Quaternary associations of acetylcholinesterase .2. The polyproline attachment domain of the collagen tail

In transfected COS cells, we analyzed the formation of heteromeric associations between rat acetylcholinesterase of type T (AChE(T)) and various constructions derived from the NH2-terminal regions of the collagen tail of asymmetric forms, Q(N). Using a series of deletions and point mutations in Q(N), we showed that the binding of AChE(T) to Q(N) does not require the cysteines that normally establish intersubunit disulfide bonds with catalytic subunits and that it essentially relies on the presence of stretches of successive proline, although adjacent residues also contribute to the interaction. We thus defined a (p) under bar roline-<(r)rich (a) under bar ttachment (d) under bar omain or PRAD, which recruits AChE(T) subunits to form heteromeric associations. Such molecules, consisting of one PRAD associated with a tetramer of AChE(T), are exported efficiently by the cells. Using the proportion of AChE(T) subunits engaged in heteromeric tetramers, we ranked the interaction efficiency of various constructions. From these experiments we evaluated the contribution of different elements of the PRAD to the quaternary assembly of AChE(T) subunits in the secretory pathway. The PRAD remained functional when reduced to six residues followed by a string of 10 prolines (Glu-Ser-Thr-Gly3-Pro(10)). We then showed that synthetic polyproline itself can associate with AChE(T) subunits, producing well defined tetramers, when added to live transfected cells or even to cell extracts. This is the first example of an in vitro assembly of AChE tetramers from monomers and dimers. These results open the way to a chemical-physical exploration of the formation of these quaternary associations, both in the secretory pathway and in vitro.