Non‐canonical inteins: Alternate mechanisms for protein splicing

Abstract

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 as a class three intein, using an internal Cys to attack the peptide bond at the N-terminal splice junction. This Cys is part of a covariant non-contiguous Trp-Cys-Thr triplet found in class three inteins. We also show that an intein from Thermobifida fusca that has the conserved triplet, but also has an N-terminal nucleophile, cannot bypass the first step of splicing, and that the internal Cys is not essential for splicing. This material is based upon work supported by the National Science Foundation under grant MCB-0950245 and by the Camille and Henry Dreyfus Foundation.