Crystal Structure of a Mini-intein Reveals a Conserved Catalytic Module Involved in Side Chain Cyclization of Asparagine during Protein Splicing*

We have determined the crystal structure of a 154-residue intein derived from the dnaB gene of Synechocystis sp. strain PCC6803 and refined it to a 2.0-Å resolution. The x-ray structure suggests that this intein possesses two catalytic sites that appear to be separately responsible for splicing and cleavage of the N- and C-terminal scissile bonds. The conserved intein block F residues are the important components of a catalytic site for side chain cyclization of the last intein residue, Asn-154. The data suggest that the imidazole ring of His-143 is involved in the activation of the side chain Nδ atom of Asn-154, leading to a nucleophilic attack on the carbonyl carbon of Asn-154. Substitution of His-143 with Ala or Gln resulted in the inhibition of C-terminal cleavage. His-153, Asp-136, and a water molecule appear to constitute an oxyanion binding site by contacting the carbonyl oxygen of Asn-154 to stabilize the transition state. The structure and mutagenesis data also support that the close contact between the hydroxyl groups of Thr-138 and Ser-155, whose side chain participates in an S → O acyl shift, plays an important role in the nucleophile orientation. Our structural modeling suggests that this catalytic module is conserved in the C-terminal subdomains of inteins from diverse organisms.