Identification of amalgamas a dominant enhancer of the Abelsontyrosine kinase mutant phenotype. E.C. Liebl ¹, E.R. Schmidt ¹, D.J. Forsthoefel ², S.C. Howard ¹, M.A. Seeger ². 1) Biology, Denison University, Granville, OH; 2) Neurobiotechnology Center, The Ohio State University, Columbus, OH.

   Screens for dominant enhancers of the Abelson (Abl)mutant phenotype have recovered alleles of several genes including Trio, failed-axon-connections,and disabled. However, the strongest dominant enhancer recovered in these screens, first identified as M109, has proven to be a missense allele of amalgam (ama^M109). Df(3R)Scrfailed to complement the M109allele. As amais removed by Df(3R)Scr,direct sequencing of the amaORF from the M109-chromosome revealed a missense mutation resulting in Cys45Tyr. Ama encodes a secreted protein composed of three immunoglobulin-like domains. Interestingly, Cys45 is normally involved in a disulfide bond that stabilizes Ama's N-terminal Ig-like domain. No animals of the genotype ama^M109, Abl¹/Abl⁴ survived to pupation. These animals had severe central nervous system disruptions of both longitudinal and commissural axon bundles, as revealed by either BP102 or 1D4 staining. A wild-type Abltransgene fully rescued these phenotypes.
    Neurotactin (Nrt) has recently been identified as an Amalgam receptor (EMBO J. 19: 4463-4472). Wild-type Ama promotes homotypic adhesion between Nrt-expressing S2 cells. Preliminary results in the S2 cell system have shown Ama^M109 protein is produced and secreted normally. We are currently addressing whether Ama^M109 binds Nrt and mediates Nrt-homotypic adhesion. In addition, RNAi experiments in embryos have shown disruption of Nrt production enhances the Abl mutant phenotype, resulting in strong CNS disruptions. These results are consistent with a model whereby cell:cell adhesion mediated by Nrt engagement of Ama forms part of a signaling network involving Abl.