Structural features of a minimal intact methyltransferase of a type I restriction-modification system

Type I restriction-modification enzymes are oligomeric proteins composed of methylation (M), DNA sequence-recognition (S), and restriction (R) subunits. The different bipartite DNA sequences of 2-4 consecutive bases are recognized by two discerned target recognition domains (TRDs) located at the two-helix bundle of the two conserved regions (CRs). Two M-subunits and a single S-subunit form an oligomeric protein that functions as a methyltransferase (M₂S₁ MTase). Here, we present the crystal structure of the intact MTase from Vibrio vulnificus YJ016 in complex with the DNA-mimicking Ocr protein and the S-adenosyl-L-homocysteine (SAH). This MTase includes the M-domain with a helix tail (M-tail helix) and the S_1/2-domain of a TRD and a CR α-helix. The Ocr binds to the cleft of the TRD surface and SAH is located in the pocket within the M-domain. The solution- and negative-staining electron microscopy-based reconstructed (M₁S_1/2)₂ structure reveals a symmetric (S_1/2)₂ assembly using two CR-helices and two M-tail helices as a pivot, which is plausible for recognizing two DNA regions of same sequence. The conformational flexibility of the minimal M₁S_1/2 MTase dimer indicates a particular state resembling the structure of M₂S₁ MTases.