Construction of a heterologous protein secretion system using the protein disulfide isomerase Pdi1p in Schizosaccharomyces pombe

The fission yeast Schizosaccharomyces pombe has been often used as a host for heterologous protein production; however, a method for extracellular secretion of heterologous protein would be advantageous for ease of purification and for native protein structure. In a previous study, overexpression of endogenous protein disulfide isomerase (PDI) genes improved the secretion of recombinant human transferrin in S. pombe. In the present study, we have explored whether Pdi1 can be used for the secretion of heterologous proteins in S. pombe. Overexpression of a fusion protein of Pdi1p and the heterologous protein EGFP (Pdi1p-EGFP), in the host S. pombe A8 strain, which lacks eight intracellular and extracellular proteases, resulted in efficient extracellular secretion of the fusion protein. To identify the optimal region of Pdi1p for use as an extracellular carrier, we compared the secretion of EGFP fused to the N-terminal Pdi1p signal domain and deletion mutants of Pdi1p. The signal sequence alone did not improve secretion, but deletion of two domains at the C-terminus did improve secretion. Notably, the x domain was important for secretion of the fusion protein. As a result of these findings, we have established a system for efficient secretion of target heterologous proteins by using optimally designed Pdi1p as a carrier for extracellular secretion.