Cell specification and division are precisely co-ordinated during neurogenesis but what controls the timing of mitotic entry? The answer for the neural precursors of the Drosophila external sensory organs, report Chang and colleagues, is the negative-feedback regulation of proneural proteins, a process that involves their degradation by the Phyl/Sina E3 ubiquitin ligase complex (see ). In Drosophila, the sensory organ precursors (SOPs) undergo asymmetric cell divisions after being specified by the proneural transcription factors Achaete (Ac) and Scute (Sc). The timing of the G2-M transition in SOP divisions controls daughter cell fate specification. In phyl mutants,the researchers report, Ac and Sc accumulation delays or blocks SOP division at the G2-M transition; ac and sc gene dose reduction rescues this defect. Other results indicate that the adaptor Phyl links the proneural proteins to the RING protein Sina. Because phyl is a transcriptional target of Ac and Sc, the researchers propose that, by initiating their own degradation, these proneural proteins control the timing of neural precursor division.
