High-level production in Pichia pastoris of an anti-p185HER-2 single-chain antibody fragment using an alternative secretion expression vector

The methylotrophic yeast Pichia pastoris has become a highly popular expression host for the recombinant production of a wide variety of proteins. Initial success with this system was greatly facilitated by the development of versatile expression vectors that were almost exclusively based on the strong, tightly regulated promoter of the P. pastoris major alcohol oxidase gene (AOX1). For example, pIB4 is an Escherichia coli-P. pastoris shuttle vector that also uses the AOX1 promoter to allow intracellular expression of endogenous and foreign genes in the latter organism. Since the eukaryotic advantages of P. pastoris would be best harnessed through the secretory targeting of the recombinant proteins, we modified the pIB4 vector by adding the Saccharomyces cerevisiae (alpha)-factor secretion signal immediately upstream of its multiple cloning site. Here we describe the construction of this modified vector, pIB4(alpha), and its successful use for the high-level expression and secretion of a functional single-chain antibody fragment (scFv), C6.5, which targets p185HER-2, a cell-surface glycoprotein overexpressed in about 30% of human breast and ovarian cancers. The PCR strategy used for the subcloning of the C6.5 construct into pIB4(alpha) also introduced a short DNA sequence coding for a C-terminal hexahistidine tag, which allowed subsequent purification of the secreted scFv, by immobilized-metal-affinity chromatography, to a yield of 70 mg·l^(-1) of shake-flask culture. In conclusion, our results suggest that the secretion expression vector pIB4(alpha) not only complements the original pIB4 vector for intracellular expression in P. pastoris, but might also constitute an attractive alternative to the commercially available secretion expression vectors.