REPLACEMENT OF THE GLYCOINOSITOL PHOSPHOLIPID ANCHOR OF DROSOPHILA ACETYLCHOLINESTERASE WITH A TRANSMEMBRANE DOMAIN DOES NOT ALTER SORTING IN NEURONS AND EPITHELIA BUT RESULTS IN BEHAVIORAL DEFECTS

Drosophila has a single glycoinositol phospholipid (GPI)-anchored form of acetylcholinesterase (AChE) encoded by the Ace locus. To assess th e role that GPI plays in the physiology of AChE, we have replaced the wild-type GPI-AChE with a chimeric transmembrane form (TM-AChE) in the nervous system of the fly. Ace null alleles provided a genetic backgr ound completely lacking in endogenous GPI-AChE, and Ace minigene P tra nsposon constructs were used to express both GPI- and TM-AChE forms in the tissues where AChE is normally expressed. Control experiments wit h the GPI-AChE minigene demonstrated a threshold between 9 and 12% of normal AChE activity for adult viability. Ace mutant flies were rescue d by GPI-AChE minigene lines that expressed 12-40% of normal activity and were essentially unchanged from wild-type flies in behavior. TM-AC hE minigene lines were able to rescue Ace null alleles, although with a slightly higher threshold than that for GPI-AChE. Although rescued f lies expressing GPI-AChE ata level of 12% of normal activity were viab le, flies expressing 13-16% of normal activity from the TM-AChE transg ene died shortly after eclosion. Flies expressing TM-AChE at about 30% of normal levels were essentially unchanged from wild-type flies in g ross behavior but had a reduced lifespan secondary to subtle coordinat ion defects. These flies also showed reduced locomotor activity and pe rformed poorly in a grooming assay. However, Light level and electron microscopic immunocytochemistry showed no differences in the localizat ion of GPI- and TM-AChE. Furthermore, endogenous and ectopic-induced e xpression of both AChEs in epithelial tissues of the adult and embryo, respectively, showed that they were sorted identically. Most epitheli al cells sorted GPI- and TM-AChE to the apical surface, but cuticle-se creting epithelia sorted both proteins basolaterally. Our data suggest that rather than having a primary role in protein sorting, the GPI an chor of AChE plays some other more subtle cellular role in neuronal ph ysiology.