Species Differences in the Carbohydrate Binding Preferences of Surfactant Protein D

Abstract

Interactions of surfactant protein D (SP-D) with micro-organisms and organic antigens involve binding to the trimeric neck plus carbohydrate recognition domain (neck+CRD). In these studies, we compared the ligand binding of homologous human, rat, and mouse trimeric neck+CRD fusion proteins, each with identical N-terminal tags remote from the ligand-binding surface. Although rat and mouse showed similar affinities for saccharide competitors, both differed markedly from the human protein. The human neck+CRD preferentially recognized N-acetyl-mannosamine, whereas the rat and mouse proteins showed greater affinity for myoinositol, maltose, and glucose. Although human neck+CRDs bound to maltosyl-agarose and fungal mannan, only rat and mouse neck+CRDs showed significant binding to maltosyl-Toyopearl beads, solid-phase maltosyl-albumin neo-glycoprotein, or the Phil82 strain of influenza A virus. Likewise, human SP-D dodecamers and trimeric subunits of full-length rat, but not full-length human SP-D trimers, bound to maltosyl-Toyopearl. Site-directed mutagenesis of the human neck+CRD demonstrated an important role of Asp324-Asp325 in the recognition of N-acetyl-mannosamine, and substitution of the corresponding murine sequence (Asn324-Asn325) conferred a capacity to interact with immobilized maltose. Thus, ligand recognition by human SP-D involves a complex interplay between saccharide presentation, the valency of trimeric subunits, and species-specific residues that flank the primary carbohydrate binding site.

Specific Sites of N-Linked Glycosylation on the Hemagglutinin of H1N1 Subtype Influenza A Virus Determine Sensitivity to Inhibitors of the Innate Immune System and Virulence in Mice

“…These findings suggest that simultaneous binding of SP-D to multiple glycans on IAV HA is likely to increase the overall affinity of binding and therefore antiviral activity. Single-site deletion of Asn 177 , but not Asn 172 , reduced sensitivity to mouse BAL/SP-D and increased virulence in mice, a finding that may reflect differences in ligand specificity between human and mouse SP-D (52). Of interest, different collectins also display a distinct specificity for oligosaccharides expressed on IAV.…”

Surfactant Protein D Increases Phagocytosis of HypocapsularCryptococcus neoformansby Murine Macrophages and Enhances Fungal Survival

“…We were unable to detect any competition for SP-D binding from lam- To test the hypothesis that SP-D interacts with more than one ligand on the surfaces of C. neoformans cells, we examined the abilities of GXM, MP1, and pustulan to inhibit SP-D binding to the encapsulated H99 strain and the acapsular cap59⌬ mutant. We found that all competitors, as well as many sugars known to bind the CRD domain of SP-D with variable affinities (11,12,37), inhibited SP-D binding to H99 cells to a much greater extent than that to cap59⌬ cells (Fig. 4).…”

Mutagenesis of Surfactant Protein D Informed by Evolution and X-ray Crystallography Enhances Defenses against Influenza A Virus in Vivo

“…Furthermore, differing interactions with these side chains contribute to known differences in saccharide selectivity. For example, the preference of human SP-D for D-N-acetylmannosamine over D-mannose can be attributed to hydrogen bonding of the Asp-325 carboxyl group with the N-acetyl substituent of the sugar (15,24).…”