His₆ tag-assisted chemical protein synthesis

To make more practical the total chemical synthesis of proteins by the ligation of unprotected peptide building blocks, we have developed a method to facilitate the isolation and handling of intermediate products. The synthetic technique makes use of a His₆ tag at the C terminus of the target polypeptide chain, introduced during the synthesis of the C-terminal peptide segment building block. The presence of a His₆ tag enables the isolation of peptide or protein products directly from ligation reaction mixtures by Ni-NTA affinity column purification. This simple approach enables facile buffer exchange to alternate reaction conditions and is compatible with direct analytical control by protein MS of the multiple ligation steps involved in protein synthesis. We used syntheses of crambin and a modular tetratricopeptide repeat protein of 17 kDa as models to examine the utility of this affinity purification approach. The results show that His₆ tag-assisted chemical protein synthesis is a useful method that substantially reduces handling losses and provides for rapid chemical protein syntheses.