A protein disulfide isomerase gene fusion expression system that increasesthe extracellular productivity of Bacillus brevis

We have developed a versatile Bacillus brevis expression and secretion syst em based on the use of fungal protein disulfide isomerase (PDI) as a gene f usion partner. Fusion with PDI increased the extracellular production of he terologous proteins (light chain of immunoglobulin G, 8-fold; geranylgerany l pyrophosphate synthase, 12-fold). Linkage to PDI prevented the aggregatio n of the secreted proteins, resulting in high-level accumulation of fusion proteins in soluble and biologically active forms. We also show that the di sulfide isomerase activity of PDI in a fusion protein is responsible for th e suppression of the aggregation of the protein with intradisulfide, wherea s aggregation of the protein without intradisulfide was prevented even when the protein was fused to a mutant PDI whose two active sites were disrupte d, suggesting that another PDI function, such as chaperone-like activity, s ynergistically prevented the aggregation of heterologous proteins in the PD I fusion expression system.