Cloning and expression of two secretory acetylcholinesterases from the bovine lungworm, Dictyocaulus viviparus.

We describe the molecular cloning, expression and biochemical characterisation of recombinant forms of two secreted acetylcholinesterases from adult Dictyocaulus viviparus. The two variants (designated Dv-ACE-1 and Dv-ACE-2) were 613 and 615 amino acids long and showed 94.7% identity to one another. The highest level of identity to other cholinesterases was with ACE-2 of Caenorhabditis elegans. Dv-ACE-1 and Dv-ACE-2 showed 48.0 and 47.7% identity to C. elegans ACE-2 over 577 amino acids, respectively. The primary structure of both enzymes showed conservation of the catalytic triad and of a tryptophan residue known to be critical for the choline-binding site, but differed in the number of potential glycosylation sites and at one amino acid in the peripheral anionic site. Southern blotting and PCR experiments indicated that the genes encoding these enzymes are distinct. When expressed in Pichia pastoris, the enzymes were active, but differed subtly in their biochemical characteristics. Both enzymes exhibited a preference for acetylcholine as substrate, but differed in the extent of excess substrate inhibition and in their optimal pH for activity. The lack of an obvious carboxy-terminal membrane anchor and the presence of an insertion at the molecular surface were other features which, thus far, appear to be characteristic of parasite secreted acetylcholinesterases.