Fig. 5. SonM—SonA-BBD—(±)SAM structure showcases closed and open conformations.

a One SonM—SonA-BBD heterodimer (green and cyan cartoon, right) has SAM bound (orange sticks) in the active site with the top and side clamps in a previously seen closed conformation. The other SonM—SonA-BBD heterodimer (magenta and yellow cartoon, left) lacks a bound cofactor and display an open conformation for both top and side clamps. b Structural overlays of the BBDs from the apo SonM—SonA-2Me complex (slate cartoon) and the heterodimer in the open conformation for the SonM—SonA-BBD—(±)SAM complex (purple cartoon). The BBDs bind similarly but appear to be translated from each other. c Structural overlays of the active sites from the apo SonM—SonA-2Me complex (white and slate cartoon) and the heterodimer in the open conformation for the SonM—SonA-BBD—(±)SAM complex (green cartoon). Loop differences and residue shifts (sticks) are highlighted. Key distances are depicted as black dashed lines and their lengths noted in italics in Ångstroms. We note that crystal contacts may contribute to the configuration of the open form of the top clamp. However, the observation of both open and closed forms of the top clamp within the same crystal strongly suggests the two conformations precede crystal packing.