Elucidation of Lipid Binding Sites on Lung Surfactant Protein A Using X-ray Crystallography, Mutagenesis, and Molecular Dynamics Simulations.
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.
Beckman Institute and Department of Physics, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
Boon Chong
BC
Goh
Division of Pulmonary, Critical Care and Sleep Medicine, Department of Internal Medicine, The University of Cincinnati , Cincinnati, Ohio 45267, United States.
Huixing
H
Wu
Department of Physiology and Biophysics, Boston University School of Medicine , Boston, Massachusetts 02118, United States.
Michael J
MJ
Rynkiewicz
Beckman Institute and Department of Physics, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
Klaus
K
Schulten
Department of Physiology and Biophysics, Boston University School of Medicine , Boston, Massachusetts 02118, United States.
Barbara A
BA
Seaton
Division of Pulmonary, Critical Care and Sleep Medicine, Department of Internal Medicine, The University of Cincinnati , Cincinnati, Ohio 45267, United States.
Francis X
FX
McCormack
D015060:1,2-Dipalmitoylphosphatidylcholine / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D018360:Crystallography, X-Ray / Q000379:methods*; D056004:Molecular Dynamics Simulation; D016297:Mutagenesis, Site-Directed; D009154:Mutation / Q000235:genetics; D011487:Protein Conformation; D011510:Proteolipids / Q000378:metabolism*; D037662:Pulmonary Surfactant-Associated Protein A / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D011663:Pulmonary Surfactants / Q000737:chemistry* / Q000378:metabolism*; D051381:Rats
D011510:Proteolipids; D037662:Pulmonary Surfactant-Associated Protein A; D011663:Pulmonary Surfactants; D015060:1,2-Dipalmitoylphosphatidylcholine