15N relaxation dispersion CPMG data for residues (a) 37, (b) 67, and (c) 83 at 180 μM ΔN6 (26% ΔN6 molecules are monomers, 48% are in dimers, 26% are in hexamers) (red) or 480 μM ΔN6 (13% ΔN6 molecules are monomers, 32% are in dimers, 55% are in hexamers) (black). Solid lines represent fits to the fast exchange model, yielding values of kexbind of 1790 ± 290 s−1 at 180 μM ΔN6 and kexbind of 1170 ± 196 s−1 at 480 μM ΔN6 (see Materials and methods). (d) Plots of Rex per residue defined as R2,eff50Hz - R2,eff680Hz. The dashed line represents one standard deviation of the mean calculated for all data points. Residues are numbered according to the WT sequence. Significant CPMG profiles are observed for residues in the N-terminus, A strand, BC, DE and FG loops, in excellent agreement with the intermolecular PRE data shown at 120 μM and 320 μM ΔN6 in Figure 3 and Figure 5—figure supplement 1. Residues which are severely broadened at 480 μM, thereby precluding accurate determination of their Rex values, are shown as black crosses. Crucially, when the protein concentration was increased the residues which show significant CPMG profiles are unchanged suggesting that the dimers and hexamers share a similar interface. (e) The structure of ΔN6 (2XKU; Eichner et al., 2011) colored according to the Rex amplitude as indicated in the scale bar. Trans Pro32 is shown in space-fill (pale blue).