A model for the structure and mechanism of action of pulmonary surfactant protein B

ABSTRACT Surfactant protein B (SP‐B), from the saposin‐like family of proteins, is essential to facilitate the formation and proper performance of surface active films at the air‐liquid interface of mammalian lungs, and lack of or deficiency in this protein is associated with lethal respiratory failure. Despite its importance, neither a structural model nor a molecular mechanism of SP‐B is available. The purpose of the present work was to purify and characterize native SP‐B supramolecular assemblies to provide a model supporting structure‐function features described for SP‐B. Purification of porcine SP‐B using detergent‐solubilized surfactant reveals the presence of 10 nm ring‐shaped particles. These rings, observed by atomic force and electron microscopy, would be assembled by oligomerization of SP‐B as a multimer of dimers forming a hydrophobically coated ring at the surface of phospholipid membranes or monolayers. Docking of rings from neighboring membranes would lead to formation of SP‐B‐based hydrophobic tubes, competent to facilitate the rapid flow of surface active lipids both between membranes and between surfactant membranes and the interface. A similar sequential assembly of dimers, supradimeric oligomers and phospholipid‐loaded tubes could explain the activity of other saposins with colipase, cytolysin, or antibiotic activities, offering a common framework to understand the range of functions carried out by saposins.—Olmeda, B., García‐Álvarez, B., Gómez, M. J., Martínez‐Calle, M., Cruz, A., Pérez‐Gil, J. Amodel for the structure and mechanism of action of pulmonary surfactant protein B. FASEB J. 29, 4236‐4247 (2015). www.fasebj.org