Proteins of frog foam nests
Proteins of frog foam nests
Many species of tropical and subtropical frog lay their eggs in foams. The foams are remarkable biomaterials in being highly stable and resist degradation by microbes and other environmental assaults, yet are compatible with naked eggs and sperm. This is paradoxical because one would normally expect a foam to have strong detergent-like properties that would destroy eggs and sperm. So, we headed off to Trinidad (someone had to do it!) to collect material from the tungara frog, Physalaemus pustulosus. It turns out that the foam matrix does not contain small molecule detergents, but does have a range of proteins that may explain the foam's remarkable properties. We cloned cDNAs encoding all six of the major proteins in the nest foam. Among these are a surfactant protein that probably explains the stability of the foams whilst not being damaging to the frog's cells, and several unusual lectins that probably protect the nests and their contents from microbial attack. Most of these lectins are of a type common in fish but has never before been found in a land vertebrate. This work has again been carried out in collaboration with Alan Cooper, and the structure of the surfactant protein was solved by Cameron Mackenzie as a PhD student working with Brian Smith.
Since then we've been looking at a strange blue-cloured proteins in the nests of a frog from the other side of the world, Polypedates leucomystax from Malaysia. Most, but not all, of the nests of this species turn dark green on the outside after laying, and if one opens a nest exposing the white interior foam to light and air, it turns blue-green within minutes. Amazing. The protein that is involved has an unusual structure, with protein-protein cross-links that have not been observed before, plus a remarkable chromphore involving an inter-molecular tyrosine-tyrosine crosslink coordinated with a zinc atom. Endearingly, the protein has been named Ranasmurfin. Our colloborators in the project are Rosalind Tan and Aishah Latiff (both of Universiti Sains Malaysia) Alan Cooper, and Jim Naismith from St Andrews University. We know an awful lot about the structure of Ranasmurfin, but are still very puzzled as to what it does.
Here is a picture of a Physalaemus foam nest.
Here is a picture of a pair of Physalaemus pustulosus contructing a nest in the wild
Here is the crystal structure of the strange and wonderful Ranasmurfin protein.
We found that Physalaemus pustulosus nests have a layer of egg-free foam ~1cm deep over their entire surface. We do not know how the male frogs arrange the eggs inside the foam nest in this way. When we let a nest develop fully on a dish without water and turn it over about three days later, all the tadpoles are confined to the central protected region. A picture ..