(b).

Structure-refinement statistics. Values in parentheses are for the highest shell.

Resolution (Å)	56–2.5 (2.565–2.500)
No. of reflections	13249 (9055)
No. of atoms	22670 [16 × 164 residues]
Observation-to-parameter ratio†	1.45
R work (%)	21.0 (39.7)
R free (%)	24.7 (40.0)
R.m.s.d., bonds (Å)	0.0066
R.m.s.d., angles (°)	1.39
Ramachandran plot (%)
Most preferred areas	97.2
Most preferred plus allowed areas	99.9
Preferred side-chain rotamers	99.7
PDB code	4lr3

^†The observation-to-parameter ratio is calculated from the number of independent observations divided by four (x, y, z and B) times the number of atoms. With such a modest ratio, we found that the removal of NCS restraints in the refinement led to large differences between working and free R factors, so we kept them until the very last two cycles of refinement. Because the amplitude differences between fourfold NCS-rotated indexes differed by 51–55% owing to the misalignment of NCS axes with respect to the crystallographic axis, the cross-validation set was chosen randomly.