Akt1 phosphorylates Nicastrin to regulate its protein stability and activity.

The gamma-secretase is a multiprotein complex that cleaves many type-I membrane proteins, such as the Notch receptor and the amyloid precursor protein. Nicastrin (NCT) is an essential component of the multimeric gamma-secretase complex and functions as a receptor for gamma-secretase substrates. In this study, we found that Akt1 markedly regulated the protein stability of NCT. Importantly, the kinase activity of Akt1 was essential for the inhibition of gamma-secretase activity through degradation of NCT. Notably, the protein level of endogenous NCT was higher in shAkt1-expressing cells than in shCon-expressing cells. Akt1 physically interacted with NCT and mediated its degradation through proteasomal and lysosomal pathways. We also found that Akt1 phosphorylates NCT at Ser437, resulting in a significant reduction in NCT protein stability. Importantly, a phospho-deficient mutation in NCT at Ser437 stabilized its protein levels. Collectively, our results reveal that Akt1 functions as a negative regulator of the gamma-secretase activity through phosphorylation and degradation of NCT. Generation of the amyloid peptide (A-beta) and the amyloid precursor protein (APP) intracellular domain (AICD) can happen by sequential proteolysis of APP by beta and gamma-secretase. The gamma-secretase complex consists of four essential proteins: presenilin (PS1 or PS2), presenilin enhancer 2 (PEN-2), anterior pharynx-defective 1 (APH-1), and the Nicastrin (NCT). NCT can interact and be phosphorylated by Akt1, and phosphorylated NCT promotes its proteasomal and lysosomal degradation. As a result, Akt1 plays role in reducing gamma-secretase activity through phosphorylation-dependent regulation of NCT protein degradation.