Small angle X-ray scattering All protein samples were prepared by dilution of the purified N TAILHN and XD solutions in buffer 10 mM Tris/HCl at pH 8, 40 mM NaCl, with 1 mM DTT as radiation scavenger. The complex N TAILHN -XD was prepared by mixing XD and N TAILHN with a molar ratio of 2:1 in the same buffer and at a final protein concentration of 10 mg/mL. The samples were filtered prior to each measurement (Millex syringe filters 0.22 mm, Millipore) to eliminate possibly existing large aggregates.