The Notch negative regulatory region

Notch signaling is a developmentally conserved pathway involved in cell fate determination. One development program in which Notch signaling plays a prominent role is in the development of the immune system, where Notch1, one of four mammalian Notch receptors, guides the choice of T-cell fate. However, mutations leading to aberrant increases in Notch1 signaling are associated with T-cell Acute Lymphoblastic Leukemia (T-ALL), indicating the importance of regulation in modulating the timing and strength of Notch1 signals. Notch receptors are synthesized as single-chain type-I transmembrane proteins. During transit through the trans -golgi, Notch receptors are typically cleaved by a furin-like protease, resulting in two subunits that are non-covalently associated through a subunit heterodimerization (HD) domain. The majority of leukemia-associated mutations in human Notch1, which are found in about 40% of T-ALLs, lie in this HD domain. This dissertation aims to understand the events surrounding maintenance of Notch receptors in the "off" state and those surrounding activation of these receptors by leukemia-associated mutations. Chapter two describes the classification of these HD mutations by biochemical and functional means. HD mutations activate Notch in a ligand-independent but protease-dependant manner and segregate into at least two mechanistic classes. Chapter three describes the cellular context of these mutations. HD mutations cause Notch receptors to become trapped in the endoplasmic reticulum, but a small fraction of molecules reach the plasma membrane, whereupon they are efficiently and autonomously activated. Chapter four answers key questions in the field surrounding the role of furin cleavage. Notch receptors that are unable to be cleaved by the protease furin were developed, and the structure and function of these receptors is indistinguishable from wild-type. Finally, chapter five describes initial efforts to form a complex between Notch and its ligands for biochemical and biophysical characterization. Together, these studies provide new understanding on how Notch receptors are regulated normally and dysregulated by leukemia-associated mutations.