Expression of a high sweetness and heat-resistant mutant of sweet-tasting protein, monellin, in Pichia pastoris with a constitutive GAPDH promoter and modified N-terminus

Expression of a single-chain monellin (MNEI) mutant with enhanced stability in transgenic mice milk

Monellin is a sweet protein that may be used as a safe and healthy sweetener. However, due to its low stability, the application of monellin is currently very limited. Here, we describe a wild-type, a double-sites mutant (E2N/E23A) and a triple-sites mutant (N14A/E23Q/S76Y) of single-chain monellin (MNEI) expressed in transgenic mice milk. Based on enzyme-linked immunoassay (ELISA), Western blot, and sweetness intensity testing, their sweetness and stability were compared. After boiling for 2 min at different pH conditions (2.5, 5.1, 6.8, and 8.2), N14A/E23Q/S76Y-MNEI showed significantly higher sweetness and stability than the wild-type and E2N/E23A-MNEI. These results suggest that N14A/E23Q/S76Y-MNEI shows remarkable potential as a sweetener in the future.

Brazzein and Monellin: Chemical Analysis, Food Industry Applications, Safety and Quality Control, Nutritional Profile and Health Impacts

Recently, customers have been keener to buy products manufactured using all-natural ingredients with positive health properties, but without losing flavor. In this regard, the objective of the current study is to review the consumption of brazzein and monellin, their nutritional profiles and health effects, and their potential applications in the food industry. This poses challenges with sustainability and important quality and safety indicators, as well as the chemical processes used to determine them. To better understand the utilization of brazzein and monellin, the chemical analysis of these two natural sweet proteins was also reviewed by placing particular emphasis on their extraction methods, purification and structural characterization. Protein engineering is considered a means to improve the thermal stability of brazzein and monellin to enhance their application in food processing, especially where high temperatures are applied. When the quality and safety of these sweet proteins are well-investigated and the approval from safety authorities is secured, the market for brazzein and monellin as food ingredient substitutes for free sugar will be guaranteed in the future. Ultimately, the review on these two natural peptide sweeteners increases the body of knowledge on alleviating problems of obesity, diabetes and other non-communicable diseases.

Establishment of a visual gene knockout system based on CRISPR/Cas9 for the rare actinomycete Nonomuraea gerenzanensis

OBJECTIVES

To develop a modified CRISPR/Cas9 system with the β-glucuronidase (GusA) reporter and a dual sgRNA cassette for Nonomuraea gerenzanensis (N. gerenzanensis).

RESULTS

With the aid of a visual GusA reporter, the complicated and tedious process of cloning and gene identification could be abandoned entirely in the genetic editing of N. gerenzanensis. Moreover, introducing a dual sgRNA cassette into the CRISPR/Cas9 system significantly improved gene deletion efficiency compared to the single sgRNA element. Furthermore, the length of the homologous flanking sequences set to the lowest value of 500 bp in this system could still reach the relatively higher conjugation transfer frequency.

CONCLUSIONS

The enhanced CRISPR/Cas9 system could efficiently perform genetic manipulation on the rare actinomycete N. gerenzanensis.

Comparison of Different Signal Peptides for the Efficient Secretion of the Sweet-Tasting Plant Protein Brazzein in Pichia pastoris

Brazzein is a small sweet-tasting protein found in the red berries of a West African evergreen shrub, Pentadiplandra brazzeana Baillon. Brazzein is highly soluble and stable over a large pH range and at high temperatures, which are characteristics that suggest its use as a natural sweetener. However, Pentadiplandra brazzeana culture is difficult at a large scale, limiting the natural source of brazzein. Heterologous expression of brazzein has been established in numerous systems, including bacteria, yeast, and transgenic plants. Brazzein requires four disulfide bonds to be active in eliciting an intense sweet taste, and the yeast Pichia pastoris appears to be one of the best options for obtaining functional brazzein in high quantities. Employing yeast secretion in the culture medium allows us to obtain fully active brazzein and facilitate purification later. To increase yeast secretion, we compared seven different signal peptides to successfully achieve brazzein secretion using the yeast P. pastoris. The brazzein proteins corresponding to these signal peptides elicited activation of the sweet taste receptor functionally expressed in a cellular assay. Among these tested signal peptides, three resulted in the secretion of brazzein at high levels.

Improved A40926 production from Nonomuraea gerenzanensis using the promoter engineering and the co-expression of crucial genes

The semi-synthetic antibiotic dalbavancin is clinically used in the treatment of severe infections caused by multidrug resistant Gram-positive pathogens. So far, fermentation has still been the only approach for the production of A40926 in the industrial scale, which is used as the precursor of dalbavancin and biosynthesized by the rare actinomycete Nonomuraea gerenzanensis (N. gerenzanensis). Therefore, it is particularly essential and necessary to enhance the yield of A40926 continually. In this paper, we firstly assessed the activity of 6 heterologous promoters using the enhanced green fluorescence protein (EGFP) reporter system in N. gerenzanensis. Furthermore, the strongest constitutive promoter gapdh confirmed in this study was applied to separately overexpress the total of ten dbv genes involved in the A40926 biosynthesis. PCR and RT-qPCR were successively carried out to verify the mutant and the overexpression of dbv genes. As a consequence, the overexpression of dbv3 and dbv20 genes both increased the A40926 production remarkably. Based on the above consequences, a mutant strain named N320 laboring the co-expression of dbv3 and dbv20 was constructed. The results of fermentation showed that the N320 strain enhanced the yield of A40926 from 163 mg/L to 272 mg/L.

Removal of the N-terminal methionine improves the sweetness of the recombinant expressed sweet-tasting protein brazzein and its mutants in Escherichia coli

The sweet-tasting protein brazzein is the smallest sweet protein with high sweet potency. Overexpression of this protein in a heterogenous host is an essential way for its production in food industry. In this study, the gene of minor form brazzein was cloned into the pET-SUMO vector with optimized codon usage and expressed in E. coli BL21-CodonPlus (DE3)-RIL. The recombinant protein in absence of the N-terminal methionine displayed a sweetness threshold about 1.5 μg/ml, which is the sweetest brazzein protein reported up to now. The unexpected sweet potency of the protein was validated by a series of mutants (7Val → Arg, 9Glu → Lys and 9Glu → Asp), in which E9K exhibited about 50% enhancement of sweetness than the wild type. The superior sweetness of recombinant brazzein and its sweeter mutants suggest their potential applications in food and beverages. PRACTICAL APPLICATIONS: The sweet-tasting proteins are natural, low-, or non-caloric and nutritive, showing to be promising replacers of sugars and artificial sweeteners, and can be used as sweet additives in the fields of food, medicine, and biotechnology. In the present study, we report that the recombinant brazzein protein expressed in E. coli exhibits superior and improved sweetness than those previously reported. Furthermore, sweeter mutants were obtained with the expression procedure. The superior sweetness of recombinant brazzein and its sweeter mutants suggest their potential applications in food, beverages, and other fields.

Metabolic Effects of the Sweet Protein MNEI as a Sweetener in Drinking Water. A Pilot Study of a High Fat Dietary Regimen in a Rodent Model

Sweeteners have become integrating components of the typical western diet, in response to the spreading of sugar-related pathologies (diabetes, obesity and metabolic syndrome) that have stemmed from the adoption of unbalanced dietary habits. Sweet proteins are a relatively unstudied class of sweet compounds that could serve as innovative sweeteners, but their introduction on the food market has been delayed by some factors, among which is the lack of thorough metabolic and toxicological studies. We have tried to shed light on the potential of a sweet protein, MNEI, as a fructose substitute in beverages in a typical western diet, by studying the metabolic consequences of its consumption on a Wistar rat model of high fat diet-induced obesity. In particular, we investigated the lipid profile, insulin sensitivity and other indicators of metabolic syndrome. We also evaluated systemic inflammation and potential colon damage. MNEI consumption rescued the metabolic derangement elicited by the intake of fructose, namely insulin resistance, altered plasma lipid profile, colon inflammation and translocation of lipopolysaccharides from the gut lumen into the circulatory system. We concluded that MNEI could represent a valid alternative to fructose, particularly when concomitant metabolic disorders such as diabetes and/or glucose intolerance are present.

Getting value from the waste: recombinant production of a sweet protein by Lactococcus lactis grown on cheese whey

Background

Recent biotechnological advancements have allowed for the adoption of Lactococcus lactis, a typical component of starter cultures used in food industry, as the host for the production of food-grade recombinant targets. Among several advantages, L. lactis has the important feature of growing on lactose, the main carbohydrate in milk and a majoritarian component of dairy wastes, such as cheese whey.

Results

We have used recombinant L. lactis NZ9000 carrying the nisin inducible pNZ8148 vector to produce MNEI, a small sweet protein derived from monellin, with potential for food industry applications as a high intensity sweetener. We have been able to sustain this production using a medium based on the cheese whey from the production of ricotta cheese, with minimal pre-treatment of the waste. As a proof of concept, we have also tested these conditions for the production of MMP-9, a protein that had been previously successfully obtained from L. lactis cultures in standard growth conditions.

Conclusions

Other than presenting a new system for the recombinant production of MNEI, more compliant with its potential applications in food industry, our results introduce a strategy to valorize dairy effluents through the synthesis of high added value recombinant proteins. Interestingly, the possibility of using this whey-derived medium relied greatly on the choice of the appropriate codon usage for the target gene. In fact, when a gene optimized for L. lactis was used, the production of MNEI proceeded with good yields. On the other hand, when an E. coli optimized gene was employed, protein synthesis was greatly reduced, to the point of being completely abated in the cheese whey-based medium. The production of MMP-9 was comparable to what observed in the reference conditions.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0974-z) contains supplementary material, which is available to authorized users.

High-level production of single chain monellin mutants with enhanced sweetness and stability in tobacco chloroplasts

MAIN CONCLUSION

Plastid-based MNEI protein mutants retain the structure, stability and sweetness of their bacterial counterparts, confirming the attractiveness of the plastid transformation technology for high-yield production of recombinant proteins. The prevalence of obesity and diabetes has dramatically increased the industrial demand for the development and use of alternatives to sugar and traditional sweeteners. Sweet proteins, such as MNEI, a single chain derivative of monellin, are the most promising candidates for industrial applications. In this work, we describe the use of tobacco chloroplasts as a stable plant expression platform to produce three MNEI protein mutants with improved taste profile and stability. All plant-based proteins were correctly expressed in tobacco chloroplasts, purified and subjected to in-depth chemical and sensory analyses. Recombinant MNEI mutants showed a protein yield ranging from 5% to more than 50% of total soluble proteins, which, to date, represents the highest accumulation level of MNEI mutants in plants. Comparative analyses demonstrated the high similarity, in terms of structure, stability and function, of the proteins produced in plant chloroplasts and bacteria. The high yield and the extreme sweetness perceived for the plant-derived proteins prove that plastid transformation technology is a safe, stable and cost-effective production platform for low-calorie sweeteners, with an estimated production of up to 25-30 mg of pure protein/plant.

Ecotoxicological survey of MNEI and Y65R-MNEI proteins as new potential high-intensity sweeteners

Low-calorie sweeteners are widespread. They are routinely introduced into commonly consumed food such as diet sodas, cereals, and sugar-free desserts. Recent data revealed the presence in considerable quantities of some of these artificial sweeteners in water samples qualifying them as a class of potential new emerging contaminants. This study aimed at evaluating the ecotoxicity profile of MNEI and Y65R-MNEI, two engineered products derived from the natural protein monellin, employing representative test organism such as Daphnia magna, Ceriodaphnia dubia, and Raphidocelis subcapitata. Potential genotoxicity and mutagenicity effects on Salmonella typhimurium (strain TA97a, TA98, TA100, and TA1535) and Escherichia coli (strain WP2 pkM101) were evaluated. No genotoxicity effects were detected, whereas slight mutagenicity was highlighted by TA98 S. typhimurium. Ecotoxicity results evidenced effects approximately up to 14 and 20% with microalgae at 500 mg/L of MNEI and Y65R-MNEI, in that order. Macrophytes and crustaceans showed no significant effects. No median effective concentrations were determined. Overall, MNEI and Y65R-MNEI can be classified as not acutely toxic for the environment.