Novel signal peptides improve the secretion of recombinant Staphylococcus aureus Alpha toxin in Escherichia coli.

Secretion of heterologous proteins into Escherichia coli cell culture medium offers significant advantages for downstream processing over production as inclusion bodies; including cost and time savings, and reduction of endotoxin. Signal peptides play an important role in targeting proteins for translocation across the cytoplasmic membrane to the periplasmic space and release into culture medium during the secretion process. Alpha toxin (AT) was selected as an antigen for vaccine development against Staphylococcus aureus infections. It was successfully secreted into culture medium of E. coli by using bacterial signal peptides linked to the N-terminus of the protein. In order to improve the level of secreted AT, we designed a series of novel signal peptides by swapping individual domains of modifying dsbA and pelB signal peptides and tested them in a fed-batch fermentation process. The data showed that some of the modified signal peptides improved the secretion efficiency of AT compared with E. coli signal peptides from dsbA, pelB and phoA proteins. Indeed, one of the novel signal peptides improved the yield of secreted AT by 3.5-fold in a fed-batch fermentation process and at the same time maintained processing at the expected site for signal peptide cleavage. Potentially, these new novel signal peptides can be used to improve the secretion efficiency of other heterologous proteins in E. coli. Furthermore, analysis of the synthetic signal peptide amino acid sequences provides some insight into the sequence features within the signal peptide that influence secretion efficiency.