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Protein SplicingProtein splicing is an intramolecular reaction of a particular protein in which an internal protein segment (called an intein) is removed from a precursor protein with a ligation of C-terminal and N-terminal external proteins (called exteins) on both sides. The splicing junction of the precursor protein is mainly a cysteine or a serine, which are amino acids containing a nucleophilic side chain. The protein splicing reactions which are known now do not require exogenous cofactors or energy sources such as adenosine triphosphate (ATP) or guanosine triphosphate (GTP). Protein splicing was reported by two groups (Anraku and Stevens) in 1990. They both discovered a Saccharomyces cerevisiae VMA1 in a precursor of a vacuolar H+-ATPase. The amino acid sequence of the N- and C-termini corresponded to 70% of that of a vacuolar H+-ATPase from other organisms, while the amino acid sequence of the central position corresponded to 30% of that of the yeast HO nuclease. The type of the splicing proteins is categorized into four classes: maxi-intein, mini-intein, trans-splicing intein, and alanine intein. The maxi-inteins are N- and C-terminal splicing domains containing an endonuclease domain. The mini-inteins are typical N- and C-terminal splicing domains; however, the endonuclease domain is not present. The trans-splicing inteins are split inteins which are divided into N-termini and C-termini. Alanine inteins have the splicing junction of an alanine instead of a cysteine or a serine, in both of which the protein splicing occurs.
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