We report the full coding sequences and the genomic organization of the four genes encoding acetylcholinesterase (AChE) in Caenorhabditis elegans and Caenorhabditis briggsae, in relation to the properties of the encoded enzymes. ace-1 and ace-2, located on chromosome X and I, respectively, encode two AChEs (ACE-1 and ACE-2) that present 35% identity. The C-terminal end of ACE-1 is homologous to the C terminus of T subunits of vertebrate AChEs. ACE-1 oligomerizes into amphiphilic tetramers. ACE-2 has a hydrophobic C terminus of H type. It associates into glycolipid-anchored dimers. In C. elegans and C. briggsae, ace-3 and ace-4 are organized in tandem on chromosome II, with only 356 nt and 369 nt, respectively, between the stop codon of ace-4 (upstream gene) and the ATG of ace-3. ace-3 produces only 5 % of the total AChE activity. It encodes an H subunit that associates into dimers of glycolipid-anchored catalytic subunits, which are highly resistant to the usual AChE inhibitors, and which hydrolyze butyrylthiocholine faster than acetylthiocholine. ACE-4 is closer to ACE-3 (54 % identity) than to ACE-1 or ACE-2. The usual sequence FGESAG surrounding the active serine residue in cholinesterases is changed to FGQSAG in ace-4. ACE-4 was not detected by our current biochemical methods, although the gene is transcribed in vivo. However the level of ace-4 mRNAs is far lower than those of ace-1, ace-2 and ace-3. The ace-2, ace-3 and ace-4 transcripts were found to be trans-spliced by both SL1 and SL2, although these genes are not included in typical operons. The molecular bases of null mutations g72 (ace-2), p1304 and dc2 (ace-3) have been identified.
Copyright 2000 Academic Press.