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Atomic-resolution crystal structures of the immune protein conglutinin from cow reveal specific interactions of its binding site with N-acetylglucosamine.

ABSTRACT: Bovine conglutinin is an immune protein that is involved in host resistance to microbes and parasites and interacts with complement component iC3b, agglutinates erythrocytes, and neutralizes influenza A virus. Here, we determined the high-resolution (0.97-1.46 Å) crystal structures with and without bound ligand of a recombinant fragment of conglutinin's C-terminal carbohydrate-recognition domain (CRD). The structures disclosed that the high-affinity ligand N-acetyl-d-glucosamine (GlcNAc) binds in the collectin CRD calcium site by interacting with the O3' and O4' hydroxyls alongside additional specific interactions of the N-acetyl group oxygen and nitrogen with Lys-343 and Asp-320, respectively. These residues, unique to conglutinin and differing both in sequence and in location from those in other collectins, result in specific, high-affinity binding for GlcNAc. The binding pocket flanking residue Val-339, unlike the equivalent Arg-343 in the homologous human surfactant protein D, is sufficiently small to allow conglutinin Lys-343 access to the bound ligand, whereas Asp-320 lies in an extended loop proximal to the ligand-binding site and bounded at both ends by conserved residues that coordinate to both calcium and ligand. This loop becomes ordered on ligand binding. The electron density revealed both ? and ? anomers of GlcNAc, consistent with the added ?/?GlcNAc mixture. Crystals soaked with ?1-2 mannobiose, a putative component of iC3b, reported to bind to conglutinin, failed to reveal bound ligand, suggesting a requirement for presentation of mannobiose as part of an extended physiological ligand. These results reveal a highly specific GlcNAc-binding pocket in conglutinin and a novel collectin mode of carbohydrate recognition.

SUBMITTER: Paterson JM

PROVIDER: S-EPMC6851296 | biostudies-literature | 2019 Nov

REPOSITORIES: biostudies-literature

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Atomic-resolution crystal structures of the immune protein conglutinin from cow reveal specific interactions of its binding site with <i>N</i>-acetylglucosamine.

Paterson Janet M JM Shaw Amy J AJ Burns Ian I Dodds Alister W AW Prasad Alpana A Reid Ken B KB Greenhough Trevor J TJ Shrive Annette K AK

The Journal of biological chemistry 20190927 45

Bovine conglutinin is an immune protein that is involved in host resistance to microbes and parasites and interacts with complement component iC3b, agglutinates erythrocytes, and neutralizes influenza A virus. Here, we determined the high-resolution (0.97-1.46 Å) crystal structures with and without bound ligand of a recombinant fragment of conglutinin's C-terminal carbohydrate-recognition domain (CRD). The structures disclosed that the high-affinity ligand <i>N</i>-acetyl-d-glucosamine (GlcNAc) ...[more]

PMID: 31562242

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Atomic-resolution crystal structures of the immune protein conglutinin from cow reveal specific interactions of its binding site with N-acetylglucosamine.

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Atomic-resolution crystal structures of the immune protein conglutinin from cow reveal specific interactions of its binding site with <i>N</i>-acetylglucosamine.

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