Amino-terminal charge affects the periplasmic accumulation of recombinant heregulin/EGF hybrids exported using the Escherichia coli alkaline phosphatase signal sequence

An Escherichia coli expression system that exploits the bacterial alkaline phosphatase (PhoA) signal sequence to translocate recombinant human epidermal growth factor (hEGF) to the periplasm was used to evaluate how changes in the composition and sequence of amino acids near the PhoA-hEGF junction influence the periplasmic accumulation of recombinant protein. A series of chimeric structural genes was generated by in vitro replacement of hEGF sequence with analogous segments from the EGF-like domain of human heregulin (HRG), significantly altering the electrostatic character of the amino-terminal region of the mature protein. Quantitation of HRG/EGF protein in E. coli periplasmic extracts, by RP-HPLC, showed a fourfold decrease after one of two acidic residues located in the amino-terminal region of the mature hEGF, near the PhoA junction, was replaced. An additional threefold decrease was observed when the second acidic residue was replaced with a positively charged lysine. Further extension of the amino-terminal HRG sequence, beyond the first six residues, resulted in net neutralization of a more distant EGF acidic residue with no additional effect on protein yield. The importance of having a negatively charged group in the amino-terminal region of the mature protein was confirmed when insertion of an aspartic acid near the amino-terminus of two poorly expressed hybrid protein sequences resulted in a five- to eightfold increase in their recovery from the periplasm. This study demonstrates the importance of having negatively charged residues near the fusion junction of recombinant proteins expressed in E. coli using the PhoA signal sequence for protein export.