Elucidation of Lipid Binding Sites on Lung Surfactant Protein A Using X‑ray Crystallography, Mutagenesis, and Molecular Dynamics Simulations

• Published in: Biochemistry (Easton) 2016-07, Vol.55 (26), p.3692-3701
• Main Authors: Goh, Boon Chong, Wu, Huixing, Rynkiewicz, Michael J, Schulten, Klaus, Seaton, Barbara A, McCormack, Francis X
• Format: Article
• Language: English
• Subjects: 1,2-Dipalmitoylphosphatidylcholine - metabolism; Animals; Binding Sites; Crystallography, X-Ray - methods; Molecular Dynamics Simulation; Mutagenesis, Site-Directed; Mutation - genetics; Protein Conformation; Proteolipids - metabolism; Pulmonary Surfactant-Associated Protein A - chemistry; Pulmonary Surfactant-Associated Protein A - genetics; Pulmonary Surfactant-Associated Protein A - metabolism; Pulmonary Surfactants - chemistry; Pulmonary Surfactants - metabolism; Rats
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/acs.biochem.6b00048

Surfactant protein A (SP-A) is a collagenous C-type lectin (collectin) that is critical for pulmonary defense against inhaled microorganisms. Bifunctional avidity of SP-A for pathogen-associated molecular patterns (PAMPs) such as lipid A and for dipalmitoylphosphatidylcholine (DPPC), the major component of surfactant membranes lining the air–liquid interface of the lung, ensures that the protein is poised for first-line interactions with inhaled pathogens. To improve our understanding of the motifs that are required for interactions with microbes and surfactant structures, we explored the role of the tyrosine-rich binding surface on the carbohydrate recognition domain of SP-A in the interaction with DPPC and lipid A using crystallography, site-directed mutagenesis, and molecular dynamics simulations. Critical binding features for DPPC binding include a three-walled tyrosine cage that binds the choline headgroup through cation−π interactions and a positively charged cluster that binds the phosphoryl group. This basic cluster is also critical for binding of lipid A, a bacterial PAMP and target for SP-A. Molecular dynamics simulations further predict that SP-A binds lipid A more tightly than DPPC. These results suggest that the differential binding properties of SP-A favor transfer of the protein from surfactant DPPC to pathogen membranes containing appropriate lipid PAMPs to effect key host defense functions.