The C terminus of N TAIL is an additional XD binding site SAXS studies suggest that the C terminus of N TAIL interacts with XD. In particular, the calculated overall envelope of the complex shows the presence of a globular cluster of invariant size at one extremity and of an elongated protuberance with varying shapes. The elongated protuberance corresponds to the 92-residue-long N-terminal region of N TAIL , while the more compact region accommodates the structure of the XD-N TAIL486-505 complex as well as the 20 C-terminal residues of N TAIL . Data from overall shape calculations thus clearly indicate that the C terminus is not protruding towards the solvent, and remains close to XD. On the other hand, attempts to more precisely model the conformation adopted by the C-terminal region of N TAIL within the complex led to an ensemble of solutions. In all these solutions, the C-terminal region of N TAIL always packs against the XD-N TAIL486-505 complex, rather than being extended and exposed to the solvent. Identification of the possible gain of regular secondary structure within the C-terminal region of N TAIL is beyond the resolution limits of SAXS. However, results provided by CD, fluorescence spectroscopy, and heteronuclear NMR all converge to suggest that the C-terminal region of N TAIL does not gain any regular secondary structure (see below). Besides supporting a role for the C-terminal region of N TAIL in the interaction with XD, these data also indicate that most of N TAIL remains disordered within the complex. The prevalent disorder of N TAIL within the complex is in agreement with other data reported in the literature where an intrinsically disordered protein largely preserves its overall extended conformation even after interaction with a binding partner (see Tompa 2002 , and references cited therein; Permyakov et al. 2003) . Finally, it is noteworthy that in the SAXS model the N terminus of XD is exposed to the solvent (see Fig. 2B ), a position that would accommodate the remaining part of P.