Drosophila Metallothionein Expression System In the

Drosophila Mt. system, the inducible Mt. promotor is used to control the expression of recombinant proteins. This system offers the benefit of a well-known host and genomic structure with a tightly regulated promotor. In contrast to the viral system BEVS, where the production peak is found when the cells already die from the viral infection, Dro-sophila cells and the Mt. expression system can be used continuously.

Despite the different expression systems for insect cell lines, development of efficient fermentation techniques and process optimization are similar to the ones already described for mammalian cell types. High-cell-density reactors to achieve a maximum yield of desired product have been developed by various groups. Cavegn et al. established a system monitoring the production of insulin-receptor tyrosine kinase domain and the soluble part of endothelial leukocyte adhesion molecule as model proteins (44) and achieved up to 3 X 107 cells/mL in a serum-free perfusion bioreactor. Other developments are immobilized systems, such as cultivation on porous microcarrier (45), or the use of simulated microgravity to reduce hydrody-namic forces, as described by O'Connor et al. applying the rotating-wall vessel (46).

Examples of heterologous proteins expressed in insect cell lines can be found in Table 5. Investigations at the laboratory level indicate interesting areas still to be exploited for biotechnological purposes. The high level of expression of measles virus protein in SF 9 facilitates research into structure, function, and immunogenicity (53). Expression of recombinant bovine /¡-lactoglobulin for the processing of milk proteins to inhibit allergic reactions is another potential application (54). Because of the ability to express two or more proteins simultaneously in the bac-ulovirus system, analysis of multisubunit formation (e.g., for bluetongue viruslike particles) is possible (49).

Research activities are also concentrating on other insect host cells to improve the production process and, as an important requirement for clinically administered compounds, obtain a correctly glycosylated and biological active protein. SF 9 cells display only a very simple glyco-sylation capacity, whereas Estigmena acrea cells are able to produce complex glycoforms very similar to their authentic counterparts (55). Other insect cell lines investigated for the manufacture of recombinant proteins are Schneider 2 (Drosophila melanogaster) and Tr 5 (Tricho-plusia ni). Apart from using insect cell lines to facilitate production, they can also be applied in screening or cyto-toxicity tests.

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