In vivo and in vitro protein ligation by naturally occurring and engineered split DnaE inteins

Background: Protein trans-splicing by naturally occurring split DnaE inteins is used for protein ligation of foreign peptide fragments. In order to widen biotechnological applications of protein trans-splicing, it is highly desirable to have split inteins with shorter C-terminal fragments, which can be chemically synthesized. Principal Findings: We report the identification of new functional split sites in DnaE inteins from Synechocystis sp. PCC6803 and from Nostoc punctiforme. One of the newly engineered split intein bearing C-terminal 15 residues showed more robust protein trans-splicing activity than naturally occurring split DnaE inteins in a foreign context. During the course of our experiments, we found that protein ligation by protein trans-splicing depended not only on the splicing junction sequences, but also on the foreign extein sequences. Furthermore, we could classify the protein trans-splicing reactions in foreign contexts with a simple kinetic model into three groups according to their kinetic parameters in the presence of various reducing agents. Conclusion: The shorter C-intein of the newly engineered split intein could be a useful tool for biotechnological applications including protein modification, incorporation of chemical probes, and segmental isotopic labelling. Based on kinetic analysis of the protein splicing reactions, we propose a general strategy to improve ligation yields by protein trans-splicing, which could significantly enhance the applications of protein ligation by protein trans-splicing.