Three genes encoding distinct pharmacological classes of acetylcholinesterase (AChE of classes A, B and C) had been defined by genetics:
ace-1,
ace-2 and
ace-3. We first cloned and sequenced
ace-1. A 2.4 kb fragment of 5' region and the coding sequence of
ace-1 were sufficient to rescue the uncoordinated phenotype
ace-1;
ace-2. GFP expression driven by this 5' region was used to define the tissue specific pattern of
ace-1 transcription: body wall and anal muscle cells,
pm5 pharyngeal muscle cells and only a few neurons in the head and retrovesicular ganglia. A comparison of 2.4 kb of promoter regions of
ace-1 in C. elegans and C. briggsae showed four blocks of conserved sequences. One of these blocks was shown to behave as a body wall muscle cells enhancer and another was found necessary for pharyngeal localization. We cloned and sequenced
ace-2 (encoding AChE of class B). ACE-2 possesses all the residues characteristic of the cholinesterase family. Whereas the C-terminus is hydrophilic in ACE-1 with a strong similarity with C-terminal sequences of T subunits of vertebrate AChEs, the C-terminus of ACE-2 is similar to that of H subunits of vertebrate AChEs, which is cleaved and exchanged to a glycolipid anchor during post-translational maturation. C-terminal sequences of ACE-1 and ACE-2 are thus totally compatible with their molecular forms described previously. It is noteworthy that in vertebrates, a single AChE gene gives rise to H or T transcripts through alternative splicing of the two last exons, whereas in C. elegans two ace genes are required for generating such a diversity.
ace-2 is located on chromosome I (Y44E3 and Y52G11). We sequenced
ace-2 in the mutant strain
g72: we found a transition G---A that resulted in the change G441E. This introduces a negative charge in immediate vicinity of the positively-charged H440 and might disrupt the normal charge relay (E327, H440, S200) that activates S200 during catalysis. GFP expression driven by 3.7 kb of 5' region showed that
ace-2 was transcribed in a large number of neurons in the head and anal ganglia and in
pm5 cells of the pharynx. In contrast with
ace-1,
ace-2 was not expressed in body wall muscle cells.