Figure 5. Brr2 mode of unwinding.

a, Domain organisation of Brr2 N-terminal helicase cassette (NHC). WH, Winged-helix; HLH, helix-loop-helix; and FN3, fibronectin3-like domains. The inactive C-terminal helicase cassette (CHC) has the same domain organisation. b, U4/U6 di-snRNP and its interaction with Brr2 in tri-snRNP. The domains of Brr2 NHC are coloured as in a. The single-stranded RNA between U4/U6 stem I and U4 3′SL is already loaded in the active site of Brr2. When the Hel308 structure⁴³ is overlaid onto the NHC of Brr2, its 10-nucleotide DNA substrate coincide with the density in the Brr2 active site, which extends to U4 snRNA 3′SL (red dotted line). The helix-loop-helix domain of Brr2 interacts with U4 snRNA 3′SL (inset). c, Superposition of the RecA1 domain of Brr2 in the crystal structure³¹ (PDB ID 4BGD, in grey) and in tri-snRNP (domains coloured as in a) shows the opening of the gap between the RecA1 and RecA2 domains (indicated by the red arrow) to accommodate the RNA substrate.