The C-terminal peptides of acetylcholinesterase: cellular trafficking, oligomerization and functional anchoring

In vertebrates, the catalytic domain of acetylcholinesterase (AChE) may be associated with several C-terminal peptides generated by alternative splicing in the 3' region of transcripts. The "readthrough" (R) variant results from a lack of splicing after the last exon encoding the catalytic domain. Such a variant has been observed in Torpedo and in mammals; its C-terminal r peptide, also called "AChE Related Peptide" (ARP), is poorly conserved between rodents and humans. In rodents, it is significantly expressed in embryonic tissues and at a very low level in the brain of adult mice; it may be increased under various stress conditions, but remains very low. The "hydrophobic" (H) variant generates glycolipid (GPI)-anchored dimers, which are expressed in muscles of Torpedo, and in blood cells of mammals; H variants exist in Torpedo and in mammals, but apparently not in other vertebrate classes, suggesting that they were lost during evolution of early vertebrates and re-appeared independently in mammals. The "tailed" (T) variant exists in all vertebrate cholinesterases and their C-terminal t peptides are strongly conserved; in mammals, AChE(T) subunits represent the major type of acetylcholinesterase in cholinergic tissues. They produce a wide variety of oligomeric forms, ranging from monomers to heteromeric assemblies containing the anchoring proteins ColQ (collagen-tailed forms) and PRiMA (membrane-bound tetramers), which constitute the major functional enzyme species in mammalian muscles and brain, respectively. The oligomerization of AChE(T) subunits depends largely on the properties of their C-terminal t peptide. These peptides contain seven conserved aromatic residues, including three tryptophans, and are organized in an amphiphilic alpha helix in which these residues form a hydrophobic cluster. The presence of a cysteine is required for dimerization, while aromatic residues are necessary for tetramerization. In the collagen-tailed molecules, four t peptides form a coiled coil around a proline-rich motif (PRAD) located in the N-terminal region of ColQ. The t peptide also strongly influences the folding and cellular trafficking of AChE(T) subunits: the presence of hydrophobic residues induces partial misfolding leading to inactive protein, while aromatic residues, organized or not in an amphiphilic helix, induce intracellular degradation through the "Endoplasmic Reticulum Associated Degradation" (ERAD) pathway, rather than secretion. It has been proposed that the r and t C-terminal peptides, or fragments of these peptides, may exert independent, non cholinergic biological functions: this interesting possibility still needs to be documented, especially in view of their various degrees of evolutionary conservation.