The crystal structure of staphylococcal superantigen-like protein 11 (SSL11) 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 aciddependent mechanism and was rapidly internalized. SSL11 also bound to sialic acid-containing glycoproteins, such as the Fc receptor for IgA (FcaRI) 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 [sLex – Neu5Aca2-3Galb1- 4(Fuc1-3)GlcNAc] coupled to bovine serum albumin gave dissociation constants of 0.7 and 7 mm respectively. Binding of SSL11 to a glycan array revealed specificity for glycans containing the trisaccharide sialyllactosamine (sLacNac – Neu5Aca2- 3Galb1-4GlcNAc). A 1.6 Å resolution crystal structure of SSL11 complexed with sLex revealed a discrete binding site in the C-terminal b-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 sialyllactosaminecontaining glycoproteins.