An intein is an amino acid sequence of a protein that can cut itself ( autocatalytically ) out of it and reconnect the remaining pieces ( exeins ) with a peptide bond . Since some inteins also contain an endonuclease , their coding genes are a form of selfish DNA .
To date, inteins have been found in all three domains (super-rich) of life ( eukaryotes , bacteria and archaea ), as well as in viruses . As of 2014, over 116 inteins are known in eukaryotes, 1137 in bacteria and 381 in archaea, and 243 in viruses including bacteriophages . (In 2002 there were 113 in eukaryotes, 289 in bacteria, 182 in archaea, and as of 2005 only 2 in viruses) The lengths are between 138 and 844 amino acids. Most inteins contain an endonuclease domain which plays a role in intein dissemination. The endonuclease only cleaves intein- free alleles of the intein-containing gene (on the homologous chromosome ), so that when the double-strand break is repaired, the intein-coding DNA is introduced into this interface by the DNA repair system . Through this process the intein was increased and the cell is homozygous for the intein-containing gene, whereby the intein is automatically propagated to all daughter cells during cell division. That is why inteins (or rather the gene segments that code for the inteins) are referred to as “self-serving genetic elements”. However, it would be more precise to describe them as parasitic .
In analogy to the RNA sequences cut out during splicing , inteins are also referred to as protein introns , while autoproteolysis and relegation are referred to as protein splicing . Three mechanisms have been described in inteins so far.
Intein-mediated protein splicing occurs after the translation of the mRNA into a protein. This precursor protein (pre-cursor) contains at least three segments: an N-extein, an intein and a C-extein. After the splicing process, the result is also referred to as extein.
The first part of the intein name consists of the scientific name of the organism in which it was found. The second part is based on the gene or extein in which it occurs. For example, the second intein from would Thermococcus fumicolans in the gene of the DNA polymerase as Tfu Pol-2 are referred to. The intein numbering starts at the 5 'end of the gene.
Separate in one
In some cases, components of a precursor protein reside on different genes. This is called separate inteins ( English split intein ). An example of this is the catalytic subunit alpha of DNA polymerase III in cyanobacteria , such as Synechocystis sp. (Designation Ssp DnaE). The DnaE protein is encoded by two genes separated by 700 kbp. One contains the N-terminus (dnaE-n) and a 123 amino acid long intein sequence, and the second codes for a second 36 aa long intein sequence and the C-terminus (dnaE-c). The two protein parts are merged into one functional protein by trans-splicing .
Inteins are used in biotechnology, for example in protein synthesis, to selectively mark protein segments in the course of molecular marking, for example with heavy atoms. This is useful when studying large proteins using NMR . In addition, polypeptides can be linked to one another ( protein ligation ). This makes it possible, among other things, to express cytotoxic proteins, perform cycling (to increase stability) and examine protein structures .
The first intein was discovered in 1988 by comparing the sequence of the ATPases from Neurospora crassa , carrots and baker's yeast . The intein sequence was erroneously classified as a calcium ion transporter . In 1990 the loss of the intein sequence after translation was described.
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