What Is an Intein

The discovery of inteins and protein splicing represented a breakthrough in our concept of the catalytic repertoire of proteins and of post-translational modification (Perler, this Vol.). Conserved residues at the intein-extein junctions facilitate splicing (Mills and Paulus, this Vol.; Perler, this Vol.). Several inteins are bifunctional proteins that not only catalyze protein splicing, but also function as endonucleases, to initiate homing of the intein gene (Figs. 1 and 2A). Additionally, several inteins have motifs suggesting an evolutionary relationship to intron-encoded homing endonucleases. The endonuclease and the protein-splicing component are genetically, structurally, and functionally separable (Dassa and Pietrokovski, this Vol.; Moure and Quiocho, this Vol.; Perler, this Vol.), supporting the hypothesis that the endonuclease genes invaded the genes of these self-splicing elements, which provided safe havens, while themselves acquiring mobile properties (Fig. 1).

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