Yeast expression platforms

Yeasts can be used to produce proteins and sugars. Yeasts differ in productivity and with respect to their capabilities to secrete, to process and to modify proteins. The different 'platforms' of types of yeast make them better suited for different cooking and industrial applications.

Use of organics in creating medicine and products

Since the onset of gene technology, a plethora of bacterial microorganisms, fungi and mammalian cells have been developed for the production of foreign proteins. These proteins are used in medicine and industry to create products such as pharmaceuticals like hepatitis B vaccines or insulin. Common organic 'platforms' for the development of medicine and products include the bacterium E. coli, and several yeasts and mammalian cells, most of them derived from Chinese hamster cells. In general a system used for production has to meet several criteria: it should be able grow rapidly in large fermenters, it should produce proteins in an efficient way, it should be safe and, in case of pharmaceuticals, it should produce and modify the products as “human” as possible.

Yeasts include a great diversity of organisms

In general, fungi are excellent hosts for the production of recombinant proteins. They offer a desired ease of genetic manipulation and rapid growth to high cell densities on inexpensive media. As eukaryotes, they are able to perform protein modifications like glycosylation (addition of sugars), thus producing even complex foreign proteins that are identical or very similar to native products from plant or mammalian sources. The first yeast expression platform was based on the commonly known baker’s yeast "Saccharomyces cerevisiae". However the baker’s yeast is only one of more than 800 different yeasts with different characteristics and capabilities. For instance some of them grow on a wide range of carbon sources and are not restricted to glucose, as it is the case with baker’s yeast. Several of them are also applied to genetic engineering and to the production of foreign proteins. Here a selection:

"Arxula adeninivorans" ("Blastobotrys adeninivorans")

A dimorphic yeast (it grows as a budding yeast like the baker’s yeast up to a temperature of 42 °C, above this threshold it grows in a filamentous form) with unusual biochemical characteristics. It can grow on a wide range of substrates and can assimilate nitrate. It has successfully been applied to the generation of strains that can produce natural plastics or the development of a biosensor for estrogens in environmental samples (see also Wikipedia article "Arxula adeninivorans").

"Candida boidinii":

A methylotrophic yeast (it can grow on methanol). Like other methylotrophic species (see "Hansenula polymorpha" and "Pichia pastoris") it provides an excellent platform for the production of foreign proteins. Yields in a multigram range of a secreted foreign protein have been reported.

"Hansenula polymorpha" ("Pichia angusta"):

A methylotrophic yeast (see "Candida boidinii"). It can furthermore grow on a wide range of other substrates; it is thermo-tolerant and can assimilate nitrate (see also Kluyveromyces lactis). It has been applied to the production of hepatitis B vaccines, insulin and interferon alpha-2a for the treatment of hepatitis C, furthermore to a range of technical enzymes (see also wickipedia article "Hansenula polymorpha").

"Kluyveromyces lactis":

A yeast regularly applied to the production of kefir. It can grow on several sugars, most importantly on lactose which is present in milk and whey. It has successfully been applied among others to the production of chymosin (an enzyme that is usually present in the stomach of calves) for the production of cheese. Production takes place in fermenters on a 40,000 L scale.

"Pichia pastoris":

A methylotrophic yeast (see "Candida boidinii" and "Hansenula polymorpha"). It provides an efficient platform for the production of foreign proteins. Platform elements are available as a kit and it is worldwide used in academia for the production of proteins. Strains have been engineered that can produce complex human N-glycan (yeast glycans are similar but not identical to those found in humans).

"Saccharomyces cerevisiae":

The traditional baker’s yeast known to all readers for its use in brewing and baking and for the production of alcohol. Often the collective term “yeast” is used for this single species. As protein factory it has successfully been applied to the production of technical enzymes and of pharmaceuticals like insulin and hepatitis B vaccines.

"Yarrowia lipolytica":

A dimorphic yeast (see "Arxula adeninivorans") that can grow on a wide range of substrates. It has a high potential for industrial applications but there are no recombinant products commercially available yet.

Do the various yeasts perform in an identical way?

The answer is “no”. They differ in productivity and with respect to their capabilities to secrete, to process and to modify proteins in particular examples. First we explain how a yeast becomes a producer of foreign proteins.Suitable yeast strains are transformed by a vector, a so-called plasmid that contains all necessary genetic elements for recognition of a transformed strain and the genetic advice for the production of a protein. The elements are summarized in the following:

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# A selection marker, required to select a transformed strain from an untransformed background – this can be done if for instance such an element enables a deficient strain to grow under culturing conditions void of a certain indispensable compound like a particular amino acid that cannot be produced by the deficient strain.
# Certain elements to propagate and to target the foreign DNA to the chromosome of the yeast (ARS and/or rDNA sequence).
# A segment responsible for the production of the desired protein compound a so-called expression cassette. Such a cassette is made up by a sequence of regulatory elements, a promoter that controls, how much and under which circumstances a following gene sequence is transcribed and as a consequence how much protein is eventually made. This means that the segment following the promoter is variable depending on the desired product – it could be for instance a sequence determining the amino acids for insulin, for hepatitis B vaccine or for IFN alpha-2a. The expression cassette is terminated by a following terminator sequence that provides a proper stop of the transcription. The promoter elements of the "H. polymorpha" system are derived from genes that are highly expressed. Some of them are not only very strong, but can also be regulated by certain addition of carbon sources like sugar, methanol or glycerol. However, most of them can only be recognized by a single yeast species.

However, since the yeasts differ in their characteristics to produce a certain protein it cannot be excluded at the beginning of a development that a selected yeast will not be able to produce the desired compound at all. This in turn can lead to costly time-consuming failures. It is therefore advisable to assess several yeast platforms in parallel for their capabilities to produce such a compound. Therefore, a plasmid system was developed that can be targeted in functional form to all yeast in parallel. The basic design of this vector system, designated CoMed, is shown in the following figure. It is composed in modular way of element for selection, a “universal” targeting sequence that is present in all yeasts (the rDNA) and it contains within the expression cassette a promoter that is active in all yeasts (Fig. 1).

References

*Gellissen G (ed) (2005) Production of recombinant proteins - novel microbial and eukaryotic expression systems. Wiley-VCH, Weinheim.ISBN 3-527-31036-3