Siglec-7 Undergoes a Major Conformational Change When Complexed with the α(2,8)-Disialylganglioside GT1b

The siglecs are a group of mammalian sialic acid binding receptors expressed predominantly in the immune system. The CD33-related siglecs show complex recognition patterns for sialylated glycans. Siglec-7 shows a preference for α(2,8)-disialylated ligands and provides a structural template for studying the key interactions that drive this selectivity. We have co-crystallized Siglec-7 with a synthetic oligosaccharide corresponding to the α(2,8)-disialylated ganglioside GT1b. The crystal structure of the complex offers a first glimpse into how this important family of lectins binds the structurally diverse gangliosides. The structure reveals that the C-C′ loop, a region implicated in previous studies as driving siglec specificity, undergoes a dramatic conformational shift, allowing it to interact with the underlying neutral glycan core of the ganglioside. The structural data in combination with mutagenesis studies show that binding of the ganglioside is driven by extensive hydrophobic contacts together with key polar interactions and that the binding site structure is complementary to preferred solution conformations of GT1b.

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Crystal structure of equine rhinitis A virus in complex with its sialic acid receptor

Journal of General Virology ◽

10.1099/vir.0.020420-0 ◽

2010 ◽

Vol 91 (8) ◽

pp. 1971-1977 ◽

Cited By ~ 11

Author(s):

Elizabeth E. Fry ◽

Tobias J. Tuthill ◽

Karl Harlos ◽

Thomas S. Walter ◽

David J. Rowlands ◽

...

Keyword(s):

Crystal Structure ◽

Sialic Acid ◽

Binding Site ◽

Foot And Mouth Disease ◽

Cell Entry ◽

Receptor Binding Site ◽

Mouth Disease Virus ◽

The Family ◽

Sialic Acid Binding ◽

High Level

Equine rhinitis A virus (ERAV) shares many features with foot-and-mouth disease virus (FMDV) and both are classified within the genus Aphthovirus of the family Picornaviridae. ERAV is used as a surrogate for FMDV research as it does not require high-level biosecurity. In contrast to FMDV, which uses integrins as cellular receptors, the receptor for ERAV has been reported to involve the sugar moiety sialic acid. This study confirmed the importance of sialic acid for cell entry by ERAV and reports the crystal structure of ERAV particles complexed with the receptor analogue 3′-sialyllactose. The receptor is attached to the rim of a capsid pit adjacent to the major immunogenic site and distinct from the sialic acid binding site used by a related picornavirus, the cardiovirus Theiler's murine encephalitis virus. The structure of the major antigenic determinant of the virus, previously identified from antibody escape mutations, is also described as the EF loop of VP1, which forms a hairpin stretching across the capsid surface close to the icosahedral fivefold axis, neighbouring the receptor-binding site, and spanning two protomeric units.

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Neutrophils do not bind to or phagocytize human immune complexes formed with influenza virus

Blood ◽

10.1182/blood.v82.5.1639.1639 ◽

1993 ◽

Vol 82 (5) ◽

pp. 1639-1646 ◽

Cited By ~ 3

Author(s):

DR Ratcliffe ◽

J Michl ◽

EB Cramer

Keyword(s):

Influenza Virus ◽

Sialic Acid ◽

Epithelial Cells ◽

Binding Site ◽

Immune Complexes ◽

Human Neutrophils ◽

Sialic Acid Binding ◽

Infected Cells ◽

Neutrophil Adherence ◽

Human Immune

Abstract Neutrophils appear to form the first line of defense against influenza virus, yet it is unclear how these leukocytes recognize influenza- infected cells. While demonstrating that neutrophils adhere specifically to the sialic acid-binding site on the hemagglutinin molecule (HA) on the surface of influenza-infected (WSN[H1N1]) epithelial cells and not to other viral or epithelial cell antigens, it was observed that human neutrophils do not recognize immune complexes formed with influenza virus. Intact antibodies (mouse monoclonal antibodies [MoAbs] IgG1 and IgG2b, human immune heat-inactivated serum [predominantly IgG1], and IgG purified from human immune serum) that block the sialic acid-binding site on HA significantly reduced (> 80%) neutrophil adherence to influenza-infected epithelial cells. Binding and phagocytosis of free influenza virions and neutrophil agglutination by influenza virus were completely prevented by these antibodies. Intact and F(ab')2 fragments of mouse MoAbs to other viral epitopes caused increased neutrophil adherence to infected cells. This binding was eliminated by F(ab'2) fragments of MoAbs against the sialic acid- binding site on HA, but not by saturating amounts of MoAbs, which block the neutrophil Fc receptors. Thus, it appears that human neutrophils show little ability to bind via their Fc receptors to the immune complexes formed with antibody and either influenza-infected epithelial cells or the free virion. These findings are in contrast to the general dogma, and are the first example of antibody opsonization reducing, rather than enhancing, neutrophil binding and phagocytosis of a pathogen.

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