Transfer and expression of heterologous genes in yeasts other than Saccharomyces cerevisiae

Bioproduction of the Recombinant Sweet Protein Thaumatin: Current State of the Art and Perspectives

There is currently a worldwide trend to reduce sugar consumption. This trend is mostly met by the use of artificial non-nutritive sweeteners. However, these sweeteners have also been proven to have adverse health effects such as dizziness, headaches, gastrointestinal issues, and mood changes for aspartame. One of the solutions lies in the commercialization of sweet proteins, which are not associated with adverse health effects. Of these proteins, thaumatin is one of the most studied and most promising alternatives for sugars and artificial sweeteners. Since the natural production of these proteins is often too expensive, biochemical production methods are currently under investigation. With these methods, recombinant DNA technology is used for the production of sweet proteins in a host organism. The most promising host known today is the methylotrophic yeast, Pichia pastoris. This yeast has a tightly regulated methanol-induced promotor, allowing a good control over the recombinant protein production. Great efforts have been undertaken for improving the yields and purities of thaumatin productions, but a further optimization is still desired. This review focuses on (i) the motivation for using and producing sweet proteins, (ii) the properties and history of thaumatin, (iii) the production of recombinant sweet proteins, and (iv) future possibilities for process optimization based on a systems biology approach.

Recombinant protein production in yeasts

Recombinant DNA (rDNA) technologies (genetic, protein, and metabolic engineering) allow the production of a wide range of peptides, proteins, and biochemicals from naturally nonproducing cells. These technologies, now approx 25 yr old, have become one of the most important technologies developed in the twentieth century. Pharmaceutical products and industrial enzymes were the first biotech products on the world market made by means of rDNA. Despite important advances in rDNA applications in mammalian cells, yeasts still represent attractive hosts for the production of heterologous proteins. In this review we summarize advantages and limitations of the main and most promising yeast hosts.

Improvement of downstream processing of recombinant proteins by means of genetic engineering methods

The rapid advancement of genetic engineering has allowed to produce an impressive number of proteins on a scale which would not have been achieved by classical biotechnology. At the beginning of this development research was focussed on elucidating the mechanisms of protein overexpression. The appearance of inclusion bodies may illustrate the success. In the meantime, genetic engineering is not only expected to achieve overexpression, but to improve the whole process of protein production. For downstream processing of recombinant proteins, the synthesis of fusion proteins is of primary importance. Fusion with certain proteins or peptides may protect the target protein from proteolytic degradation and may alter its solubility. Intracellular proteins may be translocated by means of fusions with signal peptides. Affinity tags as fusion complements may render protein separation and purification highly selective. These methods as well as similar ones for improving the downstream processing of proteins will be discussed on the basis of recent literature.

Physiological and technological aspects of large-scale heterologous-protein production with yeasts

Commercial production of heterologous proteins by yeasts has gained considerable interest. Expression systems have been developed for Saccharomyces cerevisiae and a number of other yeasts. Generally, much attention is paid to the molecular aspects of heterologous-gene expression. The success of this approach is indicated by the high expression levels that have been obtained in shake-flask cultures. For large-scale production however, possibilities and restrictions related to host-strain physiology and fermentation technology also have to be considered. In this review, these physiological and technological aspects have been evaluated with the aid of numerical simulations. Factors that affect the choice of a carbon substrate for large-scale production involve price, purity and solubility. Since oxygen demand and heat production (which are closely linked) limit the attainable growth rate in large-scale processes, the biomass yield on oxygen is also a key parameter. Large-scale processes impose restrictions on the expression system. Many promoter systems that work well in small-scale systems cannot be implemented in industrial environments. Furthermore, large-scale fed-batch fermentations involve a substantial number of generations. Therefore, even low expression-cassette instability has a profound effect on the overall productivity of the system. Multicopy-integration systems may provide highly stable expression systems for industrial processes. Large-scale fed-batch processes are typically performed at a low growth rate. Therefore, effects of a low growth rate on the physiology and product formation rates of yeasts are of key importance. Due to the low growth rates in the industrial process, a substantial part of the substrate carbon is expended to meet maintenance-energy requirements. Factors that reduce maintenance-energy requirements will therefore have a positive effect on product yield. The relationship between specific growth rate and specific product formation rate (kg product.[kg biomass]-1.h-1) is the main factor influencing production levels in large-scale production processes. Expression systems characterized by a high specific rate of product formation at low specific growth rates are highly favourable for large-scale heterologous-protein production.

Heterologous protein production in yeast

The exploitation of recombinant DNA technology to engineer expression systems for heterologous proteins represented a major task within the field of biotechnology during the last decade. Yeasts attracted the attention of molecular biologists because of properties most favourable for their use as hosts in heterologous protein production. Yeasts follow the general eukaryotic posttranslational modification pattern of expressed polypeptides, exhibit the ability to secrete heterologous proteins and benefit from an established fermentation technology. Aside from the baker's yeast Saccharomyces cerevisiae, an increasing number of alternative non-Saccharomyces yeast species are used as expression systems in basic research and for an industrial application. In the following review a selection from the different yeast systems is described and compared.

Gene expression in yeast: Pichia pastoris

Recent studies have shown the versatility and utility of the Pichia pastoris expression system. Improvements in strains have boosted the yield of proteins and peptides to the commercially feasible range. The Pichia pastoris expression system will soon be used to manufacture proteins for human clinical trials.