Fig. 1.

Schematic representation of protein splicing in cis and trans. (A) Cis-splicing inteins excise themselves from a precursor where the N- and C-exteins flank the intein on the same polypeptide. (B) Protein trans-splicing (PTS) ligates N- and C-exteins, each originating from an independent polypeptide, with a covalent peptide bond resulting in a trans-spliced product. Interaction of the N- (Int_N) and C-terminal (Int_C) split intein halves initiates the protein-splicing reaction. The N-terminal junction sequence at the front of an intein is termed as the −1 position. The +1 position after the intein sequence usually has a Cys, Ser, or Thr residue. The second residue after an intein is numbered as the +2 position.