Ectodomain Shedding and Intramembrane Cleavage of Mammalian Notch Proteins is Not Regulated Through Oligomerization

Intramembrane cleaving proteases such as site 2 protease, gamma-secretase, and signal peptide peptidase hydrolyze peptide bonds within the transmembrane domain (TMD) of signaling molecules such as SREBP, Notch, and HLA-E, respectively. All three enzymes require a prior cleavage at the juxtamembrane region by another protease. It has been proposed that removing the extracellular domain allows dissociation of substrate TMD, held together by the extracellular domain or loop. Using gamma-secretase as a model intramembrane cleaving protease and Notch as a model substrate, we investigated whether activating and inactivating mutations in Notch modulate gamma-secretase cleavage through changes in oligomerization. We find that although the Notch epidermal growth factor repeats can promote dimer formation, most surface Notch molecules in mammalian cells are monomeric as are constitutively active or inactive Notch1 proteins. Using a bacterial assay for TM dimerization, we find that the isolated TMD of Notch and amyloid precursor protein self-associate and that mutations affecting Notch cleavage by gamma-secretase cleavage do not alter TMD dimerization. Our results indicate that ligand-induced reversal of controlled TMD dimerization by the Notch extracellular domain is unlikely to underlie the regulatory mechanism of intramembranous cleavage.