PROTEIN SPLICING - A NOVEL FORM OF GENE-EXPRESSION AND PARADIGM FOR SELF-CATALYZED PROTEIN REARRANGEMENTS

Protein splicing is one of the mechanisms by which genes that are inte rrupted by intervening sequences can produce functional proteins. It i nvolves the self-catalyzed excision of an internal segment from an ina ctive precursor protein and the ligation of the flanking N- and C-term inal segments to yield an active protein. A key reaction in protein sp licing is the rearrangement of a peptide bond involving the amino grou p of serine or cysteine to an ester bond. Such N-O or N-S acyl shifts are also the basis of other self-catalyzed protein rearrangements, whi ch include the cleavage of hedgehog proteins and certain amidotransfer ases and the formation of pyruvoyl enzymes. Although N-O or N-S acyl r earrangements are thermodynamically unfavorable, their coupling to sel f-catalyzed irreversible steps drives the protein rearrangements to co mpletion. In protein splicing, these steps are intramolecular transest erification followed by asparagine cyclization and peptide bond cleava ge. All steps of protein splicing are catalyzed by the intervening seq uence, which is a composite protein with separate catalytic centers fo r protein splicing and DNA homing endonuclease activity. Experiments a re in progress to study the structure and function of the catalytic ce nter for protein splicing by the genetic dissection of the intervening sequence.