The Huck Institutes of the Life Sciences

Structural basis for methyl transfer by a radical SAM enzyme

Boal AK, Grove TL, McLaughlin MI, Yennawar NH, Booker SJ, and Rosenzweig AC (2011) Science 332(6033): 1089-92


The radical S-adenosyl-L-methionine (SAM) enzymes RlmN and Cfr methylate 23S ribosomal RNA, modifying the C2 or C8 position of adenosine 2503. The methyl groups are installed by a two-step sequence involving initial methylation of a conserved Cys residue (RlmN Cys355) by SAM. Methyl transfer to the substrate requires reductive cleavage of a second equivalent of SAM. Crystal structures of RlmN and RlmN with SAM show that a single molecule of SAM coordinates the [4Fe-4S] cluster. Residue Cys355 is S-methylated and located proximal to the SAM methyl group, suggesting the SAM that is involved in the initial methyl transfer binds at the same site. Thus, RlmN accomplishes its complex reaction with structural economy, harnessing the two most important reactivities of SAM within a single site.

The X-Ray Crystallography Facility was used to do initial X-ray screening on the crystals of radical SAM enzymes.

Boal et al. 2011 Figure 1A

The structure of E. coli RlmN with SAM (A). The α6/β6 partial barrel core is shown in dark purple, a three-strand β extension to the core (β'1-3 extension) is shown in light purple, an N-terminal helical accessory domain is shown in green, and a C-terminal extension (β7 extension) culminating in an α helix is shown in blue. The [4Fe-4S] cluster cofactor is represented as a space-filling model, and SAM is shown in stick format colored by atom type. Cys ligands to the [4Fe-4S] cluster and mechanistically important residues Cys118 and mCys355 are shown as purple and blue sticks, respectively, and colored by atom type.

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