Cobra (Naja spp.) nicotinic acetylcholine receptor exhibits resistance to Erabu sea snake (Laticauda semifasciata) short-chain α-neurotoxin

Abstract

Snake α-neutotoxins of Elapidae venoms are grouped into two structural classes, short-chain and long-chain α-neutotoxins. While these two classes share many chemical and biological characteristics, there are also distinct dissimilarities between them, including their binding site on the nicotinic acetylcholine receptor (nAChR), specificity among species of Chordata, and the associated pharmacological effects. In the present study we test the hypothesis that structural motifs that evolved to confer natural resistance against conspecific long-chain α-neurotoxins in Elapidae snakes also interfere with the biological action of short-chain α-neurotoxins. We expressed functional nAChRs that contains segments or single residues of the Elapidae nAChR ligand binding domain and tested the effect of short-chain α-neurotoxin erabutoxin-a (ETX-a) from the Erabu sea snake Laticauda semifasciata on the acetylcholine-induced currents as measured by two-microelectrode voltage clamp. Our results show that the Elapidae nAChR α subunit segment T154-L208 ligand binding domain has an inhibitory effect on the pharmacological action of ETX-a. This effect is primarily attributed to the presence of glycosylation at position N 189. If the glycosylation is removed from the T154-L 208 segment, the nAChR will be inhibited, however, to a lesser extent than seen in the mouse. This effect correlates with the variations in α-neurotoxin sensitivity of different species and, importantly, reflects the evolutionary conservation of the binding site on the nAChR polypeptide backbone per se. Phylogenetic analysis of α-neurotoxin resistance suggests that α-neurotoxin-resistant nAChR evolved first, which permitted the evolution of snake venom α-neurotoxins. A model describing α-neurotoxin resistance in Elapidae snakes is presented.