Assigning statistical significance to proteotypic peptides via database searches

Unraveling the Composition of Insecticidal Crystal Proteins in Bacillus thuringiensis: a Proteomics Approach

Bacillus thuringiensis (Bt) is the most widely used active ingredient for biological insecticides. The composition of δ-endotoxins (Cry and Cyt proteins) in the parasporal crystal determines the toxicity profile of each Bt strain. However, a reliable method for their identification and quantification has not been available, due to the high sequence identity of the genes that encode the δ-endotoxins and the toxins themselves. Here, we have developed an accurate and reproducible mass spectrometry-based method (liquid chromatography-tandem mass spectrometry-multiple reaction monitoring [LC-MS/MS-MRM]) using isotopically labeled proteotypic peptides for each protein in a particular mixture to determine the relative proportion of each δ-endotoxin within the crystal. To validate the method, artificial mixtures containing Cry1Aa, Cry2Aa, and Cry6Aa were analyzed. Determination of the relative abundance of proteins (in molarity) with our method was in good agreement with the expected values. This method was then applied to the most common commercial Bt-based products, DiPel DF, XenTari GD, VectoBac 12S, and Novodor, in which between three and six δ-endotoxins were identified and quantified in each product. This novel approach is of great value for the characterization of Bt-based products, not only providing information on host range, but also for monitoring industrial crystal production and quality control and product registration for Bt-based insecticides.IMPORTANCE Bacillus thuringiensis (Bt)-based biological insecticides are used extensively to control insect pests and vectors of human diseases. Bt-based products provide greater specificity and biosafety than broad-spectrum synthetic insecticides. The biological activity of this bacterium resides in spores and crystals comprising complex mixtures of toxic proteins. We developed and validated a fast, accurate, and reproducible method for quantitative determination of the crystal components of Bt-based products. This method will find clear applications in the improvement of various aspects of the industrial production process of Bt. An important aspect of the production of Bt-based insecticides is its quality control. By specifically quantifying the relative proportion of each of the toxins that make up the crystal, our method represents the most consistent and repeatable evaluation procedure in the quality control of different batches produced in successive fermentations. This method can also contribute to the design of specific culture media and fermentation conditions that optimize Bt crystal composition across a range of Bt strains that target different pestiferous insects. Quantitative information on crystal composition should also prove valuable to phytosanitary product registration authorities that oversee the safety and efficacy of crop protection products.

Targeted absolute quantitative proteomics with SILAC internal standards and unlabeled full-length protein calibrators (TAQSI)

RATIONALE

Liquid Chromatography/Mass Spectrometry (LC/MS)-based proteomics for absolute protein quantification has been increasingly utilized in both basic and clinical research. There is a great need to overcome some major hurdles of current absolute protein quantification methods, such as significant inter-assay variability and the high cost associated with the preparation of purified stable-isotope-labeled peptide/protein standards.

METHODS

We developed a novel targeted absolute protein quantification method, named TAQSI, utilizing full-length isotope-labeled protein internal standards generated from SILAC (stable isotope labeling by amino acid in cell culture) and unlabeled full-length protein calibrators. This approach was applied to absolute quantification of carboxylesterase 1 (CES1), the primary human hepatic hydrolase, in a large set of human liver samples. Absolute CES1 quantities were derived from the standard calibration curves established from unlabeled CES1 protein calibrators and the isotope-labeled CES1 internal standards obtained from SILAC HepG2 cells.

RESULTS

The TAQSI assay was found to be accurate, precise, reproducible, and cost-effective. Importantly, protein quantification was not affected by various protein extraction and digestion protocols, and measurement errors associated with nonsynonymous variants can be readily identified and avoided. Furthermore, the TAQSI approach significantly simplifies the procedure of identifying the best performance surrogate peptides.

CONCLUSIONS

The TAQSI assay can be widely used for targeted absolute protein quantification in various biomedical research and clinical practice settings.

PhosphoHunter: An Efficient Software Tool for Phosphopeptide Identification

Phosphorylation is a protein posttranslational modification. It is responsible of the activation/inactivation of disease-related pathways, thanks to its role of “molecular switch.” The study of phosphorylated proteins becomes a key point for the proteomic analyses focused on the identification of diagnostic/therapeutic targets. Liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) is the most widely used analytical approach. Although unmodified peptides are automatically identified by consolidated algorithms, phosphopeptides still require automated tools to avoid time-consuming manual interpretation. To improve phosphopeptide identification efficiency, a novel procedure was developed and implemented in a Perl/C tool called PhosphoHunter, here proposed and evaluated. It includes a preliminary heuristic step for filtering out the MS/MS spectra produced by nonphosphorylated peptides before sequence identification. A method to assess the statistical significance of identified phosphopeptides was also formulated. PhosphoHunter performance was tested on a dataset of 1500 MS/MS spectra and it was compared with two other tools: Mascot and Inspect. Comparisons demonstrated that a strong point of PhosphoHunter is sensitivity, suggesting that it is able to identify real phosphopeptides with superior performance. Performance indexes depend on a single parameter (intensity threshold) that users can tune according to the study aim. All the three tools localized >90% of phosphosites.

Quantifying the effect of competition for detection between coeluting peptides on detection probabilities in mass-spectrometry-based proteomics

There are many factors that contribute to the variation in detection probabilities of proteins in LC-MS/MS experiments, and currently little is known about their relative importance. In this study, we analyze the effect of competition for detection between coeluting peptides on peptide detection probability. Using a novel method for estimating peptide detection probabilities, we show that these probabilities can vary by an order of magnitude between peptides that elute from the liquid chromatograph at the same time as many other peptides and those that elute with fewer other peptides. To explore these results, we use a mathematical model to show that competition for detection between peptides is expected to be a major source of missed detections in complex mixtures because there will be many MS/MS scanning intervals that contain more coeluting peptides than can be subjected to MS/MS analysis. Our data and simulation results show that the number of coeluting peptides is a primary determinant of whether a peptide will be detected. In our data, this had a several-fold larger effect on peptide detection probability than did peptide abundance. Furthermore, the distribution of elution times for the most frequently detected peptides was strongly shifted toward values where there were few coeluting peptides, indicating that the number of coeluting peptides is a major determinant of whether a peptide is proteotypic.

Improving peptide identification sensitivity in shotgun proteomics by stratification of search space

Because of its high specificity, trypsin is the enzyme of choice in shotgun proteomics. Nonetheless, several publications do report the identification of semitryptic and nontryptic peptides. Many of these peptides are thought to be signaling peptides or to have formed during sample preparation. It is known that only a small fraction of tandem mass spectra from a trypsin-digested protein mixture can be confidently matched to tryptic peptides. If other possibilities such as post-translational modifications and single-amino acid polymorphisms are ignored, this suggests that many unidentified spectra originate from semitryptic and nontryptic peptides. To include them in database searches, however, may not improve overall peptide identification because of the possible sensitivity reduction from search space expansion. To circumvent this issue for E-value-based search methods, we have designed a scheme that categorizes qualified peptides (i.e., peptides whose differences in molecular weight from the parent ion are within a specified error tolerance) into three tiers: tryptic, semitryptic, and nontryptic. This classification allows peptides that belong to different tiers to have different Bonferroni correction factors. Our results show that this scheme can significantly improve retrieval performance compared to those of search strategies that assign equal Bonferroni correction factors to all qualified peptides.

The frontiers of mass spectrometry-based techniques in food allergenomics

In the last years proteomic science has started to provide an important contribution to the disclosure of basic aspects of food-related diseases. Among these, the identification of proteins involved in food allergy and their mechanism of activation of toxicity. Elucidation of these key issues requires the integration of clinical, immunological, genomic and proteomic approaches. These combined research efforts are aimed to obtain structural and functional information to assist the development of novel, more reliable and powerful diagnostic protocols alternative to the currently available procedures, mainly based on food challenge tests. Another crucial aspect related to food allergy is the need for methods to detect trace amounts of allergenic proteins in foods. Mass spectrometry is the only non-immunological method for high-specificity and high-sensitivity detection of allergens in foods. Nowadays, once provided the appropriate sample handling and the correct operative conditions, qualitative and quantitative determination of allergens in foods and ingredients can be efficiently obtained by MALDI-TOF-MS and LC-MS/MS methods, with limits of detection and quantification in the low-ppb range. The availability of accurate and fast alternatives to immunological ELISA tests may also enable the development of novel therapeutic strategies and food processing technologies to aid patients with food allergy or intolerance, and to support allergen labelling and certification processes, all issues where the role of proteomic science is emerging.