Protein splicing of yeast VMA1-derived endonuclease via thiazolidine intermediates.

Protein splicing precisely excises out an internal intein segment from a protein precursor, and concomitantly ligates the N- and C-terminal extein polypeptides flanking the intein. A recombinant X10SNS bearing N- and C-extein polypeptides has been prepared for the intein endonuclease derived from the Saccharomyces cerevisiae VMA1 gene. X10SNS has replacements of C284S, H362N and C738S, and forms the intein and extein segments in the crystal lattice. The crystal structure of X10SNS revealed a linkage between the N- and C-extein segments, and showed that the C284 amino group of the resultant intein segment is in interaction with the G283 O atom of the N-extein segment. A mechanism for the final S --> N acyl shift step proposes that a tetrahedral intermediate involves a five-membered thiazolidine ring at G283-C738 junction. An oxyanion of the thiazolidine intermediate is to be stabilized by the C284 N atom.