Typical PD-(D/E)XK nuclease structure. Ribbon representation for archaeal holliday junction resolvase (Hjc, pdb:1gef) with the critical motif I (E9), motif II (D33), motif III (E46, K48) and motif IV (D61) active site residues shown as balls-and-sticks. Secondary structure elements of the conserved core fold are labeled and colored blue (α-helix) and yellow (β-strand), while additional insertions to the core are colored violet. In several detected families, conserved motif III lysine migrates to motif IV where (frequently mutated to arginine) it may occupy a similar spatial position (highlighted in grey) and thus play an equivalent functional role.
Knizewski et al. BMC Structural Biology 2007 7:40 doi:10.1186/1472-6807-7-40