Expressed protein ligation is a simple and powerful method in protein engineering to introduce sequences of unnatural amino acids, posttranslational modifications, and biophysical probes into proteins of any size. This methodology has been developed based on the knowledge obtained from protein splicing. Protein splicing is a multistep biochemical reaction that includes the concomitant cleavage and formation of peptide bonds carried out by self-processing domains named inteins. The natural substrates of protein splicing are essential proteins found in intein-containing organisms; inteins are also functional in nonnative frameworks and can be used to alter nearly any protein's primary amino acid sequence. Accordingly, different reactivity features of inteins have been largely exploited to manipulate proteins in countless methods encompassing fields from biochemical research to the development of biotechnological applications including the study of disease progression and validation of potential drug candidates. Here, we review almost three decades of research to uncover the chemical and biochemical enigmas of protein splicing and the development of inteins as potent protein engineering tools.
Keywords: Conditional protein splicing; Expressed protein ligation; Extein; Intein; Split intein; Trans-splicing.