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

SummaryStaphylococcus 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 [sLe x -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 sLe x 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.

Immunoregulatory role of lactoferrin-lipopolysaccharide interactions

“…Although glycosylation has no influence on folding in the majority of the Lf species (Baker and Baker 2009), most of the glycosylation sites are exposed on the external surface of the molecule and have been supposed to play a role in Lf interaction with viruses (Valenti and Antonini 2005), toxins (Chung et al 2007), sialic acid-binding immunoglobulin superfamily lectins (Choi et al 2008) and C-type lectin receptors on immune cells (Groot et al 2005;Zimecki et al 2002). However, no evidence for the direct involvement of glysosylation in Lf binding to LPS has been provided yet.…”

Anaplasma marginale Outer Membrane Protein A Is an Adhesin That Recognizes Sialylated and Fucosylated Glycans and Functionally Depends on an Essential Binding Domain

“…1C). ApOmpA and other microbial proteins that interact with sLe x do so at cationic surface patches (18,(23)(24)(25)(26)(27)(28). Consistent with this trend, using the APBS (29) plugin for PyMOL to calculate AmOmpA surface electrostatic values predicted that amino acids 19 to 67, which contain the region that is homologous to the sLe x /6-sulfosLe x binding domain of ApOmpA, have an overall cationic surface charge (Fig.…”

Bacterial superantigens and superantigen-like toxins

“…All staphylococcal and streptococcal SAgs share a common fold with a conserved two-domain architecture and the presence of a long, solvent-accessible central α-helix [164,167] (Figures 32.2 a and b). Interestingly, the same protein fold is also found in the functionally unrelated family of SSLs [167][168][169][170][171]. The N-terminal domain is a mixed β-barrel with Greek-key topology known as an oligosaccharide/oligonucleotide binding (OB) fold.…”

“…The crystal structures of SSL4 [170], SSL5 [347], SSL7 [168], and SSL11 [169] have been published. They reveal that SSLs display remarkably high structural homology with the staphylococcal and streptococcal SAgs.…”

“…In the cocrystal structure of SSL7 bound to human C5 [382], residues near the end of this elongated face interact with the MG1 and MG5 domains of C5. Cocrystals of SSL4, SSL5, and SSL11 with the ligand sialyl Lewis X (sLeX-Neu5Acα2-3Galβ1-4(Fucα1-3)GlcNAc) reveal details of a highly conserved binding site specific for sialylated glycans [169,170,367]. Located on the side of the β-grasp domain, this binding site comprises of a shallow V-shaped depression formed between the β10 strand on the opposite edge of the β sheet to the elongated β6-β7 loop, and a distorted 3 10 -helix in the following loop.…”