The crystal structure of staphylococcal superantigen‐like protein 11 in complex with sialyl Lewis X reveals the mechanism for cell binding and immune inhibition

Abstract

Staphylococcus aureus is a major pathogen that produces a family of 14 staphylococcal superantigen‐like (SSL) proteins, which are structurally similar to superantigens but do not stimulate T cells. SSL11 is one member of the family that is found in all staphylococcal strains. Recombinant SSL11 bound to granulocytes and monocytes through a sialic acid‐dependent mechanism and was rapidly internalized. SSL11 also bound to sialic acid‐containing glycoproteins, such as the Fc receptor for IgA (FcαRI) and P‐selectin glycoprotein ligand‐1 (PSGL‐1), and inhibited neutrophil attachment to a P‐selectin‐coated surface. Biosensor analysis of two SSL11 alleles binding to sialyl Lewis X [sLe^x– Neu5Acα2‐3Galβ1‐4(Fuc1‐3)GlcNAc] coupled to bovine serum albumin gave dissociation constants of 0.7 and 7 μm respectively. Binding of SSL11 to a glycan array revealed specificity for glycans containing the trisaccharide sialyllactosamine (sLacNac – Neu5Acα2‐3Galβ1‐4GlcNAc). A 1.6 Å resolution crystal structure of SSL11 complexed with sLe^x revealed a discrete binding site in the C‐terminal β‐grasp domain, with predominant interactions with the sialic acid and galactose residues. A single amino acid mutation in the carbohydrate binding site abolished all SSL11 binding. Thus, SSL11 is a staphylococcal protein that targets myeloid cells by binding sialyllactosamine‐containing glycoproteins.