Structural comparison of the second Glu of the ARTT motif between SPNct and ADP-ribosyltransferases. (a) Superimposition of the active site residues (Glu389 and Glu391) of SPN with those of ADP-ribosyltransferases such as B. cereus VIP2 complexed with NAD, B. cereus VIP2, C. botulinum C2 at pH 3.0, C. botulinum C2 at pH 6.1, C. perfringens iota toxin, V. cholerae CT and E. coli heat-labile enterotoxin (LT). Other secondary structure elements except for the region containing the ARTT motif were removed for clarity. The regions containing the second Glu of the ARTT motif and β-NAD are marked with a dotted square. (b) A slightly different view of the dotted square in (a) for the superposition of SPNct and VIP2 complexed with NAD is shown in the box. A water molecule present in our SPNct structure is donoted by W (a purple sphere). Secondary structure elements of SPNct are labeled.