Severin E. Stevenson

Quantitation of soybean allergens using tandem mass spectrometry.

Soybean (Glycine max) seed contain some proteins that are allergenic to humans and animals. However, the concentration of these allergens and their expression variability among germplasms is presently unknown. To address this problem, 10 allergens were quantified from 20 nongenetically modified commercial soybean varieties using parallel, label-free mass spectrometry approaches. Relative quantitation was performed by spectral counting and absolute quantitation was performed using multiple reaction monitoring (MRM) with synthetic, isotope-labeled peptides as internal standards. During relative quantitation analysis, 10 target allergens were identified, and five of these allergens showed expression levels higher than technical variation observed for bovine serum albumin (BSA) internal standard (∼11%), suggesting expression differences among the varieties. To confirm this observation, absolute quantitation of these allergens from each variety was performed using MRM. Eight of the 10 allergens were quantified for their concentration in seed and ranged from approximately 0.5 to 5.7 μg/mg of soy protein. MRM analysis reduced technical variance of BSA internal standards to approximately 7%, and confirmed differential expression for four allergens across the 20 varieties. This is the first quantitative assessment of all major soybean allergens. The results show the total quantity of allergens measured among the 20 soy varieties was mostly similar.

Validation of gel-free, label-free quantitative proteomics approaches: Applications for seed allergen profiling

Plant seeds provide a significant portion of the protein present in the human diet, but are also the major contributors of allergenic proteins that cause a majority of the reported cases of food-induced anaphylaxis. New varieties of grains and nuts as well as other seeds could be screened for allergen content before they are introduced as cultivars for food production using mass spectrometry-based quantitation approaches. Here, we present a practical comparison of gel-free and label-free methods, peak integration and spectral counting, using a linear trap mass spectrometer. The results show that both methods are linear and reproducible with protein standards from 5-200 ng, however, bioinformatic analysis for spectral counting is much simpler and therefore more amenable to high-throughput sample processing. We therefore applied spectral counting towards the analysis of transgenic peanut lines targeting the reduction of a prominent allergen. Spectral count analysis of an Ara h 2 (conglutin-7) RNA-silenced line confirmed reduction of this allergen as well as Ara h 6 (conglutin), which was further confirmed by quantitative immunoblotting. Other collateral changes include an increase in Ara h 10 (oleosin 1) in one of the three lines, a decrease in conarachin as well as increased 13-lipoxygenase and Ahy-3 (arachin) in two of three lines.

Reduction of IgE Binding and Nonpromotion of Aspergillus flavus Fungal Growth by Simultaneously Silencing Ara h 2 and Ara h 6 in Peanut

The most potent peanut allergens, Ara h 2 and Ara h 6, were silenced in transgenic plants by RNA interference. Three independent transgenic lines were recovered after microprojectile bombardment, of which two contained single, integrated copies of the transgene. The third line contained multiple copies of the transgene. Ara h 2 expression was significantly suppressed in all three lines, whereas Ara h 6 was reduced in two lines. Expression of peanut allergens Ara h 1 and Ara h 3 was not noticeably affected. Significant reduction of human IgE binding to Ara h 2 and Ara h 6 also was observed. Seed weight and germination data from transgenic and nontransgenic segregants showed no significant differences. Data collected from in vitro Aspergillus flavus infection indicate no significant difference in fungal growth between the transgenic lines and the nontransgenic controls. These data suggest that silencing Ara h 2 and Ara h 6 is a feasible approach to produce hypoallergenic peanut.

Neural cell adhesion molecule (NCAM) marks adult myogenic cells committed to differentiation

Although recent advances in broad-scale gene expression analysis have dramatically increased our knowledge of the repertoire of mRNAs present in multiple cell types, it has become increasingly clear that examination of the expression, localization, and associations of the encoded proteins will be critical for determining their functional significance. In particular, many signaling receptors, transducers, and effectors have been proposed to act in higher-order complexes associated with physically distinct areas of the plasma membrane. Adult muscle stem cells (satellite cells) must, upon injury, respond appropriately to a wide range of extracellular stimuli: the role of such signaling scaffolds is therefore a potentially important area of inquiry. To address this question, we first isolated detergent-resistant membrane fractions from primary satellite cells, then analyzed their component proteins using liquid chromatography-tandem mass spectrometry. Transmembrane and juxtamembrane components of adhesion-mediated signaling pathways made up the largest group of identified proteins; in particular, neural cell adhesion molecule (NCAM), a multifunctional cell-surface protein that has previously been associated with muscle regeneration, was significant. Immunohistochemical analysis revealed that not only is NCAM localized to discrete areas of the plasma membrane, it is also a very early marker of commitment to terminal differentiation. Using flow cytometry, we have sorted physically homogeneous myogenic cultures into proliferating and differentiating fractions based solely upon NCAM expression.

Environmental Effects on Allergen Levels in Commercially Grown Non-Genetically Modified Soybeans: Assessing Variation Across North America

Soybean (Glycine max) is a hugely valuable soft commodity that generates tens of billions of dollars annually. This value is due in part to the balanced composition of the seed which is roughly 1:2:2 oil, starch, and protein by weight. In turn, the seeds have many uses with various derivatives appearing broadly in processed food products. As is true with many edible seeds, soybeans contain proteins that are anti-nutritional factors and allergens. Soybean, along with milk, eggs, fish, crustacean shellfish, tree nuts, peanuts, and wheat, elicit a majority of food allergy reactions in the United States. Soybean seed composition can be affected by breeding, and environmental conditions (e.g., temperature, moisture, insect/pathogen load, and/or soil nutrient levels). The objective of this study was to evaluate the influence of genotype and environment on allergen and anti-nutritional proteins in soybean. To address genetic and environmental effects, four varieties of non-GM soybeans were grown in six geographically distinct regions of North America (Georgia, Iowa, Kansas, Nebraska, Ontario, and Pennsylvania). Absolute quantification of proteins by mass spectrometry can be achieved with a technique called multiple reaction monitoring (MRM), during which signals from an endogenous protein are compared to those from a synthetic heavy-labeled internal standard. Using MRM, eight allergens were absolutely quantified for each variety in each environment. Statistical analyses show that for most allergens, the effects of environment far outweigh the differences between varieties brought about by breeding.

Evolution of seed allergen quantification--from antibodies to mass spectrometry.

Development of accurate, high-throughput approaches for protein allergen quantification is important for the seed industry as a means to monitor natural variability in expression and ensure introduced transgenes do not collaterally alter the expression of any known allergen. Analytical approaches for protein quantification have undergone a renaissance in recent years with the emergence of soft-ionization approaches and advanced mass spectrometers capable of achieving low attomolar sensitivity. These advances coupled with bioinformatic tools to mine mass spectral data are collectively referred to as proteomics, and allow for the large-scale study of proteins with high precision and quantitative accuracy. In this review, we discuss differential and quantitative proteomics workflows that proceed from discovery profiling to targeted, quantitative analysis of specific proteins using stable isotopically-labeled, synthetic peptide doping standards. These synthetic peptide standards, also referred to as AQUA peptides, are synthetic mimics to proteotypic peptides and allow for absolute quantification of proteins in complex biological mixtures. The approaches discussed herein are ideal for the analysis of prominently expressed proteins such as protein allergens from plant seed, as no gels or sample pre-fractionation is required. We discuss these new techniques in the context of traditional, antibody-based technologies for allergen detection and quantification.

A quantitative mass spectrometry-based approach for identifying protein kinase clients and quantifying kinase activity

The Homo sapiens and Arabidopsis thaliana genomes are believed to encode more than 500 and 1000 protein kinases, respectively. Despite this abundance, few bona fide kinase-client relationships have been described in detail. Here we describe a quantitative mass spectrometry (MS)-based approach for identifying kinase-client proteins. During method development, we used the dedicated kinase pyruvate dehydrogenase kinase (PDK) for the in vitro assays. As kinase substrate, we used synthetic peptide cocktails and, in the process, demonstrated that the assay is both sensitive and specific. The method is also useful for characterizing protein kinase-substrate kinetics once the peptide substrate is detected. Applying a label-free spectral counting method, the activity of PDK was determined using the peptide substrate YHGH(292)SMSDPGSTYR derived from the pyruvate dehydrogenase E1alpha subunit sequence. The utility of spectral counting was further validated by studying the negative effect of Met oxidation on peptide phosphorylation. We also measured the activity of the unrelated calcium-dependent protein kinase 3 (CPK3), demonstrating the utility of the method in protein kinase screening applications.

Mass spectrometry analysis of soybean seed proteins: optimization of gel-free quantitative workflow

For high-throughput quantitative mass spectrometry (MS) analysis of soybean seed proteins, a method that avoids gel fractionation is advantageous. We developed and optimized a workflow from protein isolation to MS-based quantitation without polyacrylamide matrices. The objective was to quantitatively compare extraction methods to reproducibly arrive at the highest yield and proteome coverage. Beginning with mature soybean seed, we compared four proteinextraction methods, employing either TCA/acetone, urea, urea/thiourea, or phenol. Soybean proteins were extracted, quantified for total protein content, and comparatively visualized by Coomassie–SDS-PAGE. The phenolextraction method yielded protein concentrations 2 to 7-fold higher than other extraction methods. Comparison of trypsin to protein ratios (1 : 25, 1 : 50, 1 : 75, and 1 : 100) revealed a near linear increase in spectral counts by MS with increasing trypsin levels. In-solution digestion procedures were also compared to determine optimal resuspension and digestion conditions for peptideextraction and quantitation. A resuspension buffer that contained 50 mM Tris–HCl of pH 8.0 and 5 M urea showed the highest spectral counts and protein identifications. The results of this study show the time-honored phenolextraction method consistently and unequivocally yielded the highest amounts of protein from mature soybean seed, and that buffered urea is sufficient for optimal resuspension of precipitated proteins for tryptic digestion and mass spectrometry.

Development of an isoform-specific tandem mass spectrometry assay for absolute quantitation of maize lipid transfer proteins.

Precise and accurate quantitation of maize grain allergens is important for seed and food industries. The major allergen in maize grain is Zea m 14, a lipid transfer protein (LTP). The B73 maize genome encodes for at least six LTPs sharing 15%-87% sequence identity to Zea m 14. Phylogenetic analysis of the maize LTP family revealed one gene that corresponds to Zea m 14 (denoted as LTPa) and two other genes sharing 43% (LTPc) and 74% (LTPb) identity with Zea m 14 that are putative homologues. Using stable isotope peptide mimics as internal standards for LTPs, we present a multiple reaction monitoring mass spectrometry approach for multiplexed, absolute quantitation of all three LTP proteins and alternative transcript models therein. To validate quantitative accuracy, a redundant peptide, simultaneously representing the two most abundant LTPs, was included. Analysis of 21 maize varieties revealed LTPa was most prominently expressed in maize grain, ranging from 9 to 32 μg LTP/mg protein. Proteins belonging to the LTPb and LTPc gene models were also expressed but at approximately 10- and 100-fold lower levels than LTPa, respectively. The quantitative results provided by the redundant peptide show around 95% agreement with the sum of the two unique peptides, thus providing support for the LTP gene models and validating the accuracy of this method. Though not all Zea m 14-related LTPs are abundant in grain, their high sequence homology and detectable expression in maize grain signify that LTPb and LTPc are putative allergens and should be accounted for in any quantitation strategy for maize LTP allergens.

Proteomic mapping for legume nodule organogenesis

While genetic screens have identified mutants of the model legume Lotus japonicus that can nodulate in the absence of rhizobia, the lack of a proteome map is a major hindrance to understanding the functional protein networks associated with this nodulation process. In this issue of Proteomics, Dam et al. (Proteomics 2014, 14, 230–240) developed 2D gel‐based reference maps of nodules and roots of Lotus and a spontaneous nodule formation mutant (snf1). Comparative proteomic analysis of roots and two developmental stages of nodules provide useful insights into tissue‐specific mechanisms underlying nodule organogenesis. Additionally, a comparison of interspecies nodule proteomes displays that overlapping and individual mechanisms are associated with legume nodulation.