Functional Implication of β-Carotene Hydroxylases in Soybean Nodulation

Legume-Rhizobium spp. symbiosis requires signaling between the symbiotic partners and differential expression of plant genes during nodule development. Previously, we cloned a gene encoding a putative β-carotene hydroxylase (GmBCH1) from soybean (Glycine max) whose expression increased during nodulation with Bradyrhizobium japonicum. In this work, we extended our study to three GmBCHs to examine their possible role(s) in nodule development, as they were additionally identified as nodule specific, along with the completion of the soybean genome. In situ hybridization revealed the expression of three GmBCHs (GmBCH1, GmBCH2, and GmBCH3) in the infected cells of root nodules, and their enzymatic activities were confirmed by functional assays in Escherichia coli. Localization of GmBCHs by transfecting Arabidopsis (Arabidopsis thaliana) protoplasts with green fluorescent protein fusions and by electron microscopic immunogold detection in soybean nodules indicated that GmBCH2 and GmBCH3 were present in plastids, while GmBCH1 appeared to be cytosolic. RNA interference of the GmBCHs severely impaired nitrogen fixation as well as nodule development. Surprisingly, we failed to detect zeaxanthin, a product of GmBCH, or any other carotenoids in nodules. Therefore, we examined the possibility that most of the carotenoids in nodules are converted or cleaved to other compounds. We detected the expression of some carotenoid cleavage dioxygenases (GmCCDs) in wild-type nodules and also a reduced amount of zeaxanthin in GmCCD8-expressing E. coli, suggesting cleavage of the carotenoid. In view of these findings, we propose that carotenoids such as zeaxanthin synthesized in root nodules are cleaved by GmCCDs, and we discuss the possible roles of the carotenoid cleavage products in nodulation.Legume-Rhizobium spp. symbiosis results in the formation of the root nodule, in which rhizobia fix atmospheric nitrogen. Nodule development requires diverse events, such as Nod factor synthesis in the rhizobia, perception of the Nod factor on plant roots by receptor-like kinases, endocytosis of rhizobia into plant cells, and so on (Stacey et al., 2006; Oldroyd et al., 2011; Singh and Parniske, 2012). Sequential expression of numerous plant genes occurs during nodulation, contributing to different stages including nitrogen fixation. Arbuscular mycorrhizal (AM) symbiosis exhibits many similarities to the nodulation process (Oldroyd et al., 2009). For example, SymRK, the receptor-like kinase gene, is required for both rhizobial and AM symbioses (Stracke et al., 2002). Similarly, the signal transduction pathways following perception are also in part the same, and the genes common to the two pathways have been referred to as the common symbiosis (SYM) genes (Kistner et al., 2005). These similarities may reflect common mechanisms for host plant cells to respond to symbionts, although the commonality is not globally defined yet.Plant carotenoids are mostly C₄₀ tetraterpenoid pigments with a series of double bonds (DellaPenna and Pogson, 2006; Lu and Li, 2008). They play essential roles in photosynthesis. The phytohormone abscisic acid (ABA) is synthesized from xanthophylls, oxygenated derivatives of carotenoids. The beneficial effects of carotenoids for human disease prevention and health promotion are well established and are based on their antioxidant activities (Kopsell and Kopsell, 2006; Rao and Rao, 2007; von Lintig, 2010). Metabolic engineering approaches have produced crop plants with enhanced carotenoid contents and improved nutritional value (Giuliano et al., 2008). For example, enhancement of β-carotene, provitamin A, by engineering the carotenoid biosynthetic pathway resulted in the development of cv Golden rice (Oryza sativa; Ye et al., 2000; Paine et al., 2005; Ha et al., 2010).The initial step of carotenoid biosynthesis is the production of phytoene by the enzyme phytoene synthase (Fig. 1; DellaPenna and Pogson, 2006; Cazzonelli and Pogson, 2010). The subsequent activities of desaturases, isomerase, and cyclase convert phytoene into lycopene and further into β-carotene. Xanthophyll synthesis begins with the action of β-carotene hydroxylase (BCH) on β-carotene, producing initially β-cryptoxanthin and thereafter zeaxanthin (Kim et al., 2009). Overexpression of BCH has been found to confer tolerance to light stress (Davison et al., 2002). The subsequent steps catalyzed by zeaxanthin epoxidase (ZEP) and neoxanthin synthase lead to the synthesis of ABA (Takaichi and Mimuro, 1998).Open in a separate windowFigure 1.The biosynthetic pathway of carotenoids in plants. GGPP, Geranylgeranyl diphosphate; PSY, phytoene synthase; PDS, phytoene desaturase; ZDS, ζ-carotene desaturase; CRTISO, carotene isomerase; LCYB, lycopene β-cyclase; CYP97A3 and CYP97C1, cytochrome P450 enzymes; NSY, neoxanthin synthase; LCYE, lycopene ε-cyclase; CRTR-E, ε-carotene hydroxylase. Enzymes in red were examined in this study.Various carotenoid cleavage dioxygenases (CCDs) catalyze the formation of apocarotenoids with functions as hormones, flavors, and pigments (Auldridge et al., 2006b; Strack and Fester, 2006; Tsuchiya and McCourt, 2009; Walter et al., 2010). Recently, CCD7 and CCD8 were shown to control the synthesis of strigolactones, newly discovered hormones that inhibit shoot branching (Gomez-Roldan et al., 2008; Umehara et al., 2008; Vogel et al., 2010; Ruyter-Spira et al., 2013). In addition, carotenoid cleavage products have been discovered in plant roots colonized by AM fungi (Strack and Fester, 2006). During AM symbiosis, roots synthesize apocarotenoids at the same time as activating plant genes for carotenoid metabolism. Although RNA interference (RNAi)-mediated inhibition of apocarotenoid synthesis suggests that apocarotenoids are functionally significant (Snowden et al., 2005; Floss et al., 2008), their role in AM symbiosis is unknown.In a search for genes differentially induced during soybean (Glycine max)-Rhizobium spp. symbiosis, several antioxidant genes, including a gene encoding a putative BCH, were identified. In this report, we describe genes (GmBCHs) encoding a putative BCH whose expression increased in soybean root nodules. Therefore, the biochemical activities of BCHs were investigated. RNAi inhibition of GmBCH expression interfered with nitrogen fixation as well as nodule development. Subsequent analysis of the expression and biochemical activities of GmCCDs in root nodules led us to hypothesize that GmCCD8 could be involved in the synthesis of apocarotenoids from zeaxanthin in these nodules.     

Stimulation of Lactic Streptococci in Milk by β-Galactosidase

Acid production in milk by lactic streptococci was stimulated by added beta-galactosidase. Both glucose and galactose accumulated rapidly in the presence of this enzyme. Glucose accumulation ceased as the culture entered the most rapid period of acid production, whereas galactose accumulation continued. In cultures without added beta-galactosidase, a low concentration of galactose accumulated in the milk, whereas glucose was not detected after 2 hr of incubation. Cultures grew and produced acid faster in broth containing glucose rather than galactose or lactose. These observations suggest that the lactic streptococci do not metabolize the lactose in milk efficiently enough to permit optimum acid production and that a phenomenon such as catabolite repression functions to allow for a preferential use of glucose over either galactose or lactose. In addition to providing the culture with a more readily available energy source, it is possible that the culture produced more acidic metabolites as a result of preferentially utilizing the glucose released by the action of the beta-galactosidase.     

Expression and Secretion of Bovine β-Lactoglobulin in Saccharomyces cerevisiae

A bovine β-lactoglobulin (β-LG) was expressed in Saccharomyces cerevisiae carrying bovine pre-β-LG cDNA and secreted into its growth medium. The expression plasmid was constructed by inserting the whole coding region of the cDNA encoding pre-β-LG between the promoter and terminator of the yeast glyceraldehyde 3-phosphate dehydrogenase gene of pYG100, a yeast expression vector. In the supernatant of the yeast growth medium, β-LG with a native conformation was detected by sandwich ELISA, and its amount was estimated to be 1.1 mg/l. A Western-immunoblotting analysis revealed that β-LG secrected in the growth medium had the same mobility as that of authentic bovine β-LG. The N-terminal sequence was also identical with that of authentic mature bovine β-LG.     

Antioxidant behaviour of Ubiquinone and β-Carotene Incorporated in model Membranes

Experiments with model membranes, in which ubiquinone was incorporated, were performed in order to clarify the mechanism by which ubiquinone can prevent or control chain lipid peroxidation in biomembranes.

Comparing the behaviour of ubiquinone-containing vesicles with β-carotene containing vesicles we suggest that a possible explanation of the ubiquinone antioxidant effect could be to scavenge singlet oxygen and to affect structurally the lipid bilayer inhibiting hydroperoxide decomposition.     

Involvement of β-Carotene in the Antioxidant Defense of the Bacterial Cell

Effects of recombinant -carotene on the resistance of E. coli culture to menadione and paraquat were studied. The presence of -carotene in E. coli cells prevented, to a considerable extent, an increase in superoxide dismutase activity (induced by redox mediators) without affecting the culture growth. These findings suggest that -carotene is involved in the defense of cells against oxidative stress.     

Metabolic Engineering for Production of β-Carotene and Lycopene in Saccharomyces cerevisiae

We have engineered a conventional yeast, Saccharomyces cerevisiae, to confer a novel biosynthetic pathway for the production of β-carotene and lycopene by introducing the bacterial carotenoid biosynthesis genes, which are individually surrounded by the promoters and terminators derived from S. cerevisiae. β-Carotene and lycopene accumulated in the cells of this yeast, which was considered to be a result of the carbon flow for the ergosterol biosynthetic pathway being partially directed to the pathway for the carotenoid production.     

Heat Treatment and β-Carotene Incorporation Effect in the Interaction of β-Lactoglobulin and Carboxymethylcellulose System

Encapsulation technologies using proteins or polysaccharides can be employed with the purpose of solubilizing and protecting carotenoids. However, information on the role of protein and polysaccharide interactions is still slightly limited. The aim of this work was to investigate the effect of β-carotene linked to protein β-lactoglobulin (BLG) in the interaction carboxymethylcellulose (CMC) using isothermal titration calorimetry (ITC). Firstly, BLG and CMC interaction was assessed by means of turbidity analysis. Based on the results of turbidity, the thermodynamic profile of BLG-CMC complexes at pH 4.0 was obtained using ITC analysis at 25 °C. Afterward, it was evaluated the effect of a thermal treatment applied to the BLG (68 °C for 50 min) in the interaction with CMC also using ITC and circular dichroism (CD). ITC and CD analysis showed that the heat treatment applied on BLG did not cause changes in molecular interactions. The binding isotherm of BLG-CMC complexes incorporated with β-carotene showed an increase in the molar ratio and a slight decrease in enthalpy of the system. Incorporation of β-carotene in the system did not significantly affect the BLG and CMC interaction, suggesting this system can be applied in food application as encapsulation.     

Proteome Analysis of Cytoplasmatic and Plastidic β-Carotene Lipid Droplets in Dunaliella bardawil

The halotolerant green alga Dunaliella bardawil is unique in that it accumulates under stress two types of lipid droplets: cytoplasmatic lipid droplets (CLD) and β-carotene-rich (βC) plastoglobuli. Recently, we isolated and analyzed the lipid and pigment compositions of these lipid droplets. Here, we describe their proteome analysis. A contamination filter and an enrichment filter were utilized to define core proteins. A proteome database of Dunaliella salina/D. bardawil was constructed to aid the identification of lipid droplet proteins. A total of 124 and 42 core proteins were identified in βC-plastoglobuli and CLD, respectively, with only eight common proteins. Dunaliella spp. CLD resemble cytoplasmic droplets from Chlamydomonas reinhardtii and contain major lipid droplet-associated protein and enzymes involved in lipid and sterol metabolism. The βC-plastoglobuli proteome resembles the C. reinhardtii eyespot and Arabidopsis (Arabidopsis thaliana) plastoglobule proteomes and contains carotene-globule-associated protein, plastid-lipid-associated protein-fibrillins, SOUL heme-binding proteins, phytyl ester synthases, β-carotene biosynthesis enzymes, and proteins involved in membrane remodeling/lipid droplet biogenesis: VESICLE-INDUCING PLASTID PROTEIN1, synaptotagmin, and the eyespot assembly proteins EYE3 and SOUL3. Based on these and previous results, we propose models for the biogenesis of βC-plastoglobuli and the biosynthesis of β-carotene within βC-plastoglobuli and hypothesize that βC-plastoglobuli evolved from eyespot lipid droplets.Lipid droplets are the least characterized organelles in both mammalian and plant cells, and they were considered until a few years ago as passive storage compartments for triglycerides (TAG), sterol esters, and some pigments. However, recent studies have shown that they have diverse metabolic functions (Goodman, 2008; Farese and Walther, 2009; Murphy, 2012). Proteomic analyses in plants and some microalgae have shown that lipid droplets in the cytoplasm and in the chloroplast contain a large diversity of proteins including both structural proteins and many enzymes, indicating that they take an active metabolic role in the synthesis, degradation, and mobilization of glycerolipids, sterols, and pigments as well as in regulatory functions that have not yet been clarified (Schmidt et al., 2006; Ytterberg et al., 2006; Nguyen et al., 2011; Lundquist et al., 2012b; Eugeni Piller et al., 2014). A major limitation for determining the proteomes of lipid droplets, particularly in microalgae, is the purity and the homogeneity of the preparation. Green microalgae, for example, may contain three distinct pools of lipid droplets in one cell: the cytoplasmatic lipid droplets (CLD), the major neutral lipid pool, which are induced under stress conditions such as nitrogen limitation or at the stationary growth phase (Wang et al., 2009); plastoglobules, which are smaller lipid droplets within the chloroplast that have been shown to change in size and number under stress conditions and seem to be involved in stress resistance, metabolite transport, and the regulation of photosynthetic electron transport (Bréhélin et al., 2007; Besagni and Kessler, 2013); and the eyespot structure, part of the visual system in green algae, composed of one or several layers of lipid droplets, characterized by their orange color resulting from a high content of β-carotene (Kreimer, 2009). Disruption of microalgal cells, which is required for the isolation of the lipid droplets, usually involves harsh treatments such as sonication, mixing with glass beads, or use of a French press that breaks not only the cell membrane but also the chloroplast. Therefore, it is almost impossible to separate the different lipid droplet classes by the subsequent density gradient centrifugation, making it difficult to assign the origin of identified proteins. The other major difficulty is contamination by proteins released during cell lysis and fractionation, which associate and copurify with lipid droplets. These include cytoplasmic, chloroplastic, and mitochondrial proteins (Moellering and Benning, 2010; James et al., 2011; Nguyen et al., 2011; Nojima et al., 2013). Purification of isolated lipid droplets from loosely associated proteins is possible by treatments with detergents, high salt, and chaotropic agents (Jolivet et al., 2004; Nguyen et al., 2011); however, the danger in such treatments is that they also remove native loosely associated proteins from the lipid droplets.In this work, we tried to circumvent these problems by choosing a special algal species that is suitable for controlled cell lysis and fractionation and by utilizing two different contamination filters.The alga we selected, Dunaliella bardawil, is unique in that it accumulates large amounts of two different types of lipid droplets, CLD and β-carotene-rich (βC) plastoglobuli, under stress conditions (Davidi et al., 2014). The lack of a rigid cell wall in this alga allows lysis of the plasma membrane by a gentle osmotic shock, releasing CLD but leaving the chloroplast intact (Katz et al., 1995). This enables the recovery of large quantities of the two types of highly purified lipid droplets by differential lysis. In a recent study, we described the isolation and lipid compositions of these two lipid pools and showed that they have similar TAG compositions but different lipid-associated major proteins (Davidi et al., 2014).The high nutritional and pharmacological value of β-carotene for humans has promoted intensive research aimed to clarify its biosynthesis and regulation in plants and also led to attempts to increase β-carotene levels by genetic manipulations in crop plants such as tomato (Solanum lycopersicum; Rosati et al., 2000; Giorio et al., 2007) or by the creation of Golden rice (Oryza sativa; Ye et al., 2000). However, the capacity of plants to store β-carotene is limited, and in this respect, D. bardawil is an exceptional example of an organism that can accumulate large amounts of this pigment, up to 10% of its dry weight. This is enabled by the compartmentation and storage of this lipophilic pigment in specialized plastoglobules. Also, the unusual isomeric composition, consisting of around 50% 9-cis- and 50% all-trans-isomers (Ben-Amotz et al., 1982, 1988), is probably of major importance in this respect, due to the better solubility of the cis-isomer in lipids, which enables the storage of high concentrations exceeding 50% of the lipid droplets. The localization of carotenoid biosynthesis in plants appears to be tissue specific: in green tissues, it takes place in chloroplast membranes, probably within the inner chloroplast envelope membrane (Joyard et al., 2009), whereas in carotenoid-accumulating fruits, such as tomato or bell pepper (Capsicum annuum), it takes place in specialized organelles derived from chromoplasts (Siddique et al., 2006; Barsan et al., 2010). In green microalgae, there are at least two types of carotenoid-accumulating organelles: CLD and eyespot. Algae such as Haematococcus pluvialis and Chlorella zofigiensis accumulate carotenoids within CLD. In H. pluvialis, the major pigment, astaxanthin, is synthesized initially in the chloroplast as β-carotene and then transferred to CLD, where it is oxidized and hydroxylated to astaxanthin (Grünewald et al., 2001). The eyespot, which is composed of one or several layers of small β-carotene-containing lipid droplets, has been shown by proteomic analysis to include part of the β-carotene biosynthesis enzymes, indicating that β-carotene is probably synthesized within these lipid droplets (Schmidt et al., 2006). Similarly, plant chromoplasts also contain carotenoid biosynthesis enzymes (Schmidt et al., 2006; Ytterberg et al., 2006; Schapire et al., 2009). D. bardawil and Dunaliella salina are unique in that they accumulate large amounts of β-carotene within βC-plastoglobuli. A special focus in this work was the identification of the β-carotene biosynthesis machinery in D. bardawil. It is not known if the synthesis takes place inside the lipid βC-plastoglobuli or in chloroplast envelope membranes. Since D. bardawil also contains β-carotene and xanthophylls at the photosynthetic system, it is interesting to know whether the β-carotene that accumulates under stress in βC-plastoglobuli is produced by the constitutive carotenoid biosynthetic pathway or by a different stress-induced enzymatic system.     

The Cystic Fibrosis-causing Mutation ΔF508 Affects Multiple Steps in Cystic Fibrosis Transmembrane Conductance Regulator Biogenesis

The deletion of phenylalanine 508 in the first nucleotide binding domain of the cystic fibrosis transmembrane conductance regulator is directly associated with >90% of cystic fibrosis cases. This mutant protein fails to traffic out of the endoplasmic reticulum and is subsequently degraded by the proteasome. The effects of this mutation may be partially reversed by the application of exogenous osmolytes, expression at low temperature, and the introduction of second site suppressor mutations. However, the specific steps of folding and assembly of full-length cystic fibrosis transmembrane conductance regulator (CFTR) directly altered by the disease-causing mutation are unclear. To elucidate the effects of the ΔF508 mutation, on various steps in CFTR folding, a series of misfolding and suppressor mutations in the nucleotide binding and transmembrane domains were evaluated for effects on the folding and maturation of the protein. The results indicate that the isolated NBD1 responds to both the ΔF508 mutation and intradomain suppressors of this mutation. In addition, identification of a novel second site suppressor of the defect within the second transmembrane domain suggests that ΔF508 also effects interdomain interactions critical for later steps in the biosynthesis of CFTR.     

Mutation in the Structural Gene for Diphtheria Toxin carried by Temperate Phage β

Corynebacterium diphtheriae strains lyso-genic for phage β are able to produce diphtheria toxin. This article describes evidence suggesting that the toxin structural gene is part of the phage genome.     

CAROTENOID CLEAVAGE DIOXYGENASE4 Is a Negative Regulator of β-Carotene Content in Arabidopsis Seeds

Experimental approaches targeting carotenoid biosynthetic enzymes have successfully increased the seed β-carotene content of crops. However, linkage analysis of seed carotenoids in Arabidopsis thaliana recombinant inbred populations showed that only 21% of quantitative trait loci, including those for β-carotene, encode carotenoid biosynthetic enzymes in their intervals. Thus, numerous loci remain uncharacterized and underutilized in biofortification approaches. Linkage mapping and genome-wide association studies of Arabidopsis seed carotenoids identified CAROTENOID CLEAVAGE DIOXYGENASE4 (CCD4) as a major negative regulator of seed carotenoid content, especially β-carotene. Loss of CCD4 function did not affect carotenoid homeostasis during seed development but greatly reduced carotenoid degradation during seed desiccation, increasing β-carotene content 8.4-fold relative to the wild type. Allelic complementation of a ccd4 null mutant demonstrated that single-nucleotide polymorphisms and insertions and deletions at the locus affect dry seed carotenoid content, due at least partly to differences in CCD4 expression. CCD4 also plays a major role in carotenoid turnover during dark-induced leaf senescence, with β-carotene accumulation again most strongly affected in the ccd4 mutant. These results demonstrate that CCD4 plays a major role in β-carotene degradation in drying seeds and senescing leaves and suggest that CCD4 orthologs would be promising targets for stabilizing and increasing the level of provitamin A carotenoids in seeds of major food crops.     

Characterization of the Role of β-Carotene 9,10-Dioxygenase in Macular Pigment Metabolism

A family of enzymes collectively referred to as carotenoid cleavage oxygenases is responsible for oxidative conversion of carotenoids into apocarotenoids, including retinoids (vitamin A and its derivatives). A member of this family, the β-carotene 9,10-dioxygenase (BCO2), converts xanthophylls to rosafluene and ionones. Animals deficient in BCO2 highlight the critical role of the enzyme in carotenoid clearance as accumulation of these compounds occur in tissues. Inactivation of the enzyme by a four-amino acid-long insertion has recently been proposed to underlie xanthophyll concentration in the macula of the primate retina. Here, we focused on comparing the properties of primate and murine BCO2s. We demonstrate that the enzymes display a conserved structural fold and subcellular localization. Low temperature expression and detergent choice significantly affected binding and turnover rates of the recombinant enzymes with various xanthophyll substrates, including the unique macula pigment meso-zeaxanthin. Mice with genetically disrupted carotenoid cleavage oxygenases displayed adipose tissue rather than eye-specific accumulation of supplemented carotenoids. Studies in a human hepatic cell line revealed that BCO2 is expressed as an oxidative stress-induced gene. Our studies provide evidence that the enzymatic function of BCO2 is conserved in primates and link regulation of BCO2 gene expression with oxidative stress that can be caused by excessive carotenoid supplementation.     

Bovine β-defensins: Identification and characterization of novel bovine β-defensin genes and their expression in mammarygland tissue

-Defensin genes code for multifunctional peptides with a broad-range antimicrobial activity. In this project we hypothesized that -defensin genes may be candidate genes for resistance to mastitis. In this article we describe the identification and genomic characterization of eight bovine -defensin genes, including six novel defensin genes and two pseudogenes. Expression in the bovine mammary gland of one of the novel genes, DEFB401, has been demonstrated, as well as the expression of LAP, TAP, DEFB1, BNBD3, BNBD9, and BNBD12. For genomic characterization, 20 BACs from two different bovine BAC libraries (RZPD numbers 750 and 754) were isolated by PCR screening with -defensin consensus primers derived from published sequences. PCR products from BACs generated with consensus primers have been subcloned and sequenced, revealing a total of 16 genes and two pseudogenes. Six novel -defensin genes share the typical exon–intron structure and are highly homologous to published bovine -defensin genes. They are named DEFB401–DEFB405 and LAP-like, and two novel pseudogenes are named EBD-P and EBD-P2. Analysis of mammary gland tissue-derived cDNA from nine cows with different clinical findings demonstrated the expression of several -defensin genes mentioned above. First results indicate that the lactational status of the cow presumably has no influence on gene expression. Competent knowledge of antimicrobial activity of -defensins from literature, the abundance of -defensin mRNA in the bovine mammary gland, and the inducibility of some genes give first evidence that -defensins may play a role in local host defense during udder infections.The nucleotide sequence data reported in this article have been submitted to EMBL and have been assigned the accession numbers AJ563279–AJ563283, AJ567353–AJ567365, AJ567990–AJ567993, and AJ620296.     

Supersensitivity to β-Lactam Antibiotics in Escherichia coli Caused by D-Alanine Carboxypeptidase IA Mutation

Escherichia coli cells acquired supersensitivity to various β-lactam antibiotics by dacA mutation, a defect in D-alanine carboxypeptidase IA activity. The mutant cells were rather less sensitive to mecillinam than the dacA⁺ cells. This mutation did not result in either thermosensitivity of cell growth or appreciable increase of the generation times in usual rich media, but the resulting appearance of supersensitivity to β-lactam antibiotics suggests that the cell wall or envelope of this mutant is somewhat abnormal and thus that D-alanine carboxypeptidase IA is involved in cell wall or envelope synthesis.     

The Cancer Mutation D83V Induces an α-Helix to β-Strand Conformation Switch in MEF2B

MEF2B is a major target of somatic mutations in non-Hodgkin lymphoma. Most of these mutations are non-synonymous substitutions of surface residues in the MADS-box/MEF2 domain. Among them, D83V is the most frequent mutation found in tumor cells. The link between this hotspot mutation and cancer is not well understood. Here we show that the D83V mutation induces a dramatic α-helix to β-strand switch in the MEF2 domain. Located in an α-helix region rich in β-branched residues, the D83V mutation not only removes the extensive helix stabilization interactions but also introduces an additional β-branched residue that further shifts the conformation equilibrium from α-helix to β-strand. Cross-database analyses of cancer mutations and chameleon sequences revealed a number of well-known cancer targets harboring β-strand favoring mutations in chameleon α-helices, suggesting a commonality of such conformational switch in certain cancers and a new factor to consider when stratifying the rapidly expanding cancer mutation data.