| Summary: | 碩士 === 國立陽明大學 === 生化暨分子生物研究所 === 97 === The serine/threonine protein kinase Akt, also known as PKB, is a downstream effector molecule of phosphoinositide 3-kinase (PI3K) and is thought to mediate many biological functions including cell survival, proliferation, and carcinogenesis. Upon stimulation with a growth factor, phosphorylations of Akt at Thr308 and Ser473 is required for full activation of the kinase. Heat shock protein 90 (Hsp90) binds to Thr308 and Ser473-phosphorylated Akt, by which its kinase activity is stabilized. Inhibition of Hsp90 ATPase activity by Hsp90 inhibitor contributes to Akt ubiquitination and proteasome-dependent degradation; however, the mechanism of degradation and the involvement of a potential E3 ubiquitin ligase in this process remain uncertain. In our study we show that phosphorylations of Thr308 and Ser473 induced by Hsp90 specific inhibitor geldanamycin (GA) or by growth factor insulin promote Akt to enter the ubiquitin-proteasome pathway. Mutations at either Thr308 or Ser473 or both sites result in decreased rate of ubiquitination. We also show that CHIP (C-terminus of Hsc70 Interacting Protein) is a major E3 ubiquitin ligase that contributes to Akt ubiquitination and degradation. Mutations of Akt at either Thr308 or Ser473 or both sites weaken its ability to bind to CHIP, which is correlated with ubiquitin phenomenon. Our study unveils that the well-known phosphorylations, in addition to responsible for Akt activation, also turn out to transduce recognition signals for CHIP-mediated ubiquitination and proteasomal degradation.
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