The murine N-domain strand arrangement derived form crystal structure depicts the CC′ loop to assume a convoluted conformation. The previous model of human CEACAM1 contained a flat CC′ loop like the Ig-folds on Fig. 10 . Binding of bacterial ligands to CEACAM1-Fc constructs as determined in the current study. A. Receptor recognition categories are colour coded according to percent binding relative to the native molecule*. B and C. Diagrams showing CEACAM-binding meningococcal ligand structures. As only the Opa protein structure has been analysed in details so far with respect to CEACAM binding (de Jonge et al., 2003) , for clarity and ease of discussion, a 2D Opa protein structure (B) and relationship of the three variable domains of strain C751 Opa proteins are shown (C). SV, semivariable domains of strain C751 Opa-A, -B and -D are identical as are the hypervariable structures HV1 of OpaA and OpaB and the HV2 of OpaB and OpaD. which it was based. This resulted in Val-39 and Gly-41 being located at a distance from the binding focus Ile-91. Both Val-39 and Gly-41 have been implicated in bacterial binding from alanine scanning and homologue scanning mutagenesis (Bos et al., 1999; Virji et al., 1999) . The remodelling shown here of CEACAM1 produces the convoluted structure of the CC′ loop relocating Val-39 close to Ile-91. Further, the aromatic ring of Tyr-34 suggested to be required to maintain the convoluted structure of the CC′ loop (Tan et al., 2002) when substituted with Phe almost always supported bacterial binding. Indeed, Y34F provides a better environment for most bacterial strains tested. On the other hand, Y34A frequently abrogated receptor recognition. However and surprisingly, Y34S is tolerated by a substantial number of bacterial ligands. Whether this is due to the flexible variable loop domains of the bacterial ligands which may produce an induced fit around the receptor needs consideration. Interestingly, Tyr-34 is conserved in the majority of the human CEACAMs with the exception of CEACAM4 which contains His at this site. However, the importance of Tyr-34 in human CEACAM1 maintaining the three dimensional structure requires human receptor crystallographic data.