Inteins are selfish DNA elements inserted in-frame and translated together with their host proteins. This precursor protein undergoes an autocatalytic protein splicing reaction resulting in two products: the host protein and the intein. ( Pietrokovski S. (2001). Inteins- Protein Introns. http://bioinfo.weizmann.ac.il/~pietro/inteins)

An intein is a segment of a protein that is able to excise itself and rejoin the remaining portions (the exteins) with a peptide bond. Inteins have also been called "protein introns". (Anraku Y, Mizutani R, Satow Y (2005). "Protein splicing: its discovery and structural insight into novel chemical mechanisms". IUBMB Life 57 (8): 563-74.)

Intein-mediated protein splicing occurs after mRNA has been translated into a protein. This precursor protein contains three segments - an N-extein followed by the intein followed by a C-extein. After splicing has taken place, the result is also called an extein. (de Grey, Aubrey D. N. J. (2000): Mitochondrial gene therapy: an arena for the biomedical use of inteins. Trends in Biotechnology 18(9): 394-399).

Inteins can contain a homing endonuclease gene domain in addition to the splicing domains. This domain is responsible for the spread of the intein by cleaving DNA at an intein free allele on the homologous chromosome, triggering the DNA double-stranded break repair (DSBR) system, which then repairs the break, thus copying the intein into a previously intein free site. The HEG domain is not necessary for intein splicing, and so it can be lost, forming a minimal, or mini intein. Several studies have demonstrated the modular nature of inteins by adding or removing HEG domains and determining the activity of the new construct.

Sometimes, the intein of the precursor protein comes from two genes. In this case, the intein is said to be a split intein. For example, in Cyanobacteria, DnaE, the catalytic subunit alpha of DNA polymerase III, is encoded by two separate genes, dnaE-n and dnaE-c. The dnaE-n product consists of an N-extein sequence followed by a 123-aa (amino acid) intein sequence, whereas the dnaE-c product consists of a 36-aa intein sequence followed by a C-extein sequence. (de Grey, Aubrey D. N. J. (2000): Mitochondrial gene therapy: an arena for the biomedical use of inteins. Trends in Biotechnology 18(9): 394-399).

Intein is the protein equivalent of intron and has been discovered in increasing numbers of organisms and host proteins. A self-splicing intein catalyzes its own removal from the host protein through a posttranslational process of protein splicing. A mobile intein displays a site-specific endonuclease activity that confers genetic mobility to the intein through intein homing. Recent findings of intein structure and the mechanism of protein splicing illuminated how inteins work and yielded clues regarding intein's origin, spread, and evolution. Inteins can evolve into new structures and new functions, such as split inteins that do trans-splicing. The structural basis of intein function needs to be identified for a full understanding of the origin and evolution of this marvelous genetic element. (Xiang-Qin Liu: Protein-splicing intein: Genetic mobility, origin, and evolution: Vol. 34: 61-76 (Volume publication date December 2000)).

Inteins are found in all three domains of life and in viruses, but have a very sporadic distribution. Only a small number of intein coding sequences have been identified in eukaryotic nuclear genes, and all of these are from ascomycete or basidiomycete fungi.

The result of intein excision is two proteins derived from a single initial translation product: (i) the free intein sequence, and (ii) the mature form of the host protein, with the two halves (the N-terminal and C-terminal external proteins, or exteins) ligated by a peptide bond. The reactions in which the intein is excised from the precursor protein and the flanking exteins are joined are mediated primarily by the intein itself, although the first residue of the C-extein also has an important role. The term intein strictly refers to a protein molecule, but the gene segment encoding the intein is also often referred to as an intein. (Timothy JD Goodwin, Margaret I Butler: Multiple, non-allelic, intein-coding sequences in eukaryotic RNA polymerase genes: Volume 4; 2006, Published online 2006 October 27)

Protein splicing was controlled by splitting precursor proteins within the Psp Pol-1 intein and expressing the resultant fragments in separate hosts. Reconstitution of an active intein was achieved by in vitro assembly of precursor fragments. Both splicing and intein endonuclease activity were restored. Complementary fragments from two of the three fragmentation positions tested were able to splice in vitro. Fragments resulting in redundant overlaps of intein sequences or containing affinity tags at the fragmentation sites were able to splice. Fragment pairs resulting in a gap in the intein sequence failed to splice or cleave (M W Southworth, E Adam, D Panne, R Byer, R Kautz, and F B Perler: Control of protein splicing by intein fragment reassembly: Volume 17(4); February 16, 1998)

(Structure des proteins: BCM-514: http://pages.usherbrooke.ca/bcm-514-bl/2c.html)

One interesting thing about this whole process is that it involves only the rearrangement of the peptide bonds and not the cleavage of them, and therefore requires no external energy (ATP for example).( Steve-O: Inteins: 03 Apr 2007)

Most of the inteins that have been described are in the 400- to 500-aa range with little absolute sequence conservation among the elements. However, Cys or Ser residues are required at the amino termini of both the intein and the second extein, and a His and Asn are present at the carboxy terminus of the intein. Most inteins contain eight conserved sequence blocks (A-H), two of these being the LAGLIDADG motifs (blocks C and E) that define a family of intron-homing endonucleases. Consistent with the occurrence of these motifs, several inteins have been shown to have site-specific endonuclease activity, and PI-SceI, the VMA1 intein of Saccharomyces cerevisiae, is capable of homing into a cognate inteinless allele. The sporadic distribution of inteins in all three biological kingdoms is consistent with their being mobile elements

Inteins are protein-intervening sequences that can self-excise and concomitantly splice together the flanking polypeptides. Two-piece split inteins capable of protein trans-splicing have been found in nature and engineered in laboratories, but they all have a similar split site corresponding to the endonuclease domain of the intein. Can inteins be split at other sites and do trans-splicing? After testing 13 split sites engineered into a Ssp DnaB mini-intein, we report the finding of three new split sites that each produced a two-piece split intein capable of protein trans-splicing. These three functional split sites are located in different loop regions between beta-strands of the intein structure, and one of them is just 11 amino acids from the beginning of the intein. Because different inteins have similar structures and similar beta-strands, these new split sites may be generalized to other inteins. We have also demonstrated for the first time that a three-piece split intein could function in protein trans-splicing. These findings have implications for intein structure-function, evolution, and uses in biotechnology.(Wenchang Sun, Jing Yang, and Xiang-Qin Liu: Synthetic two-piece and three-piece split inteins for protein trans-splicing: Vol. 279 Issue 34, 35281-35286, May 17, 2004)

Protein trans-splicing involving naturally or artificially split inteins results in two polypeptides being linked together by a peptide bond. While this phenomenon has found a variety of applications in chemical biology and biotechnology, precious little is known about the molecular recognition events governing the initial fragment association step. In this study, fluorescence approaches have been used to measure the dissociation constant for the Ssp DnaE split intein interaction and to determine the on and off rates of fragment association. The DnaE fragments bind with low nanomolar affinity, and our data suggest that electrostatics make an important contribution to the very rapid association of the fragments at physiological pH. This information was used to develop a tandem trans-splicing system based on native and engineered split inteins. This novel system allows the one-pot assembly of three polypeptides under native conditions and can be performed in crude cell lysates. The technology should provide a convenient approach to the segmental isotopic or fluorogenic labeling of specific domains within the context of large multidomain proteins

(Jianxin Shi and Tom W. Muir: Development of a Tandem Protein Trans-Splicing System Based on Native and Engineered Split Inteins: J. Am. Chem. Soc., 127 (17); April 8, 2005)