We analyzed the production of Torpedo marmorata acetylcholinesterase (AChE) in transfected COS cells. We report that the presence of an aspartic acid at position 397, homologous to that observed in other cholinesterases and related enzymes (Krejci, E., N. Duval, A. Chatonnet, P. Vincens, and J. Massoulié. 1991. Proc. Natl. Acad. Sci. USA. 88:6647-6651), is necessary for catalytic activity. The presence of an asparagine in the previously reported cDNA sequence (Sikorav, J.L., E. Krejci, and J. Massoulié. 1987. EMBO (Eur. Mol. Biol. Organ.) J. 6:1865-1873) was most likely due to a cloning error (codon AAC instead of GAC). We expressed the T and H subunits of Torpedo AChE, which differ in their COOH-terminal region and correspond respectively to the collagen-tailed asymmetric forms and to glycophosphatidylinositol-anchored dimers of Torpedo electric organs, as well as a truncated T subunit (T delta), lacking most of the COOH-terminal peptide. The transfected cells synthesized similar amounts of AChE immunoreactive protein at 37 degrees and 27 degrees C. However AChE activity was only produced at 27 degrees C and, even at this temperature, only a small proportion of the protein was active. We analyzed the molecular forms of active AChE produced at 27 degrees C. The H polypeptides generated glycophosphatidylinositol-anchored dimers, resembling the corresponding natural AChE form. The cells also released non-amphiphilic dimers G2na. The T polypeptides generated a series of active forms which are not produced in Torpedo electric organs: G1a, G2a, G4a, and G4na cellular forms and G2a and G4na secreted forms. The amphiphilic forms appeared to correspond to type II forms (Bon, S., J. P. Toutant, K. Méflah, and J. Massoulié. 1988. J. Neurochem. 51:776-785; Bon, S., J. P. Toutant, K. Méflah, and J. Massoulié. 1988. J. Neurochem. 51:786-794), which are abundant in the nervous tissue and muscles of higher vertebrates (Bon, S., T. L. Rosenberry, and J. Massoulié. 1991. Cell. Mol. Neurobiol. 11:157-172). The H and T catalytic subunits are thus sufficient to account for all types of known AChE forms. The truncated T delta subunit yielded only non-amphiphilic monomers, demonstrating the importance of the T COOH-terminal peptide in the formation of oligomers, and in the hydrophobic character of type II forms.
The "tailed" molecules of Electrophorus (electric eel) acetylcholinesterase aggregate under conditions of low ionic strength. These aggregates have been studied by sedimentation analysis and high-resolution electron microscopy. They consist of bundles of at least half a dozen molecules, the tails of which are packed side by side, to form the core of the structure. Although aggregation is normally fully reversible, aggregates were irreversibly stabilized by methylene blue-sensitized photo-oxidation. This process was shown to consist of a singlet oxygen oxidation reaction and probably involves methionine or histidine residues. It did not modify the structural or hydrodynamic characteristics of the aggregates.