In order to assess the importance of these residues in interactions with mucosal pathogens, substitutions were introduced by site-directed mutagenesis at three sites (91, 34 and 89) on the N-domain. The substitutions introduced in CEACAM1 [NA1B]-Fc are shown in Fig. 2 . Chimeric receptors proteins with the native sequence or with sub- (Virji et al., 1999) . (B) New model based on the murine CEACAM1a N-domain crystal structure (Tan et al., 2002) . The models are presented as Van der Waals surface representation. In the latter case V39 locates more centrally with the other critical residues for bacterial binding. (C-E, left) Stereo pairs presented are ribbon diagrams for CEACAM1 N-domain in the native form (C), with Y34F (D) or Q89N (E) substitutions show a flattening of the bacterial binding region on CEACAM1 and improving accessibility of the primary binding residue I91. The side-chain atom colouring (C-green, O-red, N-blue) show the increased hydrophobic nature of the binding site around I91 in the mutant structures. (C-E, right) Surface presentations of the binding site (views corresponding to a 90°rotation of the stereo images in the horizontal axis) coloured according to hydrophobicity. stitutions were produced by transient transfection of COS cells for functional studies described below.