Non‐canonical inteins: Alternate mechanisms for protein splicing

Protein splicing involves the post‐translational excision of an intervening polypeptide (the intein) from flanking domains (the exteins), as well as ligation of the exteins. The first step of splicing is an amide‐ester rearrangement of the peptide bond that links the N‐terminal extein and intein. However, the intein that interrupts a putative phage terminase in C. thermocellum lacks an N‐terminal nucleophile and should not be able to promote this step. We show that this intein can splice, and have examined the influence of conserved residues on efficient splicing. The third step of splicing is cyclization of Asn coupled to peptide bond cleavage. However, the PolII intein from P. abyssi has a C‐terminal Gln and can still promote splicing. We compare the effects of C‐terminal Asn and Gln on splicing efficiency, as well as the influence of conserved residues on the catalysis of splicing. This material is based upon work supported by the National Science Foundation under Grant No. 0320824 and CAREER grant No. 0447647.