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

Snake alpha-neutotoxins of Elapidae venoms are grouped into two structural classes, short-chain and long-chain alpha-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 alpha-neurotoxins in Elapidae snakes also interfere with the biological action of short-chain alpha-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 alpha-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 a subunit segment T-154 -L-208 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 T-154 -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 alpha-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 alpha-neurotoxin resistance suggests that alpha-neurotoxin-resistant nAChR evolved first, which permitted the evolution of snake venom alpha-neurotoxins. A model describing alpha-neurotoxin resistance in Elapidae snakes is presented.