Abstract

The Notch pathway functions in diverse developmental processes as well as contributing to human diseases. In each context it regulates different sets of target genes. We are interested in the fundamental mechanisms involved in making genes Notch responsive. Following activation, the Notch intracellular domain (NICD) is released and enters the nucleus where it forms a complex with the DNA -binding transcription factor, CSL (CBF1, Suppressor of Hairless, Lag-1). The activity of this complex needs to be sensitive to the levels of Notch activity. CSL occupancy, measured by chromatin immunoprecipitation, is increased following Notch activation, suggesting that NICD stimulates CSL movement and/or binding kinetics. These observations inspired us to investigate CSL dynamics in the nucleus using live imaging techniques. The results reveal that, in Notch-off cells, only a small fraction of CSL is bound and that it has short residency times. While the general properties of CSL are unchanged in Notch-on cells, increased levels of CSL are detected at specific loci, reflecting a local increase in residency that is directly dependant on interactions with NICD . These changes in the binding dynamics suggest new models to explain how the core transcription complex responds to NICD /pathway activity.