Acetylcholinesterases of the cat flea Ctenocephalides felis: identification of two distinct genes and biochemical characterization of recombinant and in vivo enzyme activities

Acetylcholinesterase (AChE, EC3.1.1.7.) is a prime target for insecticides and is the site of action of carbamate and organophosphate drugs used to combat the cat flea Ctenocephalides felis. In this paper we report the identification and cDNA cloning of two AChE-encoding genes from the cat flea, cface1 and cface2. Functional heterologous expression of the catalytic domains in Pichia pastoris shows that both genes encode functional enzymes, CfAChE1 and CfAChE2. Bioinformatical analysis of the predicted translation products and heterologous expression of the full length cDNAs in the human cell line HEK293 demonstrate, that CfAChE1 and CfAChE2 possess glycosylphosphatidylinositol membrane anchors and are transported to the cell surface. Recombinant CfAChE1 and CfAChE2 share high sensitivity towards the anti-flea carbamates propoxur and carbaryl, but can be distinguished by their specificity for different acylthiocholine AChE substrates and, particularly, by their differential sensitivity to the non-covalent inhibitor galanthamine. Comparison of substrate specificities and inhibitor sensitivities of both recombinant enzymes with those of AChE activities extracted from adult fleas suggest that CfAChE1, and not CfAChE2, is the dominant activity in C. felis imagoes. Three-dimensional structure models of CfAChE1 and CfAChE2 reveal similarities, but also differences, and compound docking experiments on these models provide potential rationales for the differential substrate and the inhibitor specificities observed experimentally.