We have previously reported the crystal structure of the C-terminal globular domain of MV P (XD, aa 459-507): It consists of a monomeric protein composed of three a-helices, forming an anti-parallel three-helix bundle . We have also shown that N TAIL undergoes an induced folding in the presence of XD and that this unstructured-to-structured transition implies a gain of a-helicity . Using a combination of computational and biochemical approaches, we have identified within N TAIL an a-helical molecular recognition element (a-MoRE, aa 488-499) involved in binding to P and in the induced folding of N TAIL (Bourhis et al. 2004) . The a-MoRE has been modeled in the crystal structure of XD, in the long hydrophobic cleft delimited by helices a2 and a3 ). In this model, the contact of N TAIL with a hydrophobic patch at the surface of XD would be the driving force in the induced folding of the a-MoRE of N TAIL by burying apolar residues at the protein-protein interface. Very recently, Kingston et al. (2004a) have reported the crystal structure of a chimeric protein composed of MV XD and a peptide corresponding to residues 486-505 of MV N. The structure of the complex is a four-helix bundle in which the ahelix of N is bound in the reverse orientation with respect to the model proposed by Johansson et al. (2003) . Using NMR and crystallographic studies, Kingston et al. (2004a) deduced that the induced folding of N TAIL is restricted to only 18 residues (out of 125), although the analysis was restricted to the N region encompassing residues 477-505. (B) . The encapsidated RNA is shown as a dotted line. PMD is represented with a dumbbell shape according to Tarbouriech et al. (2000) . The tetrameric P (Rahaman et al. 2004 ) is shown bound to N NUC through three of its four C-terminal XD ''arms,'' as in the model of Curran and Kolakofsky (1999) . The L protein is shown as a rectangle contacting P through PMD by analogy with SeV (Smallwood et al. 1994) .