REGULATION OF BHLH-PAS PROTEIN SUBCELLULAR-LOCALIZATION DURING DROSOPHILA EMBRYOGENESIS

The Drosophila Single-minded and Tango basic-helix-loop-helix-PAS prot ein heterodimer controls transcription and embryonic development of th e CNS midline cells, while the Trachealess and Tango heterodimer contr ols tracheal cell and salivary duct transcription and development. Exp ression of both single-minded and trachealess is highly restricted to their respective cell lineages, however tango is broadly expressed, Th e developmental control of subcellular localization of these proteins is investigated because of their similarity to the mammalian basic-hel ix-loop-helix-PAS Aromatic hydrocarbon receptor whose nuclear localiza tion is dependent on ligand binding. Confocal imaging of Single-minded and Trachealess protein localization indicate that they accumulate in cell nuclei when initially synthesized in their respective cell linea ges and remain nuclear throughout embryogenesis, Ectopic expression ex periments show that Single-minded and Trachealess are localized to nuc lei in cells throughout the ectoderm and mesoderm, indicating that nuc lear accumulation is not regulated in a cell-specific fashion and unli kely to be ligand dependent. In contrast, nuclear localization of Tang o is developmentally regulated; it is localized to the cytoplasm in mo st cells except the CNS midline, salivary duct, and tracheal cells whe re it accumulates in nuclei. Genetic and ectopic expression experiment s indicate that Tango nuclear localization is dependent on the presenc e of a basic-helix-loop-helix-PAS protein such as Single-minded or Tra chealess, Conversely, Drosophila cell culture experiments show that Si ngle-minded and Trachealess nuclear localization is dependent on Tango since they are cytoplasmic in the absence of Tango, These results sug gest a model in which Single-minded and Trachealess dimerize with Tang o in the cytoplasm of the CNS midline cells and trachea, respectively, and the dimeric complex accumulates in nuclei in a ligand-independent mode and regulates lineage-specific transcription. The lineage-specif ic action of Single-minded and Trachealess derives from transcriptiona l activation of their genes in their respective lineages, not from ext racellular signaling.