Abstract
The fish-hunting groups of venomous marine cone snails (genus Conus) possess a high diversity of conotoxins that can be attributed to positive selection of venom peptide encoding genes and a high degree of post-translational modification of the mature peptides. Of three distinct clades of fish-hunters represented by the species C. catus, C. purpurascens, and C. tulipa, the C. catus and C. purpurascens groups are exclusive harpooners that use neuroexcitatory peptides to subdue prey. The C. tulipa group, which includes C. geographus and C. obscurus, has a range of feeding behaviors from harpooners to netfeeders. In addition, C. tulipa differs in prey capture behavior in that it does not hold on to its prey after injection, unlike other harpooners. Thus, it is uncertain whether neuroexcitatory peptides exist in the C. tulipa group of fish-hunting cone snails. In this study, we looked at the A-superfamily conotoxin genes, which encode α-conotoxins and αA-conotoxins that cause flaccid paralysis by inhibiting nicotinic acetylcholine receptors, and neuroexcitatory peptides that cause tetanic paralysis by a mechanism under investigation. Neuroexcitatory peptides have only been discovered in fish-hunting Conus species to date. While A-superfamily neuroexcitatory homologs in C. tulipa were not identified in the initial screen, seven novel C. tulipa α-conotoxins and two novel C. purpurascens α4/7 conotoxins were identified in the genome of these species. The C. tulipa sequences define two subfamilies of α4/7 conotoxins, with one subfamily showing distinct amino acid variability between the conserved intercysteine loops. Previous studies have shown that α4/7 conotoxins target a variety of neuronal-type nicotinic acetylcholine receptor (nAChR) subtypes, which could provide insight into the physiological roles of these receptor subtypes in pain, inflammation, nicotine addiction, Alzheimer?s disease, and Parkinson?s disease. The diversity of α4/7 conotoxin sequences observed in C. tulipa warrants further investigation into highly selective inhibitors of nAChR subtypes. In addition, the absence of A-superfamily neuroexcitatory peptide sequences in C. tulipa thus far encourages a more in-depth search. We have recently adapted the use of a massively parallel sequencing approach to obtain a more extensive library of A-superfamily genes from representatives of each distinct fish-hunting group. Feeding behavior differences likely contribute to the variation in venom composition of fish-hunting Conus species.
