Crystal Structure of the N-Terminal Domain of Sialoadhesin in Complex with 3′ Sialyllactose at 1.85 Å Resolution  A.P. May, R.C. Robinson, M. Vinson, P.R. Crocker, E.Y. Jones  Molecular Cell  Volume 1, Issue 5, Pages 719-728 (April 1998) DOI: 10.1016/S1097-2765(00)80071-4

Figure 1 Electron Density Maps Portions of the final (2|Fo|-|Fc|)φc electron density map, contoured at 1σ. (A) 3′ sialyllactose bound to molecule B. Solvent molecules are shown as red spheres. (B) The intrasheet disulphide bond formed between residues Cys-22 and Cys-79 can be seen stacked against the side chain of Phe-96. Figures were drawn using BOBSCRIPT (Esnouf 1997) and RASTER3D (Bacon and Anderson 1988; Merrit and Murphy 1994). All subsequent molecular representations were also drawn using this method. Molecular Cell 1998 1, 719-728DOI: (10.1016/S1097-2765(00)80071-4)

Figure 2 The Structure of the N-Terminal Domain of Sialoadhesin in Complex with 3′ Sialyllactose Each strand is labeled. The 3′ sialyllactose lies along strand G and makes interactions with residues from the A,G, and F strands. Molecular Cell 1998 1, 719-728DOI: (10.1016/S1097-2765(00)80071-4)

Figure 3 Superposition of the V-Set Domain from P0 with SnD1 The Cα trace of SnD1 is shown in green; the Cα trace of P0 is shown in yellow. Molecular Cell 1998 1, 719-728DOI: (10.1016/S1097-2765(00)80071-4)

Figure 4 Sequence Comparisons The amino acid sequence of domain 1 of sialoadhesin (Sn) was aligned manually with the predicted N-terminal domains of all the available sequence homologs of MAG, CD22, and CD33. Species are indicated as follows: h, human; m, mouse; and r, rat. The sequence is also aligned on the basis of structural superposition with the amino acid sequences of V-set Ig domains from P0 and human CD2 (domain 1) (PDB accession codes 1NEU and 1HNF, respectively). Identical residues within the siglec family are in red, and residues important in stabilizing the Ig fold are in blue. Residues with side chains involved in the sialic acid–binding template are in green and are highlighted. Cysteine residues, which are involved in disulphide bond formation, are in yellow. Those residues that have Cα positions, which match structurally equivalent residues in SnD1 within 2.5 Å, are boxed. Other residues are aligned so as to maximize pairwise alignments. Molecular Cell 1998 1, 719-728DOI: (10.1016/S1097-2765(00)80071-4)

Figure 5 Interactions at the 3′ Sialyllactose Binding Site (A) Stereo view of the binding site of SnD1 molecule B. 3′ sialyllactose makes a number of interactions with sialoadhesin. The carboxylate forms a salt bridge with the side chain of Arg-97. Hydrophobic interactions are made between the acetamido methyl group and the side chain of Trp-2; C9 of the glycerol side chain and the side chain of Trp-106; C6 of galactose and the side chain of Leu-107. Hydrogen bonds (shown as broken yellow lines) are made between sialic acid and main chain atoms of Ser-103, Leu-107, and Arg-105, and between galactose and the side chain of Tyr-44. A number of water molecules are also involved in the SnD1/3′ sialyllactose interaction and are depicted as red spheres. The glucose does not interact with the SnD1 molecule to which sialic acid is bound. (B) Interactions between the B molecule of SnD1 and 3′ sialyllactose. Solvent molecules are depicted as ovals. 3′ sialyllactose is shown in thick black lines. Molecular Cell 1998 1, 719-728DOI: (10.1016/S1097-2765(00)80071-4)

Figure 6 Site-Directed Mutagenesis (A) Residues that disrupt sialic acid–dependent adhesion when mutated. 3′ sialyllactose is shown with white carbon atoms. Mutations carried out previously (Vinson et al. 1996) are shown in mustard. Residues mutated subsequent to structure determination are also shown: Trp-2 is shown in green, and Tyr-44 is shown in light gray. (B) Binding assays of the sialoadhesin mutants W2Q and Y44A. Assays were carried out as described previously (Vinson et al. 1996). Binding to erythrocytes of (I) W2Q; filled circles, wild-type protein; open circles, mutant W2Q; crosses, NCAM-Fc (II) Y44A; filled circles, wild-type protein; open circles, mutant Y44A; crosses, NCAM-Fc. Molecular Cell 1998 1, 719-728DOI: (10.1016/S1097-2765(00)80071-4)