Biofoams and natural protein surfactants

Abstract

Naturally occurring foam constituent and surfactant proteins with intriguing structures and functions are now being identified from a variety of biological sources. The ranaspumins from tropical frog foam nests comprise a range of proteins with a mixture of surfactant, carbohydrate binding and antimicrobial activities that together provide a stable, biocompatible, protective foam environment for developing eggs and embryos. Ranasmurfin, a blue protein from a different species of frog, displays a novel structure with a unique chromophoric crosslink. Latherin, primarily from horse sweat, but with similarities to salivary, oral and upper respiratory tract proteins, illustrates several potential roles for surfactant proteins in mammalian systems. These proteins, together with the previously discovered hydrophobins of fungi, throw new light on biomolecular processes at air-water and other interfaces. This review provides a perspective on these recent findings, focussing on structure and biophysical properties.

Fig. 1

Foam nests of the African foam nesting tree frog, Chiromantis xerampelina. Usually found adjacent to water after heavy rain, in overhanging vegetation or, as here, on old tree stumps (MalaMala, Mpumalanga Province, South Africa, January 2010. Photo: Alan Cooper).

Fig. 2

Cartoon showing the possible arrangement of protein/carbohydrate assemblies at the air–water interface conferring stability to natural biofoams. This hypothesis is developed from detailed studies of the foam nest components of the túngara frog , with approximate dimensions (not to scale) estimated from neutron scattering of both the natural mixture and isolated recombinant ranaspumin-2 .

Fig. 3

(a) NMR solution structure of ranaspumin-2 (RSN-2), the surfactant protein from foam nests of the túngara frog. (b) Hypothetical “open” conformation of RSN-2 that might be adopted at the air–water interface. (Adapted from [50]). The colour coding signifies chain progression from N- (blue) to C-terminal (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

(a) X-ray structure of the ranasmurfin dimer, isolated from foam nests of the Malaysian tree frog, Polypedates leucomystax. (b) Expanded view of the unusual Lys–Tyr–N–Tyr–Lys chromophore linking the two subunits. (Adapted from [48]).

Fig. 5

Comparison of a 49aa peptide sequence predicted from the archaean Methanobrevibacter smithii genome (Genbank accession EFC92631) and a segment of ranasmurfin from the frog, Polypedates leucomystax (113aa; SwissProt P85511.1). Alignment was obtained using the BLOSUM62 substitution matrix; identical residues are shown in bold (red), with similarities indicated by colons in the consensus sequence. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)