The accommodation of the 486-505 region of N within XD (Kingston et al. 2004a ) triggers some minor rearrangements at the surface of the latter, compared to the crystal structure of the uncomplexed form . In particular, the largest movements are observed for the side chains of residues Arg15 and Glu17 of XD (corresponding to residues 472 and 474 of P). The Arg15 NH2 atom moves 9.7 Å away in the chimeric structure compared to the structure of XD alone, while the Glu17 C atom undergoes a shift of 2.5 Å . These two residues are both located within the loop connecting a1 and a2 helices, and occur at least 9 Å away from Box2, thus ruling out the possibility that the observed spatial rearrangements they undergo could be ascribed to Box2 embedding. The significant displacement they undergo in the complex therefore indicates that binding of Box2 induces local conformation changes in XD that could favor interaction with Box3. The surface displaced residues of XD could, in fact, be part of a Box3 binding site.