Protein Trans-Splicing of an Atypical Split Intein Showing Structural Flexibility and Cross-Reactivity

Inteins catalyze a protein splicing reaction to excise the intein from a precursor protein and join the flanking sequences (exteins) with a peptide bond. In a split intein, the intein fragments (I N and I C ) can reassemble non-covalently to catalyze a trans -splicing reaction that joins the exteins from separate polypeptides. An atypical split intein having a very small I N and a large I C is particularly useful for joining synthetic peptides with recombinant proteins, which can be a generally useful method of introducing site-specific chemical labeling or modifications into proteins. However, a large I C derived from an Ssp DnaX intein was found recently to undergo spontaneous C-cleavage, which raised questions regarding its structure-function and ability to trans -splice. Here, we show that this I C could undergo trans -splicing in the presence of I N , and the trans -splicing activity completely suppressed the C-cleavage activity. We also found that this I C could trans -splice with small I N sequences derived from two other inteins, showing a cross-reactivity of this atypical split intein. Furthermore, we found that this I C could trans -splice even when the I N sequence was embedded in a nearly complete intein sequence, suggesting that the small I N could project out of the central pocket of the intein to become accessible to the I C . Overall, these findings uncovered a new atypical split intein that can be valuable for peptide-protein trans -splicing, and they also revealed an interesting structural flexibility and cross-reactivity at the active site of this intein.