(a) Arrangement of conserved structural domains. Conserved motifs of the helicase core are shown in dark gray. The domain arrangements of DEAH/RHA and Ski2-like families shown are based on Mtr4 and Prp43 structures, respectively (domains not to scale). The winged helix and ratchet domains are conserved in DEAH/RHA and Ski2-like helicases and the OB fold domain is conserved in DEAH/RHA helicases, while the arch domain is present in only a subset of Ski2-like helicases. Individual proteins from all three families may include other, non-conserved domains that are not shown in the figures.
(b) Crystal structures of Ski2-like (Mtr4)(28), DEAH/RHA (Prp43)(30) and DEAD-box (Mss116)(54) helicases. Domains are colored as in (a). The nucleotide (ADP for Mtr4 and Prp43, and ADPNP for Mss116) is shown in red and co-crystallized ssRNA bound to Mtr4 and Mss116 is shown in black. The β hairpin within D2 in Ski2-like and DEAH/RHA helicases that is thought to function as a pin during RNA unwinding is highlighted in pink.