Two novel dosage-sensitive modifiers of the Abelson tyrosine kinase (Abl) mutant phenotype have been identified. Amalgam (Ama) is a secreted protein that interacts with the transmembrane protein Neurotactin (Nrt) to promote cell:cell adhesion. We have identified an unusual missense ama allele, ama M109 , which dominantly enhances the Abl mutant phenotype, affecting axon pathfinding. Heterozygous null alleles of ama do not show this dominant enhancement, but animals homozygous mutant for both

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... and Abl show abnormal axon outgrowth. Cell culture experiments demonstrate the Ama M109 mutant protein binds to Nrt, but is defective in mediating Ama/Nrt cell adhesion. Heterozygous null alleles of nrt dominantly enhance the Abl mutant phenotype, also affecting axon pathfinding. Furthermore, we have found that all five mutations originally attributed to disabled are in fact alleles of nrt. These results suggest Ama/Nrtmediated adhesion may be part of signaling networks involving the Abl tyrosine kinase in the growth cone.