The C-terminal t peptide of acetylcholinesterase forms an alpha helix that supports homomeric and heteromeric interactions

Acetylcholinesterase subunits of type T (AChE(T)) possess an alternatively spliced C-terminal peptide (t peptide) which endows them with amphiphilic properties, the capacity to form various homo-oligomers and to associate, as a tetramer, with anchoring proteins containing a proline rich attachment domain (PRAD). The t peptide contains seven conserved aromatic residues. By spectroscopic analyses of the synthetic peptides covering part or all of the t peptide of Torpedo AChE(T), we show that the region containing the aromatic residues adopts an alpha helical structure, which is favored in the presence of lipids and detergent micelles: these residues therefore form a hydrophobic cluster in a sector of the helix. We also analyzed the formation of disulfide bonds between two different AChE(T) subunits, and between AChE(T) subunits and a PRAD-containing protein [the N-terminal fragment of the ColQ protein (Q(N))] possessing two cysteines upstream or downstream of the PRAD. This shows that, in the complex formed by four T subunits with Q(N) (T-4-Q(N)), the t peptides are not folded on themselves as hairpins but instead are all oriented in the same direction, antiparallel to that of the PRAD. The formation of disulfide bonds between various pairs of cysteines, introduced by mutagenesis at various positions in the t peptides, indicates that this complex possesses a surprising flexibility.