Designing Signal Peptides for Efficient Periplasmic Expression of Human Growth Hormone in Escherichia coli

The secretion efficiency of a protein in a Sec-type secretion system is mainly determined by an N-terminal signal peptide and its combination with its cognate protein. Five signal peptides, namely, two synthetic Sec-type and three Bacillus licheniformis alpha-amylase-derived signal peptides, were compared for periplasmic expression of the human growth hormone (hGH) in E. coli. Based on in silico predictions on the signal peptides' cleavage efficiencies and their corresponding mRNA secondary structures, a number of amino acid substitutions and silent mutations were considered in the modified signal sequences. The two synthetic signal peptides, specifically designed for hGH secretion in E. coli, differ in their N-terminal positively charged residues and hydrophobic region lengths. According to the mRNA secondary structure predictions, combinations of the protein and each of the five signal sequences could lead to different outcomes, especially when accessibility of the initiator ATG and ribosome binding sites were considered. In the experimental stage, the two synthetic signal peptides displayed complete processing and resulted in efficient secretion of the mature hGH in periplasmic regions, as was demonstrated by protein analysis. The three alpha-amylase-derived signal peptides, however, were processed partially from their precursors. Therefore, to achieve efficient secretion of a protein in a heterologous system, designing a specific signal peptide by using a combined approach of optimizations of the mRNA secondary structure and the signal peptide H-domain and cleavage site is recommended.