High-resolution Structural Insights into Ligand binding and Immune Cell Recognition by Human Lung Surfactant Protein D

• Published in: Journal of molecular biology Vol. 331; no. 2; pp. 509 - 523
• Main Authors: Shrive, Annette K, Tharia, Hazel A, Strong, Peter, Kishore, Uday, Burns, Ian, Rizkallah, Pierre J, Reid, Kenneth B.M, Greenhough, Trevor J
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 08.08.2003
• Subjects: Amino Acid Sequence; Aspergillus fumigatus; Aspergillus fumigatus - metabolism; Binding Sites; Calcium - metabolism; carbohydrate recognition; Collagen - chemistry; collectin; Crystallography, X-Ray; Glutamine - chemistry; high-resolution structure; Humans; Hydrogen Bonding; Ions; Ligands; Lipopolysaccharides - metabolism; Lung - metabolism; lung surfactant protein; Lysine - chemistry; Maltose - chemistry; Models, Molecular; Molecular Sequence Data; Protein Binding; Protein Structure, Tertiary; Pulmonary Surfactant-Associated Protein D - chemistry; Pulmonary Surfactant-Associated Protein D - metabolism; Recombinant Proteins - chemistry; Sequence Homology, Amino Acid; Tyrosine - chemistry; SP-D, lung surfactant protein D; carbohydrate recognition; high-resolution structure; MBL, mannan binding lectin; hSP-D, human lung surfactant protein D; hSP-A, human lung surfactant protein A; rfhSP-D, recombinant fragment of human lung surfactant protein D; MBP, mannose binding protein; collectin; lung surfactant protein; CRD, carbohydrate recognition domain; MPD, 2,4-methylpentane diol; SP-A, lung surfactant protein A; PEG, polyethyleneglycol
• ISSN: 0022-2836; 1089-8638
• DOI: 10.1016/S0022-2836(03)00761-7

Lung surfactant protein D (SP-D) can directly interact with carbohydrate residues on pulmonary pathogens and allergens, stimulate immune cells, and manipulate cytokine and chemokine profiles during the immune response in the lungs. Therapeutic administration of rfhSP-D, a recombinant homotrimeric fragment of human SP-D comprising the α-helical coiled-coil neck plus three CRDs, protects mice against lung allergy and infection caused by the fungal pathogen Aspergillus fumigatus. The high resolution crystal structures of maltose-bound rfhSP-D to 1.4 Å, and of rfhSP-D to 1.6 Å, define the fine detail of the mode and nature of carbohydrate recognition and provide insights into how a small fragment of human SP-D can bind to allergens/antigens or whole pathogens, and at the same time recruit and engage effector cells and molecules of humoral immunity. A previously unreported calcium ion, located on the trimeric axis in a pore at the bottom of the funnel formed by the three CRDs and close to the neck–CRD interface, is coordinated by a triad of glutamate residues which are, to some extent, neutralised by their interactions with a triad of exposed lysine residues in the funnel. The spatial relationship between the neck and the CRDs is maintained internally by these lysine residues, and externally by a glutamine, which forms a pair of hydrogen-bonds within an external cleft at each neck–CRD interface. Structural links between the central pore and the cleft suggest a possible effector mechanism for immune cell surface receptor binding in the presence of bound, extended natural lipopolysaccharide and phospholipid ligands. The structural requirements for such an effector mechanism, involving both the trimeric framework for multivalent ligand binding and recognition sites formed from more than one subunit, are present in both native hSP-D and rfhSP-D, providing a possible explanation for the significant biological activity of rfhSP-D.