The overall envelope of the complex was restored ab initio from its scattering profile using the program GASBOR ). Several independent runs yielded different shapes with recurrent features: they were all elongated, with a globular cluster of always the same size at one extremity and an elongated protuberance with varying bends and cross-sections. The quality of the fit to the experimental data was similar in all cases, with a 2 of 1.3 to 1.6. The globular part most probably corresponds to XD packed against the folded region of N TAIL , while the outgrowth corresponds to unfolded regions of the latter. The crystal structure of the XD-N TAIL486-505 complex (PDB code 1T6O), which encompasses the a-MoRE (residues 488-499), was inserted in the shape using the program SUPCOMB (Kozin and Svergun 2001) (Fig. 2B ). Interestingly, only one protuberance was observed, corresponding to the N-terminal area of N TAIL , and no other protuberance was restored from the shape determination in the opposite region of the globular cluster, thus suggesting that the C-terminal region of N TAIL (aa 506-525), although not visible per se, is packed inside the bulky portion of the complex (Fig. 2B ). In order to get further insights into the conformation that the regions of N TAIL encompassing residues 401-485 and 506-525 adopt in the complex with XD, we used the program package CREDO, which is an extension of GASBOR and calculates the position of missing loops in crystal structures. The results obtained with several independent runs using the crystal structure of XD-N TAIL486-505 as the template, revealed at one side a peptide chain in an extended conformation and at the other side a chain packed against the bulky portion of the XD-N TAIL486-505 complex. The extended conformation, which protrudes from the globular cluster of the complex and points to the solvent, corresponds to the 92-residue-long N-terminal region of the N TAILHN construct. In contrast, the more compact peptide chain, corresponding to the 20-residue-long C-terminal region of N TAIL (aa 506-525), always packs against the XD-N TAIL486-505 complex but at varying distances and positions. An example of one of the calculated conformations is shown in Figure 2C . The experimental data were fitted by these models with similar 2 of about 0.8-1.0 (see Fig. 2C , inset). The various possible conformations provided by the program CREDO together with the extended structural properties of the complex (as indicated by the distance distribution function, the R g and the restored shape, with respect to the number of amino acids of the complex) are typical of disordered polypeptide chains lacking a stable regular structure. Accordingly, the structure shown in Figure 2C represents only one possible conformation found by CREDO among many others. Indeed, CREDO restores only one conformation per run, and therefore cannot account for multiple conformations due to disorder when fitting the data. Therefore, although a unique conformation of the N TAIL region encompassing residues 506-525 cannot be derived, all the solutions provided by CREDO consistently revealed a packing of the Cterminal region of N TAIL against XD. The recurrence of these packed conformations suggests that the Cterminal region of N TAIL interacts with XD.