Possible variation in the protein-splicing reaction

Pietrokovski S.
"Identification of a virus intein and a possible variation in the protein-splicing reaction"
Current Biology, 8, R634-R635, (1998)
Glutamine is the carboxy-terminal residue of the CIV RIR1 intein and of the P.horikoshii polC intein. Only asparagine has previously been described at this position. This asparagine plays a key role in the protein-splicing reaction: it undergoes cyclization, releasing the amino-terminally cleaved intein from the ligated host flanks in the penultimate step of the reaction. I suggest that these two inteins, integrated in highly conserved protein motifs, are active and splice using a variation of the previously described protein-splicing reaction in which their carboxy-terminal glutamines undergo cyclization analogous to that of the intein carboxy-terminal asparagine forming a glutarimide ring. Such a cyclization is chemically plausible and has been predicted (Geiger and Clarke 1987, Daniel et al. 1996). These two inteins probably evolved their peculiar carboxy-terminal ends independently, as they are not particularly similar by sequence and are also different in the highly conserved penultimate residue.

Geiger, T. & Clarke, S. (1987). 
Deamidation, isomerization, and racemization at asparaginyl and aspartyl 
residues in peptides. Succinimide-linked reactions that contribute to 
protein degradation. 
J Biol Chem 262, 785-794.


Daniel, R. M., Dines, M. & Petach, H. H. (1996). 
The denaturation and degradation of stable enzymes at high temperatures. 
Biochem J 317, 1-11.
Inteins C-terminal end Third step in protein splicing reaction:
branch resolution by intein C-terminal residue cyclization
Typical inteins inteins C-terminal end current accepted model

CIV RIR1

Pho polC		 suggested variation of model
The C-terminal end of inteins and its cleavage from the ligated host protein. Top: The sequences found in the C-terminal end of a typical intein (left) and a scheme for the release of the semi-cleaved intein from the ligated host flanks (exteins) by asparagine cyclization (right). The sequences are represented by a logo, where the height of each residue is related to its frequency. In the scheme, X denotes the side-chain of the residue C-terminal to the asparagine: a sulfur atom (residue is cysteine) or an oxygen atom (residue is serine or threonine). Bottom: The C-terminal ends of the two known inteins with glutamine at the C-end and the mechanism proposed for their release from the ligated host flanks.
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Page last modified July 1998
Shmuel Pietrokovski <pietro@weizmann.ac.il>