In vitro splicing of erythropoietin by the Mycobacterium tuberculosis RecA intein without substituting amino acids at the splice junctions

Protein splicing is a self-catalyzed process involving the excision of an intervening polypeptide sequence, the intein, and joining of the flanking polypeptide sequences, the extein, by a peptide bond. We have studied the in vitro splicing of erythropoietin (EPO) using a truncated form of the Mycobacterium tuberculosis RecA mini-intein in which the homing endonuclease domain was replaced with a hexahistidine sequence (His-tag). The intein was inserted adjacent to cysteine residues to assure that the spliced product had the natural amino acid sequence. When expressed in Escherichia coli, intein-containing EPO was found entirely as inclusion bodies but could be refolded in soluble form in the presence of 0.5 M arginine. Protein splicing of the refolded protein could be induced with a reducing agent such as DTT or tris(2-carboxyethyl)phosphine and led to the formation of EPO and mini-intein along with some cleavage products. Protein splicing mediated by the RecA intein requires the presence of a cysteine residue adjacent to the intein insertion site. We compared the efficiencies of protein splicing adjacent to three of the four cysteine residues of EPO (Cys29, Cys33 and Cys161) and found that insertion of intein adjacent to Cys29 allowed far more efficient protein splicing than insertion adjacent to Cys33 or Cys161. For ease of purification, our experiments involved a His-tagged EPO fusion protein and a His-tagged intein and the spliced products (25 kDa EPO and 24 kDa mini-intein) were identified by Western blotting using anti-EPO and anti-His-tag antibodies and by mass spectroscopy. The optimal splicing yield at Cys29 (40%) occurred at pH 7.0 after refolding at 4 degrees C and splicing for 18 h at 25 degrees C in the presence of 1 mM DTT.