title: Recognition of mannosylated ligands and influenza A virus by human surfactant protein D: contributions of an extended site and residue 343.
abstract: Surfactant protein D (SP-D) plays important roles in antiviral host defense. Although SP-D shows a preference for glucose/maltose, the protein also recognizes d-mannose and a variety of mannose-rich microbial ligands. This latter preference prompted an examination of the mechanisms of mannose recognition, particularly as they relate to high-mannose viral glycans. Trimeric neck plus carbohydrate recognition domains from human SP-D (hNCRD) preferred alpha1-2-linked dimannose (DM) over the branched trimannose (TM) core, alpha1-3 or alpha1-6 DM, or D-mannose. Previous studies have shown residues flanking the carbohydrate binding site can fine-tune ligand recognition. A mutant with valine at 343 (R343V) showed enhanced binding to mannan relative to wild type and R343A. No alteration in affinity was observed for D-mannose or for alpha1-3- or alpha1-6-linked DM; however, substantially increased affinity was observed for alpha1-2 DM. Both proteins showed efficient recognition of linear and branched subdomains of high-mannose glycans on carbohydrate microarrays, and R343V showed increased binding to a subset of the oligosaccharides. Crystallographic analysis of an R343V complex with 1,2-DM showed a novel mode of binding. The disaccharide is bound to calcium by the reducing sugar ring, and a stabilizing H-bond is formed between the 2-OH of the nonreducing sugar ring and Arg349. Although hNCRDs show negligible binding to influenza A virus (IAV), R343V showed markedly enhanced viral neutralizing activity. Hydrophobic substitutions for Arg343 selectively blocked binding of a monoclonal antibody (Hyb 246-05) that inhibits IAV binding activity. Our findings demonstrate an extended ligand binding site for mannosylated ligands and the significant contribution of the 343 side chain to specific recognition of multivalent microbial ligands, including high-mannose viral glycans.
authors.affiliation: Department of Pathology and Immunology, Washington University School of Medicine, St. Louis, Missouri 63110, USA. crouch@path.wustl.edu
authors.forename: Erika
authors.initials: E
authors.lastname: Crouch
authors.forename: Kevan
authors.initials: K
authors.lastname: Hartshorn
authors.forename: Tim
authors.initials: T
authors.lastname: Horlacher
authors.forename: Barbara
authors.initials: B
authors.lastname: McDonald
authors.forename: Kelly
authors.initials: K
authors.lastname: Smith
authors.forename: Tanya
authors.initials: T
authors.lastname: Cafarella
authors.forename: Barbara
authors.initials: B
authors.lastname: Seaton
authors.forename: Peter H
authors.initials: PH
authors.lastname: Seeberger
authors.forename: James
authors.initials: J
authors.lastname: Head
mesh_terms: D001665:Binding Sites / Q000235:genetics; D018360:Crystallography, X-Ray; D006801:Humans; D009980:Influenza A virus / Q000737:chemistry / Q000378:metabolism*; D008024:Ligands; D008358:Mannose / Q000737:chemistry / Q000378:metabolism*; D009154:Mutation; D009844:Oligosaccharides / Q000737:chemistry / Q000378:metabolism; D011134:Polysaccharides / Q000737:chemistry / Q000378:metabolism; D017434:Protein Structure, Tertiary / Q000235:genetics; D037663:Pulmonary Surfactant-Associated Protein D / Q000737:chemistry / Q000378:metabolism*; D014633:Valine / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D008024:Ligands; D009844:Oligosaccharides; D011134:Polysaccharides; D037663:Pulmonary Surfactant-Associated Protein D; D014633:Valine; D008358:Mannose