaction with glycoproteins and other microvillar membrane constituents along the digestive tract of the larvae. New findings have shown that following interaction with the microvilli, the vicilins are absorbed across the intestinal epithelium and thus reach the internal environment of the larvae. In the present paper we studied the insecticidal activity of the variant vicilins purified from a resistant cowpea variety (IT81D-1053). Bioassays showed that the seeds of this genotype affected larval growth, causing developmental retardation and 100% mortality. By feeding C. maculatus larvae on susceptible and IT81D-1053 derived vicilins (FITC labelled or unlabelled), followed by fluorescence and immunogold cytolocalization, we were able to demonstrate that both susceptible and variant forms are internalized in the midgut cells and migrate inside vesicular structures from the apex to the basal portion of the enterocytes. However, when larvae were fed with the labelled vicilins for 24h and then returned to a control diet, the concentration of the variant form remained relatively high, suggesting that variant vicilins are not removed from the cells at the same rate as the non-variant vicilins. We suggest that the toxic effects of variant vicilins on midgut cells involve the binding of these proteins to the cell surface followed by internalization and interference with the normal physiology of the enterocytes, thereby affecting larval development in vivo.

Identification and analysis of the germin-like gene family in soybean

BACKGROUND

Germin and germin-like proteins constitute a ubiquitous family of plant proteins. A role of some family members in defense against pathogen attack had been proposed based on gene regulation studies and transgenic approaches. Soybean (G. max L. Merr.) germin genes had not been characterized at the molecular and functional levels.

RESULTS

In the present study, twenty-one germin gene members in soybean cultivar 'Maple Arrow' (partial resistance to Sclerotinia stem rot of soybean) were identified by in silico identification and RACE method (GmGER 1 to GmGER 21). A genome-wide analyses of these germin-like protein genes using a bioinformatics approach showed that the genes located on chromosomes 8, 1, 15, 20, 16, 19, 7, 3 and 10, on which more disease-resistant genes were located on. Sequence comparison revealed that the genes encoded three germin-like domains. The phylogenetic relationships and functional diversity of the germin gene family of soybean were analyzed among diverse genera. The expression of the GmGER genes treated with exogenous IAA suggested that GmGER genes might be regulated by auxin. Transgenic tobacco that expressed the GmGER 15 [corrected] gene exhibited high tolerance to the salt stress. In addition, the GmGER mRNA increased transiently at darkness and peaked at a time that corresponded approximately to the critical night length. The mRNA did not accumulate significantly under the constant light condition, and did not change greatly under the SD and LD treatments.

CONCLUSIONS

This study provides a complex overview of the GmGER genes in soybean. Phylogenetic analysis suggested that the germin and germin-like genes of the plant species that had been founded might be evolved by independent gene duplication events. The experiment indicated that germin genes exhibited diverse expression patterns during soybean development. The different time courses of the mRNAs accumulation of GmGER genes in soybean leaves appeared to have a regular photoperiodic reaction in darkness. Also the GmGER genes were proved to response to abiotic stress (such as auxin and salt), suggesting that these paralogous genes were likely involved in complex biological processes in soybean.

The fate of vicilins, 7S storage globulins, in larvae and adult Callosobruchus maculatus (Coleoptera: Chrysomelidae: Bruchinae)

The fate of vicilins ingested by Callosobruchus maculatus and the physiological importance of these proteins in larvae and adults were investigated. Vicilins were quantified by ELISA in the haemolymph and fat body during larval development (2nd to 4th instars), in pupae and adults, as well as in ovaries and eggs. Western blot analysis demonstrated that the majority of absorbed vicilins were degraded in the fat body. Tracing the fate of vicilins using FITC revealed that the FITC-vicilin complex was present inside cells of the fat body of the larvae and in the fat bodies of both male and female adult C. maculatus. Labelled vicilin was also detected in ovocytes and eggs. Based on the results presented here, we propose that following absorption, vicilins accumulate in the fat body, where they are partially degraded. These peptides are retained throughout the development of the insects and eventually are sequestered by the eggs. It is possible that accumulation in the eggs is a defensive strategy against pathogen attack as these peptides are known to have antimicrobial activity. Quantifications performed on internal organs from larvae of C. maculatus exposed to extremely dry seeds demonstrated that the vicilin concentration in the haemolymph and fat body was significantly higher when compared to larvae fed on control seeds. These results suggest that absorbed vicilins may also be involved in the survival of larvae in dry environments.

An engineered C-terminal disulfide bond can partially replace the phaseolin vacuolar sorting signal

Seed storage proteins accumulate either in the endoplasmic reticulum (ER) or in vacuoles, and it would appear that polymerization events play a fundamental role in regulating the choice between the two destinies of these proteins. We previously showed that a fusion between the Phaseolus vulgaris vacuolar storage protein phaseolin and the N-terminal half of the Zea mays prolamin gamma-zein forms interchain disulfide bonds that facilitate the formation of ER-located protein bodies. Wild-type phaseolin does not contain cysteine residues, and assembles into soluble trimers that transiently polymerize before sorting to the vacuole. These transient interactions are abolished when the C-terminal vacuolar sorting signal AFVY is deleted, indicating that they play a role in vacuolar sorting. We reasoned that if the phaseolin interactions directly involve the C terminus of the polypeptide, a cysteine residue introduced into this region could stabilize these transient interactions. Biochemical studies of two mutated phaseolin proteins in which a single cysteine residue was inserted at the C terminus, in the presence (PHSL*) or absence (Delta 418*) of the vacuolar signal AFVY, revealed that these mutated proteins form disulphide bonds. PHSL* had reduced protein solubility and a vacuolar trafficking delay with respect to wild-type protein. Moreover, Delta 418* was in part redirected to the vacuole. Our experiments strongly support the idea that vacuolar delivery of phaseolin is promoted very early in the sorting process, when polypeptides are still contained within the ER, by homotypic interactions.

Phylogenetic footprints in fern spore- and seed-specific gene promoters

Spermatophyte seed-storage proteins have descended from a group of proteins involved in cellular desiccation/hydration processes. Conserved protein structures are found across all plant phyla and in the fungi and Archaea. We investigated whether conservation in the coding region sequence is paralleled by common gene regulatory processes. Seed- and spore-specific gene promoters of three phylogenetically diverse plants were analysed by transient and transgenic expression in Arabidopsis thaliana and tobacco. The transcription factors FUS3 and ABI3, which are central regulators of seed maturation processes, interact with cis-motifs of seed-specific promoters from distantly related plants. The promoter of a fern spore-specific gene encoding a seed-storage globulin-like protein exhibits strong seed-specific activity in both Arabidopsis and tobacco. The existence of phylogenetic footprints indicates good conservation of regulatory pathways controlling gene expression in fern spores and in gymnosperm and angiosperm seeds, reflecting the concerted evolution of coding and regulatory regions.

The multigene family encoding germin-like proteins of barley. Regulation and function in Basal host resistance

Germin-like proteins (GLPs) have been shown to be encoded by multigene families in several plant species and a role of some subfamily members in defense against pathogen attack has been proposed based on gene regulation studies and transgenic approaches. We studied the function of six GLP subfamilies of barley (Hordeum vulgare) by selecting single mRNAs for gene expression studies as well as overexpression and gene-silencing experiments in barley and Arabidopsis (Arabidopsis thaliana). Expression of all six subfamilies was high in very young seedlings, including roots. The expression pattern gradually changed from developmental to conditional with increasing plant age, whereby pathogen attack and exogenous hydrogen peroxide application were found to be the strongest signals for induction of several GLP subfamilies. Transcripts of four of five GLP subfamilies that are expressed in shoots were predominantly accumulating in the leaf epidermis. Transient overexpression of HvGER4 or HvGER5 as well as transient silencing by RNA interference of HvGER3 or HvGER5 protected barley epidermal cells from attack by the appropriate powdery mildew fungus Blumeria graminis f. sp. hordei. Silencing of HvGER4 induced hypersusceptibility. Transient and stable expression of subfamily members revealed HvGER5 as a new extracellular superoxide dismutase, and protection by overexpression could be demonstrated to be dependent on superoxide dismutase activity of the encoded protein. Data suggest a complex interplay of HvGER proteins in fine regulation of basal resistance against B. graminis.

C-Terminal 23 kDa polypeptide of soybean Gly m Bd 28 K is a potential allergen

Gly m Bd 28 K is a major soybean (Glycine max Merr.) glycoprotein allergen. It was originally identified as a 28 kDa polypeptide in soybean seed flour. However, the full-length protein is encoded by an open reading frame (ORF) of 473 amino acids, and contains a 23 kDa C-terminal polypeptide of as yet unknown allergenic and structural characteristics. IgE-binding (allergenic potential) of the Gly m Bd 28 K protein including the 23 kDa C-terminal portion as well as shorter fragments derived from the full-length ORF were evaluated using sera from soy-sensitive adults. All of these sera contained IgE that efficiently recognized the C-terminal region. Epitope mapping demonstrated that a dominant linear C-terminal IgE binding epitope resides between residues S256 and A270. Alanine scanning of this dominant epitope indicated that five amino acids, Y260, D261, D262, K264 and D266, contribute most towards IgE-binding. A model based on the structure of the beta subunit of soybean beta-conglycinin revealed that Gly m Bd 28 K contains two cupin domains. The dominant epitope is on the edge of the first beta-sheet of the C-terminal cupin domain and is present on a potentially solvent-accessible loop connecting the two cupin domains. Thus, the C-terminal 23 kDa polypeptide of Gly m Bd 28 K present in soy products is allergenic and apparently contains at least one immunodominant epitope near the edge of a cupin domain. This knowledge could be helpful in the future breeding of hypoallergenic soybeans.

Vicilin-like storage globulin from buckwheat (Fagopyrum esculentum Moench) seeds

An 8S storage globulin from buckwheat seed, which resembles the structure and features common to the vicilin-like family of seed storage proteins, was analyzed for this paper. It was found that expression of the 8S globulin gene precedes that of the 13S globulin (the main buckwheat storage protein) and starts from an early stage of buckwheat seed development (9-11 days after flowering), continuing to accumulate throughout seed development to contribute approximately 7% of total seed proteins. This protein fraction might be more interesting for biotechnological application than the 13S buckwheat legumin consisting of 23-25 kDa subunits reported to be the major buckwheat allergen. A partial cDNA was also isolated, showing high homology with cDNAs coding for vicilin-like storage proteins from various plant species, and its expression profile throughout seed development as well as in different buckwheat tissues was analyzed.

Legumin- and vicilin-like proteins from spores of the fern Matteuccia struthiopteris

Legumin- and vicilin-like proteins have been isolated from spores of the fern Matteuccia struthiopteris. Their relationship with seed legumin and vicilin was demonstrated by cross-reactivities of antibodies directed against respective storage globulins from Vicia faba as evidenced by Western blotting. The Matteuccia legumin-like protein was characterised as a 300-340 kDa holoprotein preferentially consisting of a 32 kDa alpha-chain and a 24 kDa beta-chain. Patterns of limited proteolysis of the spore legumin-like protein and seed legumins were similar as well. In contrast to seed legumins, the Matteuccia legumin-like protein is devoid of disulfide bridges between alpha- and beta-chains. A 52 kDa polypeptide of the Matteuccia vicilin-like protein, first detected by SDS gel electrophoresis, is probably encoded by a vicilin-like gene specifically expressed in Matteuccia struthiopteris spores (Shutov et al. 1998). The vicilin-like holoprotein was found to form a complex of 600 kDa apparent molecular mass, presumably composed of four vicilin-like trimers.

Plant lectins: occurrence, biochemistry, functions and applications

Growing insights into the many roles of glycoconjugates in biorecognition as ligands for lectins indicates a need to compare plant and animal lectins. Furthermore, the popularity of plant lectins as laboratory tools for glycan detection and characterization is an incentive to start this review with a brief introduction to landmarks in the history of lectinology. Based on carbohydrate recognition by lectins, initially described for concanavalin A in 1936, the chemical nature of the ABH-blood group system was unraveled, which was a key factor in introducing the term lectin in 1954. How these versatile probes are produced in plants and how they are swiftly and efficiently purified are outlined, and insights into the diversity of plant lectin structures are also given. The current status of understanding their functions calls for dividing them into external activities, such as harmful effects on aggressors, and internal roles, for example in the transport and assembly of appropriate ligands, or in the targeting of enzymatic activities. As stated above, attention is given to intriguing parallels in structural/functional aspects of plant and animal lectins as well as to explaining caveats and concerns regarding their application in crop protection or in tumor therapy by immunomodulation. Integrating the research from these two lectin superfamilies, the concepts are discussed on the role of information-bearing glycan epitopes and functional consequences of lectin binding as translation of the sugar code (functional glycomics).

Crystal structures of recombinant and native soybean beta-conglycinin beta homotrimers

The crystal structures of recombinant and native beta homotrimers of soybean beta-conglycinin were determined by X-ray crystallography at 2.7 and 2.8 A resolutions, respectively. The crystals of the recombinant and native beta homotrimers belong to space group P21 with cell parameters a = 80.51 A, b = 63.48 A, c = 131.43 A, and beta = 90.01 degrees and with cell parameters a = 82.78 A, b = 69.47 A, c = 125.33 A and beta = 97.22 degrees, respectively. The beta monomers consist of amino-terminal and carboxyl-terminal modules that are very similar to each other and are related by a pseudo-dyad axis. Each module of the beta monomer is subdivided into a core and a loop domain. These structural features of both beta homotrimers are consistent with those of canavalin and phaseolin, which are similar vicilin class proteins. The superposition of the models of the native and recombinant beta monomers shows a root mean square deviation of 0.43-0.51 A for 343 common Calpha atoms within 2.0 A. This result indicates that the N-linked glycans do not influence the final structure of the beta homotrimer. Comparison of the models of beta-conglycinin, phaseolin and canavalin indicates that beta-conglycinin resembles canavalin rather than phaseolin, and that canavalin and phaseolin differ the most among them. The evolutional relationships are discussed.

A pea nuclear protein that is induced by dehydration belongs to the vicilin superfamily

The purification to homogeneity of p16, a protein with an electrophoretic mobility compatible with an apparent molecular mass of 16 kDa, from nuclei of ungerminated pea embryonic axes is described. A cDNA clone of its gene, which was designated psp54, was also isolated. The psp54 cDNA contains an open reading frame coding for a 54.4-kDa polypeptide (p54). p16 corresponds to the C-terminal third of p54, although the mechanisms by which the primary polypeptide could be processed are not yet known. The sequence of p54 is 60% identical with that of the precursor of a sucrose-binding soybean protein, and, to a lesser extent (31-34%), it shares homology with some storage proteins. p16 is also 30% homologous with Nhp2p, a yeast nuclear protein. The psp54 gene, present in a single copy in pea genome, starts being expressed during seed desiccation. Soon after rehydration in seed germination, p54 mRNA disappears and is no longer detectable in vegetative tissues, except in response to hydric stress (exposure to abscisic acid, osmolites or desiccation). p16 can be recovered from nuclei cross-linked to histone H3, when the disulfide bridges that occur in vivo are preserved. On the other hand, p16 shares some properties with dehydrins, which are thought to protect cellular structures against desiccation. We propose that the possible precursor polypeptide p54 belongs to the vicilin superfamily, members of which play a variety of roles. The function of p16 may be related to the protection of chromatin structure against desiccation during seed development.

Microbial relatives of the seed storage proteins of higher plants: conservation of structure and diversification of function during evolution of the cupin superfamily

This review summarizes the recent discovery of the cupin superfamily (from the Latin term "cupa," a small barrel) of functionally diverse proteins that initially were limited to several higher plant proteins such as seed storage proteins, germin (an oxalate oxidase), germin-like proteins, and auxin-binding protein. Knowledge of the three-dimensional structure of two vicilins, seed proteins with a characteristic beta-barrel core, led to the identification of a small number of conserved residues and thence to the discovery of several microbial proteins which share these key amino acids. In particular, there is a highly conserved pattern of two histidine-containing motifs with a varied intermotif spacing. This cupin signature is found as a central component of many microbial proteins including certain types of phosphomannose isomerase, polyketide synthase, epimerase, and dioxygenase. In addition, the signature has been identified within the N-terminal effector domain in a subgroup of bacterial AraC transcription factors. As well as these single-domain cupins, this survey has identified other classes of two-domain bicupins including bacterial gentisate 1, 2-dioxygenases and 1-hydroxy-2-naphthoate dioxygenases, fungal oxalate decarboxylases, and legume sucrose-binding proteins. Cupin evolution is discussed from the perspective of the structure-function relationships, using data from the genomes of several prokaryotes, especially Bacillus subtilis. Many of these functions involve aspects of sugar metabolism and cell wall synthesis and are concerned with responses to abiotic stress such as heat, desiccation, or starvation. Particular emphasis is also given to the oxalate-degrading enzymes from microbes, their biological significance, and their value in a range of medical and other applications.

GAMETOPHYTE DEVELOPMENT IN FERNS

▪ Abstract  The fern gametophyte has interested plant biologists for the past century because its structure and development is simple and amenable to investigation. Past studies have described many aspects of its development, including germination of the spore, patterns of cell division and differentiation, photomorphogenic or light-regulated responses, sex determination and differentiation of gametangia, hormone and pheromone responses, and fertilization. Several genes that are predicted to regulate some of these processes have been recently cloned, making it possible to analyze how these processes are controlled at a molecular level. The emergence of the fern Ceratopteris richardii as a model organism for readily identifying and characterizing mutations that affect key developmental processes in gametophytes makes it a powerful tool for dissecting the molecular mechanisms underlying these processes. If advances in gene cloning techniques and transformation are forthcoming in Ceratopteris, it is likely that the study of developmental processes in ferns will significantly contribute to our understanding of plant development and evolution beyond that which can be learned solely from studying angiosperms.

Barley oxalate oxidase is a hexameric protein related to seed storage proteins: evidence from X-ray crystallography

The oxalate oxidase enzyme expressed in barley roots is a thermostable, protease-resistant enzyme that generates H2O2. It has great medical importance because of its use to assay plasma and urinary oxalate, and it has also been used to generate transgenic, pathogen-resistant crops. This protein has now been purified and three types of crystals grown. X-ray analysis shows that the symmetry present in these crystals is consistent with a hexameric arrangement of subunits, probably a trimer of dimers. This structure may be similar to that found in the related seed storage proteins.