Data Collection and Refinement Statistics.

Values in parentheses refer to the highest resolution shell. Rmerge = ∑hkl ∑i (|Ii(hkl) − |)/∑hkl ∑i Ii(hkl) and Rpim = ∑hkl (√(1/(N-1)) ∑i (|Ii(hkl) − |)/∑hkl ∑i I(hkl) where I is the measured intensity and is the averaged intensity of each unique reflection with indices hkl. I/σI indicates the average of the intensity divided by its average standard deviation. Rwork = ∑hkl ||Fo| − |Fc||/∑hkl |Fo| where Fo and Fc are the observed and calculated structure factors, respectively. Rfree is the same as Rwork, calculated for the 5% of the data that was randomly omitted from refinement. The Ramachandran statistics indicate the fraction of residues in the most favored, allowed and disallowed regions of the Ramachandran plot, respectively, in addition to the fraction of rotamer outliers observed.