Arg343 in Human Surfactant Protein D Governs Discrimination between Glucose and N-Acetylglucosamine Ligands

Date
2004-08-01
Authors
Allen, Martin
Laederach, Alain
Reilly, Peter
Reilly, Peter
Mason, Robert
Voelker, Dennis
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Chemical and Biological Engineering
Abstract

Surfactant protein D (SP-D), one of the members of the collectin family of C-type lectins, is an important component of pulmonary innate immunity. SP-D binds carbohydrates in a calcium-dependent manner, but the mechanisms governing its ligand recognition specificity are not well understood. SP-D binds glucose (Glc) stronger than N-acetylglucosamine (GlcNAc). Structural superimposition of hSP-D with mannose- binding protein C (MBP-C) complexed with GlcNAc reveals steric clashes between the ligand and the side chain of Arg343 in hSP-D. To test whether Arg343contributes to Glc > GlcNAc recognition specificity, we constructed a computational model of Arg343→Val (R343V) mutant hSP-D based on homology with MBP-C. Automated docking of α-Me-Glc and α-Me-GlcNAc into wild-type hSP-D and the R343V mutant of hSP-D suggests that Arg343 is critical in determining ligand-binding specificity by sterically prohibiting one binding orientation. To empirically test the docking predictions, an R343V mutant recombinant hSP-D was constructed. Inhibition analysis shows that the R343V mutant binds both Glc and GlcNAc with higher affinity than the wild-type protein and that the R343V mutant binds Glc and GlcNAc equally well. These data demonstrate that Arg343 is critical for hSP-D recognition specificity and plays a key role in defining ligand specificity differences between MBP and SP-D. Additionally, our results suggest that the number of binding orientations contributes to monosaccharide binding affinity.

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This is a post-print of an article from Glycobiology, 14, no. 8 (2004): 693–700, doi: 10.1093/glycob/cwh088.

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