Ranaspumin-2: structure and function of a surfactant protein from the foam nests of a tropical frog

Ranaspumin-2 (Rsn-2) is a monomeric, 11 kDa surfactant protein identified as one of the major foam nest components of the túngara frog (Engystomops pustulosus), with an amino acid sequence unlike any other protein described so far. We report here on its structure in solution as determined by high-resolution NMR analysis, together with investigations of its conformation and packing at the air-water interface using a combination of infrared and neutron reflectivity techniques. Despite the lack of any significant sequence similarity, Rsn-2 in solution adopts a compact globular fold characteristic of the cystatin family, comprising a single helix over a four-stranded sheet, in a motif not previously associated with surfactant activity. The NMR structure of Rsn-2 shows no obvious amphiphilicity that might be anticipated for a surfactant protein. This suggests that it must undergo a significant conformational change when incorporated into the air-water interface that may involve a hinge-bending, clamshell opening of the separate helix and sheet segments to expose hydrophobic faces to air while maintaining the highly polar surfaces in contact with the underlying water layer. This model is supported by direct observation of the relative orientations of secondary structure elements at the interface by infrared reflection absorption spectroscopy, and by protein packing densities determined from neutron reflectivity profiles.