SAXS.

(i) Data recording and processing. SAXS data were collected at the BioSAXS beamline BM29 (ESRF, Grenoble, Switzerland) using an in-line analytical SEC S200 column (10/300 GL; GE Healthcare). SAXS measurements were performed every second with a Pilatus 1M detector at a distance of 2.867 m, allowing measurements for momentum transfer (Q) ranging from 0.03 to 4.94 nm, with a wavelength of 0.9919 Å [Q = (4 π sinθ)/λ, where θ = scattering angle and λ = wavelength]. The scattering measured for the buffer before and after each sample measurement was used for background subtraction. Data were analyzed using Primus (ATSAS) (61) and the ScÅtter (Bioisis) program suite (62). The volume of correlation was used to determine the molecular mass (41). The radius of gyration (R_g) was calculated using both the Guinier approximation and the pair distribution function. SAXS data sets were scaled and averaged to produce a single I(Q) curve.

(ii) Model generation and selection. An initial ensemble of all atom conformers suitable for fitting the SAXS data was built by the use of flexible-meccano (44). The known structure was used for P_CTD, and residues in the N-terminal disordered tail were stochastically assigned amino acid-specific backbone dihedral angles in agreement with random-coil statistics. Side chains in this disordered region(s) were constructed using the program SCCOMP (63). Scattering curves were calculated with CRYSOL (64), and selections were performed with the program GAJOE (45).