Quaternary associations of acetylcholinesterase. I. Oligomeric associations of T subunits with and without the amino-terminal domain of the collagen tail

Abstract

We investigated the production of acetylcholinesterase of type T (AChET) in COS cells during transient transfection. When expressed alone, Torpedo AChET remains essentially intracellular, forming dimers and tetramers; in contrast, rat AChET is secreted and produces mostly amphiphilic monomers (G1a) and dimers (G2a), together with smaller proportions of nonamphiphilic (G4na) tetramers, amphiphilic tetramers (G4a), and an unstable higher polymer (13.7 S). The latter two forms have not been described before. We show that secreted G1a and G2a forms differ from their cellular counterparts and that proteolytic cleavage occurs at the COOH terminus of "flagged" subunits. The binding proteins QN/HC and QN/stop are constructed by associating the NH2-terminal domain of the collagen tail (QN) with a functional or truncated signal for addition of a glycolipidic anchor (glycophosphatidylinositol). Coexpression with QN/stop recruits monomers and dimers to form soluble tetramers (G4na), increasing the yield of secreted rat AChE and allowing secretion of Torpedo AChE. Using antibodies against QN or addition of a flag epitope, we showed that the secreted tetramers contain the attachment domain. Coexpression with QN/HC modifies the distribution of AChET in subcellular compartments and allows the externalization of glycophosphatidylinositol-anchored tetramers at the cell surface.