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We demonstrated the successful optimization of a recombinant multi-subunit malaria vaccine candidate protein for production in the methylotrophic yeast Pichia pastoris by the identification and subsequent removal of two protease cleavage sites. After observing protein degradation in the culture supernatant of a fed-batch fermentation, the predominant proteolytic fragment of the secreted recombinant protein was analyzed by mass spectrometry. The MS data indicated the cleavage of an amino acid sequence matching the yeast KEX2-protease consensus motif EKRE. The cleavage in this region was completely abolished by the deletion of the EKRE motif in a modified variant. This modified variant was produced, purified, and used for immunization of rabbits, inducing high antigen specific antibody titers (2 × 10(6) ). Total IgG from rabbit immune sera recognized different stages of Plasmodium falciparum parasites in immunofluorescence assays, indicating native folding of the vaccine candidate. However, the modified variant was still degraded, albeit into different fragments. Further analysis by mass spectrometry and N-terminal sequencing revealed a second cleavage site downstream of the motif PEVK. We therefore removed a 17-amino-acid stretch including the PEVK motif, resulting in the subsequent production of the full-length recombinant vaccine candidate protein without significant degradation, with a yield of 53 mg per liter culture volume. We clearly demonstrate that the proteolytic degradation of recombinant proteins by endogenous P. pastoris proteases can be prevented by the identification and removal of such cleavage sites. This strategy is particularly relevant for the production of recombinant subunit vaccines, where product yield and stability play a more important role than for the production of a stringently-defined native sequence which is necessary for most therapeutic molecules.

Metabolic engineering of plant natural product pathways in heterologous systems requires the highly concerted action of several biosynthetic genes. Besides the functional heterologous expression of the genes encoding the natural product biosynthetic pathway, often additional extensive modifications in the host primary metabolism are also needed, in order to obtain efficient supply of the required biosynthetic building blocks to support the engineered natural product biosynthesis. Selection markers in heterologous expression systems, like baker's yeast (Saccharomyces cerevisiae), are often limited and the chromosomal insertion prevents later modifications of engineered pathway, e.g. exchange of gene promoters, or the introduction of additional genetic regulatory elements in a timely manner. Thus the construction of biosynthetic gene clusters on episomal expression vectors seems a logical solution for this dilemma. Although manipulation of long DNA fragments still represents a challenge, by using PCR and in vitro homologous recombination, we assembled a biosynthetic gene cluster for the concerted heterologous expression of three important genes for the metabolic engineering of taxoid biosynthesis in yeast.

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