STRUCTURAL AND FUNCTIONAL-ANALYSIS OF ECORI DNA METHYLTRANSFERASE BY PROTEOLYSIS

Native EcoRI DNA methyltransferase (Mtase, M(r) 38 050) is proteolyzed by trypsin to generate an intermediate 36-kDa fragment (p36) followed by the formation of two polypeptides of M(r) 23 000 and 13 000 (p23 and p13, respectively). Protein sequence analysis of the tryptic fragments indicates that p36 results from removal of the first 14-or 16 amino acids, p23 spans residues 15-216, and p13 spans residues 217-325. The relative resistance to further degradation of p23 and p13 suggests stable domain structures. This is further supported by the generation of similar fragments with SV8 endoprotease which has entirely different peptide specificities. Our results suggest the Mtase is a two-domain protein connected by a highly flexible interdomain hinge. The putative hinge region encompasses previously identified peptides implicated in AdoMet binding [Reich, N. O., & Everett, E. (1990) J. Biol. Chem. 265, 8929-8934] and catalysis [Everett et al. (1990) J. Biol. Chem. 265, 17713-17719]. Protection studies with DNA, S-adenosylmethionine (AdoMet), S-adenosylhomocysteine (AdoHcy), and sinefungin (AdoMet analogue) show that the Mtase undergoes significant conformational changes upon ligand binding. Trypsinolysis of the AdoMet-bound form of the Mtase generates different fragments, and the AdoMet-bound form is over 800 times more stable than unbound Mtase. The sequence-specific ternary complex (Mtase-DNA-sinefungin) is 2000 times more resistant to degradation by trypsin; cleavage eventually generates 26- and 12-kDa fragments which span residues 104-325 and 1-103, respectively (p26 and p12). The first 14 or 16 amino acids of the Mtase are not essential since p36 retains activity. Activity analysis of the p26 and p12 mixture also indicates retention of activity. Therefore, either p26 is catalytically active or the two fragments remain associated to create a functional enzyme. The former rationale is supported by the fact that for EcoRI Mtase, all peptide regions implicated in DNA binding, AdoMet binding, and catalysis reside in the p26 fragment.