Identification of neurotactin as a dominant enhancer of the Abelson tyrosine kinase mutant phenotype. E.C. Liebl ¹, R.G. Rowe ¹, D.J. Forsthoefel ², A.L. Stammler ¹, M.A. Seeger ². 1) Dept Biol, Denison Univ, Granville, OH; 2) Neurobiotechnology Center, The Ohio State University, Columbus, OH.

   We have previously reported identifying a strong dominant enhancer of the Abl mutant phenotype as a missense allele of amalgam (ama[M109]). Wild-type ama encodes a secreted protein, composed of three immunoglobulin-like domains, that binds to the trans-membrane receptor Neurotactin (Nrt), mediating cell:cell adhesion (EMBO J. 19: 4463-4472). We now show that ama[M109] encodes a secreted protein that binds to Nrt, but does not mediate efficient cell:cell adhesion. This altered biological activity is responsible for ama[M109]'s dominant enhancement of the Abl mutant phenotype, as we have recovered an ama[null] allele in screens for reversion of ama[M109]'s dominant enhancement of the Abl mutant phenotype. In further genetic analysis, ama[M109] shows neomorphic activity; ama[M109]/ama[null] in an Abl mutant background shows much more severe central nervous system defects as compared to ama[null]/ama[null] in an Abl mutant background. The genetic interaction between ama and Abl led us to ask whether there is any interaction between nrt and Abl. Previously, RNAi experiments in embryos had shown disruption of Nrt production enhances the Abl mutant phenotype, resulting in strong CNS disruptions. We now report that mutations in nrt are strong dominant enhancers of the Abl mutant phenotype as assayed by either viability or CNS architecture. We have identified this effect in three null alleles and two missense alleles of nrt. We are currently assaying whether the proteins produced by these missense alleles of nrt can bind Ama and mediate cell:cell adhesion. Additional genetic and cell biological characterization of the nrt:Abl interaction will be presented. These results are all consistent with a model whereby cell:cell adhesion mediated by Nrt engagement of Ama forms part of a signaling network involving Abl during axon pathfinding.