Myeloid Translocation Gene (MTG)-16 Binds Intracellular Domains of Notch Receptors to Coordinate Notch-Dependent Cell Fate Specification.

Abstract 257

The Notch signaling pathway regulates gene expression programs to control the specification of cell fate in diverse tissues. In response to ligand binding, the intracellular domain of Notch receptors is cleaved by the γ-secretase complex and then translocates to the nucleus. There it binds the transcriptional repressor CSL, triggering its conversion to an activator of Notch target gene expression. The events that control this conversion are poorly understood. We show that the transcriptional co-repressor, MTG16, interacts with both CSL and the intracellular domains of Notch receptors. The MTG16 NHR3 domain contributes to CSL binding, while N-ICD binding sites lie within the PST1 and PST2 domains. The Notch intracellular domain disrupts the MTG16—CSL interaction, suggesting a pivotal role in regulating the Notch transcription complex. Using co-culture of Lin-/Sca-1+/c-Kit+ (LSK) cells on OP9-DL1 stromal cells, we show that ex vivo fate specification in response to Notch signal activation is altered in Mtg16 (−/−) hematopoietic progenitors. While Notch signal activation specifies lymphoid fate in Mtg16 (+/+) LSK cells, Mtg16 (−/−) LSK cells display cell surface marker expression reminiscent of myeloid differentiation. We used this lineage allocation assay to assess the contribution of MTG16 to cell fate determination. Lymphoid fate specification is restored by MTG16WT expression in Mtg16 (−/−) LSK cells. However, an MTG16 mutant deficient in N1-ICD binding is defective in this assay, suggesting this region is important to Notch-dependent lineage allocation. These data suggest that MTG family proteins interface with critical components of the Notch transcription complex and intimate a functional relationship between MTG proteins and Notch signaling in normal and malignant hematopoiesis.