Intein
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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".
Most reported inteins also contain an endonuclease domain that plays a role in intein propagation. In fact, many genes have unrelated intein-coding segments inserted at different positions. For these and other reasons, inteins (or more properly, the gene segments coding for inteins) are sometimes called selfish genetic elements but it may be more accurate to call them parasitic.
Intein-mediated protein splicing occurs after mRNA has been translated into a protein. This precursor protein contains three segements - an N-extein followed by the intein followed by a C-extein. After splicing has taken place, the result is also called an extein.
The first intein was discovered in 1987. Since then, inteins have been found in all three domains of life (eukaryotes, bacteria, and archaea). The mechanism for the splicing effect is nature's analogy to the technique for chemically generating large proteins called native chemical ligation, which was developed at the same time as inteins were discovered.
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Inteins in biotechnology
Inteins are very efficient at protein splicing and they have accordingly found an important role in biotechnology. Inteins have been engineered for particular applications such as protein synthesis, and the selective labeling of protein segments, which is useful for NMR studies of large proteins.
Pharamceutical inhibition of intein excision may be useful tool for drug development, the protein that contains the intein will not carry out its normal function if the intein does not excise since its structure will be disrupted.
Intein naming conventions
The first part of an intein name is based on the scientific name of the organism in which it is found, and the second part is based on the name of the corresponding gene or extein. For example, the intein found in Thermoplasma acidophilum and associated with 'Vacuolar ATPase subunit A' (VMA) is called 'Tac VMA'.
Normally, as in this example, just three letters suffice to specify the organism, but there are variations. For example, additional letters may be added to indicate a strain. If more than one intein is encoded in the corresponding gene, the inteins are given a numerical suffix starting from 5' to 3' or in order of their identification. For example, "Msm dnaB-1".
The segment of the gene that encodes the intein is usually given the same name as the intein, but to avoid confusion, the name of the intein proper is usually capitalized (e.g. Pfu RIR1-1), whereas the name of the corresponding gene segment is italicized.
Full and mini inteins
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 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.
Split inteins
Sometimes, the intein of the pre-cursor 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.
External links
- The Intein Database (http://www.neb.com/neb/intein_intro.html)
- Shmuel Pietrokovski's Intein database (http://bioinformatics.weizmann.ac.il/~pietro/inteins/)