Amalgam is a ligand for the transmembrane receptor neurotactin and is required for neurotactin-mediated cell adhesion and axon fasciculation in Drosophila

Neurotactin (NRT), a member of the cholinesterase-homologous protein family , is a heterophilic cell adhesion molecule that is required for proper axon guidance during Drosophila development. In this study, we identify amalgam (AMA), a member of the immunoglobulin superfamily, as a ligand for the NRT receptor. Using transfected Schneider 2 cells and embryonic primary cultur es, we demonstrate that AMA is a secreted protein. Furthermore, AMA is nece ssary for NRT-expressing cells both to aggregate with themselves and to ass ociate with embryonic primary culture cells. Aggregation assays performed w ith truncated NRT molecules reveal that the integrity of the cholinesterase -like extracellular domain was not required either for AMA binding or for a dhesion, with only amino acids 347-482 of the extracellular domain being ne cessary for both activities. Moreover, the NRT cytoplasmic domain is requir ed for NRT-mediated adhesion, although not for AMA binding. Using an ama-de ficient stock, we find that ama function is not essential for viability, Pu pae deficient for ama do exhibit defasciculation defects of the ocellar ner ves similar to those found in nrt mutants.