Fig. 1. Stereo view of the ribbon drawing of msCEACAM1a[1,4], which contains two Ig-like domains. The CC′ loop in the N-terminal domain (D1), which is involved in binding of MHV and other ligands, is highlighted in yellow. The predicted key virus-binding residue Ile41 on the CC′ loop is shown in red in ball-and-stick representation. The FG loop of D1, another biologically important element, is shown in violet. The carbohydrate moieties are drawn in gray in ball-and-stick representation. The glycan at Asn70 that is conserved in the whole CEA family is labeled. The figure was prepared using MOLSCRIPT (Kraulis, 1991).

Fig. 2. (A) Superposition of D1 of msCEACAM1a[1,4], CD2, CD4 and Bence-Jones protein REI using C_α atoms of β-sheets. Each molecule is shown in C_α trace, with msCEACAM1a in cyan, CD2 in purple, CD4 in brown and REI in green, respectively. The unique convoluted conformation of the CC′ loop in msCEACAM1a[1,4] is striking. The sequence alignment of the CC′ loop regions of these four molecules are shown using the same color code. (B) Stereo view of the exposed residues on the CFG face of D1 of msCEACAM1a[1,4]. The C_α trace of the CC′ loop is highlighted in yellow. Displayed side chains and carbohydrates are drawn in ball-and-stick representation. (C) Electrostatic potential surface representation of the same view as (B). The electrostatic potential is colored blue for positive and red for negative, and was calculated in the absence of carbohydrates and solvent molecules. (A) and (B) were prepared with MOLSCRIPT (Kraulis, 1991), and (C) with GRASP (Nicholls et al., 1991).

Fig. 3. A comparative view of structures of several virus receptors, including: msCEACAM1a, the receptor for murine coronavirus MHV; ICAM1, the receptor for the major group of rhinoviruses; CD4, the primary receptor for HIV; and CD46, the receptor for measles virus. Only their N-terminal domains are shown here. The key virus-binding motifs with unique topological features are highlighted in red.

Fig. 4. Sequence alignment of D1 and D4 of murine CEACAM1 with corresponding domains of human CEA family members. Residues invariant throughout all sequences shown are colored yellow, while physico-chemically conserved residues (with no more than two exceptions) are colored blue. The β-strands are indicated with arrows below the appropriate sequence. (A) D1 of murine CEACAM1a is aligned with D1 of murine CEACAM1b (top), as well as the human CEA members found in the SWISSPROT database (bottom). (B) The I1 set Ig-fold of D4 of murine CEACAM1a is aligned with D2 of the same molecule (top). These sequences are compared with the presumed I1 set of domains A1, A2, A3 and presumed I2 set of domains B1, B2, B3 of CEA (CD66e) (bottom). The asterisks indicate potential N-glycosylation sites.

Fig. 5. Topology diagram for D1 of msCEACAM1a with β-strands shown as arrows. The diagram is colored according to the degree of variability in sequence of N-terminal domain for all available mammalian CEA molecules. The variability was measured using Shannon’s entropy value (H) (Stewart et al., 1997). The least variable, or most conserved, residues (H < 1) are colored green, while the most variable ones (H > 2) are colored red. Residues in between (1 < H < 2) are colored yellow. The difference in the degree of sequence conservation between the ABED and CFG faces is striking. On the ABED face, the glycan at Asn70 and the shielded hydrophobic residues are marked.

Fig. 6. Backbone worm representation of the ‘parallel’ interaction between the dyad-related msCEACAM1a[1,4] molecules seen in the crystal structure, prepared with GRASP (Nicholls et al., 1991). (A) Two monomers related by a crystallographic 2-fold axis are shown in blue and green, respectively. Carbohydrates are drawn in ball-and-stick representation. (B) Stereo view of the close-up view across the dimer interface. The residues potentially involved in interactions are shown in ball-and-stick representation.