Bovine brain acetylcholinesterase primary Sequence involved in intersubunit disulfide linkages

Three distinct classes of membrane-bound acetylcholinesterases (AChEs) have been identified. A₁₂ AChE is composed of 12 catalytic subunits that are linked to noncatalytic collagen-like subunits through intersubunit disulfide bonds. G₂ AChE is localized in membranes by a glycoinositol phospholipid covalently linked to the C-terminal amino acid. Brain G4 AChE involves two catalytic subunits linked by a direct intersubunit disulfide bond while the other two are disulfide-linked to a membrane-binding 20-kDa noncatalytic subunit. Molecular cloning studies have so far failed to find evidence of more than one AChE gene in any organism although alternative splicing of torpedo AChE mRNA results in different C-terminal sequences for the A₁₂ and G₂ AChE forms. Support for a single bovine AChE gene is provided in this report by amino acid sequencing of the N-terminal domains from the G2 erythrocyte, G₄ fetal serum, and G₄ brain AChE. Comparison of the 38-amino acid sequences reveals virtually complete identity among the three AChE forms. Additional extensive identity between the fetal serum and brain AChEs was demonstrated by sequencing several brain AChE peptides isolated by high performance liquid chromatography after trypsin digestion of nitrocellulose blots of brain AChE catalytic subunits. Cysteines involved in intersubunit disulfide linkages in brain AChE were reduced selectively with dithiothreitol in the absence of denaturants and radioalkylated with iodoacetamide. The observed sequence of the major radiolabeled tryptic peptide was C*SDL, where C* was the radioalkylated cysteine residue. This sequence is precisely the same as that observed at the C terminus of fetal bovine serum AChE and shows close homology to the C-terminal sequence of torpedo A₁₂ AChE. We conclude that the mammalian brain G₄ AChEs utilize the same exon splicing pattern as the A₁₂ AChEs and that factors other than the primary sequence of the AChE catalytic subunits dictate assembly with either the collagen-like or the 20-kDa noncatalytic subunits.