Ambivalent partnership of the Drosophila posterior class Hox protein Abdominal-B with the Extradenticle and Homothorax cofactors

Hox proteins, a sub-group of the homeodomain (HD) transcription factor family, provide positional information for axial patterning in development and evolution. Hox protein functional specificity is reached, at least in part, through Pbc (Extradenticle (Exd) in Drosophila) and Meis/Prep (Homothorax (Hth) in Drosophila) cofactor interactions. Most of our current knowledge of Hox protein specificity stems from the study of anterior and central Hox proteins, identifying the molecular and structural bases for Hox/Pbc/Meis-Prep cooperative action. Posterior Hox class proteins, Abd-B in Drosophila and Hox9-13 in vertebrates, have been comparatively less studied. They strongly diverge from anterior and central class Hox proteins, with a low degree of HD sequence conservation and the absence of a core canonical Pbc interaction motif. Here we explore how Abd-B function interface with that of Exd/Hth using several developmental contexts, studying mutual expression control, functional dependency and intrinsic protein requirements. Results identify cross regulatory interactions setting relative expression and activity levels required for proper development. They also reveal organ-specific requirement and a binary functional interplay with Exd and Hth, either synergistic or antagonistic. This highlights context specific use of Exd/Hth cofactors, and a similar context specific use of Abd-B protein intrinsic protein requirements.