The duck-billed Platypus is such an odd creature that one could get the idea that its survival depends on potential predators laughing themselves to death, but in fact it can rely on a far more potent defense. It carries a venomous sting on its hind legs. Envenoming by a male Ornithorhynchus anatinus causes not only immediate excruciating pain, but also a long-lasting hypersensitivity, probably due to nerve damage.
The venom itself is a complex mixtures of peptides, for example defensin-like peptides similar to those found in venomous reptiles. Curiously those don’t go back to a common ancestor but evolved independently in both lines, as genomic analyses show. Another class of toxins consists of peptides related to C-type natriuretic peptides. CNPs are vasorelaxant peptide hormones that are widely distributed many tissues, notably the central nervous system.
The amino acid sequences of mammalian CNPs, such as human and mouse CNP are highly conserved, indicating that it fills some vital function within the body. The OvCNP from the Platypus poison, however, forms fast cation channels in lipid bilayers, which plays havoc with signal transduction in nerve cells. It also promotes histamine release from mast cells and promotes inflammation. So far, so usual.
But recently Japanese and Australian scientists came across something odd: They discovered a number of small peptides homologous to parts of OvCNP and its precursor in the venom. The interesting part is that these smaller peptides have a similar effect – they promote calcium influx – but they seem to work by an entirely different mechanism. They don’t form pores in bilayers. It appears that they interact with either the calcium channels or some other transmembrane receptors.
I don’t know how those small peptides are formed, but it appears that they must somehow be a product of the process that spawns the complete OvCNP. Probably they are evolutionary recent, but that’s a question still to be sorted out. But I wonder what will happen to them during the course of evolution? Will they increase in length and structural complexity until they form a separate class of full-fledged proteins?
Kita, M., Black, D., Ohno, O., Yamada, K., Kigoshi, H., & Uemura, D. (2009). Duck-Billed Platypus Venom Peptides Induce Ca
Influx in Neuroblastoma Cells
Journal of the American Chemical Society, 131 (50), 18038-18039 DOI: 10.1021/ja908148z