(a, Left panel) Active site of Prp43 with the bound ATP-analog ADP•BeF3- as present in ctPrp43ΔN•ADP•BeF3-(HR) (PDBid: 5ltj). The RecA1 domain is colored in light gray, the RecA2 domain in dark gray, carbon atoms of the ADP in yellow, oxygen in red, nitrogen in blue, phosphorus in orange, beryllium in chartreuse, fluoride in light blue, magnesium in light green and water molecules in pale pink. Nucleotide-interacting motifs are shown in blue, nucleic acid-binding motifs in ruby and motif III, which couples ATP hydrolysis to RNA unwinding, in green. Residues, which are involved in base-stacking with the adenine moiety, are presented in light orange. Residues from motif I, which are involved in Mg2+ coordination (Thr 126), and from motif II, which coordinate water molecules at the active site (Asp 218 and Glu 219), are presented as sticks. The water molecule 388 is perfectly positioned for the nucleophilic attack on the γ-phosphate and thus its hydrolysis. The conserved SF2 helicase motifs I-VI are numbered according to convention. (a, Right panel) Schematic representation of the active site in ctPrp43ΔN•ADP•BeF3-(HR). Motifs and residues are labeled as introduced in the left panel. Water molecules are numbered according to the PDB entry. Stacking interactions are shown by double lines, polar interactions between ADP•BeF3- and Prp43 via red dotted lines, interactions between Prp43 and water molecules via light blue lines and the coordination of the central Mg2+ by dark green lines. (b) Active site of Prp43 with bound ADP (PDBid: 5d0u) (Tauchert et al., 2016). (Left and right panel) Labeling and numbering according to a. Numerous rearrangements and conformational changes are noticeable between the ATP- and ADP-bound state.