The structure of a haemopexin-fold protein from cow pea (Vigna unguiculata) suggests functional diversity of haemopexins in plants

Seed Storage Protein, Functional Diversity and Association with Allergy

Plants are essential for humans as they serve as a source of food, fuel, medicine, oils, and more. The major elements that are utilized for our needs exist in storage organs, such as seeds. These seeds are rich in proteins, show a broad spectrum of physiological roles, and are classified based on their sequence, structure, and conserved motifs. With the improvements to our knowledge of the basic sequence and our structural understanding, we have acquired better insights into seed proteins and their role. However, we still lack a systematic analysis towards understanding the functional diversity associated within each family and their associations with allergy. This review puts together the information about seed proteins, their classification, and diverse functional roles along with their associations with allergy.

PsEND1 Is a Key Player in Pea Pollen Development Through the Modulation of Redox Homeostasis

Redox homeostasis has been linked to proper anther and pollen development. Accordingly, plant cells have developed several Reactive Oxygen Species (ROS)-scavenging mechanisms to maintain the redox balance. Hemopexins constitute one of these mechanisms preventing heme-associated oxidative stress in animals, fungi, and plants. Pisum sativum ENDOTHECIUM 1 (PsEND1) is a pea anther-specific gene that encodes a protein containing four hemopexin domains. We report the functional characterization of PsEND1 and the identification in its promoter region of cis-regulatory elements that are essential for the specific expression in anthers. PsEND1 promoter deletion analysis revealed that a putative CArG-like regulatory motif is necessary to confer promoter activity in developing anthers. Our data suggest that PsEND1 might be a hemopexin regulated by a MADS-box protein. PsEND1 gene silencing in pea, and its overexpression in heterologous systems, result in similar defects in the anthers consisting of precocious tapetum degradation and the impairment of pollen development. Such alterations were associated to the production of superoxide anion and altered activity of ROS-scavenging enzymes. Our findings demonstrate that PsEND1 is essential for pollen development by modulating ROS levels during the differentiation of the anther tissues surrounding the microsporocytes.

High resolution structural and functional analysis of a hemopexin motif protein from Dolichos

It is increasingly evident that seed proteins exhibit specific functions in plant physiology. However, many proteins remain yet to be functionally characterized. We have screened the seed proteome of Dolichos which lead to identification and purification of a protein, DC25. The protein was monomeric and highly thermostable in extreme conditions of pH and salt. It was crystallized and structure determined at 1.28 Å resolution using x-ray crystallography. The high-resolution structure of the protein revealed a four-bladed β-propeller hemopexin-type fold containing pseudo four-fold molecular symmetry at the central channel. While the structure exhibited homology with 2S albumins, variations in the loops connecting the outermost strands and the differences in surface-charge distribution may be relevant for distinct functions. Comparative study of the protein with other seed hemopexins revealed the presence of four conserved water molecules in between the blades which cross-link them and maintain the tertiary structure. The protein exhibited intrinsic peroxidase activity, which could be inhibited by binding of a heme analog. The identification of redox-sensitive cysteine and inhibition of peroxidase activity by iodoacetamide facilitated characterization of the possible active site. The determined peroxidase activity of DC25 may be responsible for rescuing germinating seeds from oxidative stress.

Crystal Structure of Mg(2+) Containing Hemopexin-Fold Protein from Kabuli Chana (Chickpea-White, CW-25) at 2.45 Å Resolution Reveals Its Metal Ion Transport Property

Plant seeds contain a number of proteins which play important roles in the protection and the process of germination of seeds. We have isolated and purified a 25 kDa protein from Kabuli Chana (Cicer arietinum L., Chickpea-white, CW-25). The CW-25 protein was crystallized using 0.5 M magnesium acetate, 0.1 M sodium cacodylate and 20 % (w/v) polyethylene glycol 8000, pH 6.5. The crystals of CW-25 belonged to space group P3 with unit cell dimensions, a = b = 80.5 Å, and c = 69.2 Å. The structure of CW-25 was determined using molecular replacement method and refined to an R factor of 0.152. The buried surface area between two molecules was found to be approximately 653 Å(2) indicating the formation of a weak homodimer. The polypeptide chain of CW-25 adopted a hemopexin-fold with four-bladed β-propellers. The structure formed a central tunnel-like architecture. A magnesium ion was observed in the centre of the tunnel. It was located at distances varying between 2.3 and 2.7 Å from five oxygen atoms of which four were backbone oxygen atoms belonging to residues, Asn7, Asp65, Asp121 and Asp174 while the fifth oxygen atom, O(δ1) was from the side chain of Asn7. The approximate length of the tunnel was 30 Å. Furthermore, a series of carbonyl oxygen atoms were present along the internal face of the tunnel. The diameter of the tunnel varied from 4.6 to 6.2 Å. The diameter and chemical environment of the tunnel clearly indicated that it might be used for the transport of various metal ions across the molecule.

Crystal structure of a plant albumin from Cicer arietinum (chickpea) possessing hemopexin fold and hemagglutination activity

Crystal structure of a reported PA2 albumin from Cicer arietinum shows that it belongs to hemopexin fold family, has four beta-propeller motifs and possesses hemagglutination activity, making it different from known legume lectins. A plant albumin (PA2) from Cicer arietinum, presumably a lectin (CAL) owing to its hemagglutination activity which is inhibited by complex sugars as well as glycoproteins such as fetuin, desialylated fetuin and fibrinogen. The three-dimensional structure of this homodimeric protein has been determined using X-ray crystallography at 2.2 Å in two crystal forms: orthorhombic (P21212) and trigonal (P3). The structure determined using molecular replacement method and refined in orthorhombic crystal form reached R-factors R free 22.6 % and R work 18.2 % and in trigonal form had 22.3 and 17.9 % in the resolution range of 20.0-2.2 and 35.3-2.2 Å, respectively. Interestingly, unlike the known legume lectin fold, the structure of this homodimeric hemagglutinin belonged to hemopexin fold that consisted of four-bladed β-propeller architecture. Each subunit has a central cavity forming a channel, inside of which is lined with hydrophobic residues. The channel also bears binding sites for ligands such as calcium, sodium and chloride ions, iodine atom in the case of iodine derivative and water molecules. However, none of these ligands seem important for the sugar recognition. No monosaccharide sugar specificity could be detected using hemagglutination inhibition. Chemical modification studies identified a potential sugar-binding site per subunit molecule. Comparison of C-alpha atom positions in subunit structures showed that the deviations between the two crystal forms were more with respect to blades I and IV. Differences also existed between subunits in two forms in terms of type and site of ligand binding.

Differential response to sulfur nutrition of two common bean genotypes differing in storage protein composition

It has been hypothesized that the relatively low concentration of sulfur amino acids in legume seeds might be an ecological adaptation to nutrient poor, marginal soils. SARC1 and SMARC1N-PN1 are genetically related lines of common bean (dry bean, Phaseolus vulgaris) differing in seed storage protein composition. In SMARC1N-PN1, the lack of phaseolin and major lectins is compensated by increased levels of sulfur-rich proteins, resulting in an enhanced concentration of cysteine and methionine, mostly at the expense of the abundant non-protein amino acid, S-methylcysteine. To identify potential effects associated with an increased concentration of sulfur amino acids in the protein pool, the response of the two genotypes to low and high sulfur nutrition was evaluated under controlled conditions. Seed yield was increased by the high sulfate treatment in SMARC1N-PN1. The seed concentrations of sulfur, sulfate, and S-methylcysteine were altered by the sulfur treatment in both genotypes. The concentration of total cysteine and extractible globulins was increased specifically in SMARC1N-PN1. Proteomic analysis identified arcelin-like protein 4, lipoxygenase-3, albumin-2, and alpha amylase inhibitor beta chain as having increased levels under high sulfur conditions. Lipoxygenase-3 accumulation was sensitive to sulfur nutrition only in SMARC1N-PN1. Under field conditions, both SARC1 and SMARC1N-PN1 exhibited a slight increase in yield in response to sulfur treatment, typical for common bean.

Spatio-temporal regulation of the OsHFP gene promoter establishes the involvement of this protein in rice anther development

Anther development leading to pollen maturation, anther dehiscence and pollen dispersal depends upon the precise timing of programmed cell death (PCD) in specified anther tissues. The PCD necessitates a properly tuned transcriptional regulation of some crucial genes. However, the detailed genetic regulation of this PCD in rice anther is yet to be deciphered. Recently, we have established that the OsHFP, a structurally novel hemopexin fold protein of rice is a flower-specific heme binding protein, and plays a role in chlorophyll degradation. Here, we report the spatio-temporal transcriptional regulation of the OsHFP gene, which is proposed to be involved in anther PCD. The OsHFP was immunodetected in rice anthers, and OsHFP-related proteins were also found to be present in anthers of other monocot (lily) and dicot (tobacco) plant species. Unique cis-acting elements, possibly involved in the activation and anther-specificity of the OsHFP promoter were identified based upon in silico prediction and in planta expression profiling of the reporter gene driven by the OsHFP promoter (2051 bp) and its two deleted versions (1057 bp and 437 bp). The temporal regulation of the OsHFP promoter in different developmental stages of tobacco anther implies the physiological function of this protein in anther PCD.

A structurally novel hemopexin fold protein of rice plays role in chlorophyll degradation

Proteins containing hemopexin fold domain are suggested to have diverse functions in various living organisms. In order to investigate the structure and function of this type of protein in rice plant (Oryza sativa), the gene encoding a hemopexin fold protein (OsHFP) was cloned, analyzed in silico and characterized. Molecular modeling revealed that the OsHFP is closely related to other hemopexin fold proteins, but is unique with a cylindrical central tunnel as well as extended N- and C-terminal domains. The recombinant OsHFP was found to bind hemin, the oxidized form of heme in vitro. The expression of the single copy OsHFP gene was detected in rice flower buds. Heterologous expression of OsHFP in green leaf tissues resulted in chlorophyll degradation; however, stable expression of OsHFP was observed in transgenic hairy roots, a non-green tissue. The possible role of OsHFP in regulating programmed cell death in anther green tissues of rice is proposed.