kiintein

Kiintein, known in English as an intein, is a protein segment that can excise itself from a precursor protein and ligate the remaining portions, the exteins. This protein-splicing reaction typically occurs post-translationally within a single polypeptide chain, yielding a mature, functional protein without the intein sequence.

Inteins are embedded within host proteins across bacteria, archaea, and some eukaryotes. They can occur as contiguous sequences (intein within a single chain) or as split inteins, where the N- and C-terminal halves are expressed separately and only come together to perform splicing. In natural settings, inteins often contain a homing endonuclease domain that assists mobility and propagation of the element, though many modern inteins lack this domain.

Mechanistically, intein splicing proceeds through a series of nucleophilic attacks at conserved sites. In class 1 inteins, the N-terminal extein junction provides a nucleophile (Cys/Ser/Thr) that attacks to form a branched intermediate, followed by transesterification with the C-extein, then cyclization of an asparagine at the intein's C-terminus to excise the intein, and a final rearrangement that joins the exteins. Other classes (class 2 and class 3) differ in the initial steps and catalytic residues but converge on the same outcome.

Biotechnological use of inteins is widespread. Split and engineered inteins enable protein ligation, cyclization, and conditional protein splicing in vivo or in vitro, useful for tagless purification, segmental labeling, and construction of protein libraries. Research continues into expanding the repertoire, improving kinetics, and minimizing off-target effects.