Assembly of acetylcholinesterase tetramers by peptidic motifs from the proline-rich membrane anchor, PRiMA -: Competition between degradation and secretion pathways of heteromeric complexes

The membrane-bound form of acetylcholinesterase (AChE) constitutes the major component of this enzyme in the mammalian brain. These molecules are hetero-oligomers, composed of four AChE catalytic subunits of type T (AChE(T)), associated with a transmembrane protein of type 1, called PRiMA (proline-rich membrane anchor). PRiMA consists of a signal peptide, an extracellular domain that contains a proline-rich motif (14 prolines with an intervening leucine, P4LP10), a transmembrane domain, and a cytoplasmic domain. Expression of AChET subunits in transfected COS cells with a truncated PRiMA, without its transmembrane and cytoplasmic domains (P-stp54 mutant), produced secreted heteromeric complexes (T-4-P-stp54), instead of membrane-bound tetramers. In this study, we used a series of deletions and point mutations to analyze the interaction between the extracellular domain of PRiMA and AChET subunits. We confirmed the importance of the polyproline stretches and defined a peptidic motif (Rp(4)LP(10)RL), which induces the assembly and secretion of a heteromeric complex with four AChET subunits, nearly as efficiently as the entire extracellular domain of PRiMA. It is noteworthy that deletion of the N-terminal segment preceding the prolines had little effect. Interestingly, short PRiMA mutants, truncated within the proline-rich motif, reduced both cellular and secreted AChE activity, suggesting that their interaction with AChET subunits induces their intracellular degradation.