Synthesis and sequence-specific proteolysis of a hybrid protein (colicin A :: growth hormone releasing factor) produced in Escherichia coli

DNA constructs coding for human growth hormone (hGH)-releasing factor (hGRF) preceded by the specific recognition sequence for the activated blood coagulation factor X (FXa), fused in frame to the N-terminal 172-amino acid residues of colicin A, have been expressed in Escherichia coli. The construct was placed under the control of the inducible caa promoter in an operon containing a downstream gene coding for the cell lysis protein, Cal. Induction resulted in excretion of only the processed colicin A fragment. Replacement of Cal by the terminator from phage fd resulted in high expression of the hybrid protein, which was recovered as cytoplasmic aggregates. Enzymatic cleavage of the purified and renatured hybrid protein using FXa allowed the recovery of authentic hGRF.