Data collection and refinement statistics.

Rmerge = ΣhΣi |Ih,i−Ih|/ΣhΣiIh,i for the intensity (I) of i observation of reflection h. R factor = Σ∥Fobs|−|Fcalc∥/Σ|Fobs|, where Fobs and Fcalc are the observed and calculated structure factors, respectively. Rfree = R factor calculated using 5% of the reflection data chosen randomly and omitted from the start of refinement. RMSD, root-mean-square deviations from ideal geometry. Data for the highest resolution shell are shown in parentheses.