The spineless-Aristapedia and tango bHLH-PAS proteins interact to control antennal and tarsal development in Drosophila

The Drosophila spineless (ss) gene encodes a basic-helixloop-helix-PAS tran scription factor that is required for proper specification of distal antenn al identity, establishment of the tarsal regions of the legs, and normal br istle growth. ss is the closest known homolog of the mammalian aryl hydroca rbon receptor (Ahr), also known as the dioxin receptor. Dioxin and other ar yl hydrocarbons bind to the PAS domain of Ahr, causing Ahr to translocate t o the nucleus, where it dimerizes with another bHLH-PAS protein, the aryl h ydrocarbon receptor nuclear translocator (Amt). Ahr:Amt heterodimers then a ctivate transcription of target genes that encode enzymes involved in metab olizing aryl hydrocarbons. In this report, we present evidence that Ss func tions as a heterodimer with the Drosophila ortholog of Amt, Tango (Tgo), We show that the ss and tgo genes have a close functional relationship: loss- of-function alleles of tgo were recovered as dominant enhancers of a ss mut ation, and tgo-mutant somatic clones show antennal, leg, and bristle defect s almost identical to those caused by ss(-) mutations. The results of yeast two-hybrid assays indicate that the Ss and Tgo proteins interact directly, presumably by forming heterodimers, Coexpression of Ss and Tgo in Drosophi la SL2 cells causes transcriptional activation of reporters containing mamm alian Ahr:Arnt response elements, indicating that Ss:Tgo heterodimers are v ery similar to Ahr:Amt heterodimers in DNA-binding specificity and transcri ptional activation ability During embryogenesis, Tgo is localized to the nu cleus at sites of ss expression. This localization is lost in a ss null mut ant, suggesting that Tgo requires heterodimerization for translocation to t he nucleus. Ectopic expression of ss causes coincident ectopic nuclear loca lization of Tgo, independent of cell type or developmental stage. This sugg ests that the interaction of Ss and Tgo does not require additional signals , unlike the ligand-dependent interaction of Ahr and Amt. Despite the very different biological roles of Ahr and Amt in insects and mammals, the molec ular mechanisms by which these proteins function appear to be largely conse rved.