The Murine Coronavirus Hemagglutinin-esterase Receptor-binding Site: A Major Shift in Ligand Specificity through Modest Changes in Architecture
Figure 5
MHV-S HE has a unique receptor-binding site that binds specifically 4-O-acetylated sialic acid.
(A) Surface and (B) stick representation of the MHV-S HE receptor-binding site in complex with a receptor analogue. The ligand bound to the HE receptor-binding site is shown in stick representation and the potassium ion as a magenta sphere, indicated by a black arrow in panel A. Hydrogen bonds between HE and the receptor are shown as black dashed lines. Surface representation of the MHV-S HE receptor-binding site reveals two pockets accommodating the 4-O- and 5-N-acetyl groups of the receptor, respectively. Note that crystals were soaked with αNeu4,5,9Ac32Me, but most likely as a result of the low pH crystallization conditions, the 9-O-Ac group was lost [42]. (C) Surface and (D) stick representation of the BCoV-Mebus HE receptor-binding site. Note that the topology of the two hydrophobic pockets is conserved, except they bind different substituents of the receptor analogue. (E) The effect of Ala substitutions on receptor binding. Relative binding affinity of wild-type HE0 (wt) and its derivatives was assessed by hemagglutination assay with rat erythrocytes and twofold serial dilutions of each of the HE0-Fc chimeras (5,000 to 10 ng per well, arrow). (F) Binding of twofold serial dilutions of wild-type (wt) HE0-Fc chimera and its derivatives in a solid-phase lectin-binding assay towards horse serum glycoproteins (HSG) as described in Figure 2A.