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Akt regulates growth by directly phosphorylating Tsc2
Christopher J Potter1, Laura G Pedraza, Tian Xu
1Howard Hughes Medical Institute, Department of Genetics, Yale University School of Medicine, Boyer Center for Molecular Medicine, 295 Congress Avenue, New Haven, CT 06536-0812, USA.
Abstract:
The direct mechanism by which the serine/threonine kinase Akt (also known as protein kinase B (PKB)) regulates cell growth is unknown. Here, we report that Drosophila melanogaster Akt/PKB stimulates growth by phosphorylating the tuberous sclerosis complex 2 (Tsc2) tumour suppressor and inhibiting formation of a Tsc1-Tsc2 complex. We show that Akt/PKB directly phosphorylates Drosophila Tsc2 in vitro at the conserved residues, Ser 924 and Thr 1518. Mutation of these sites renders Tsc2 insensitive to Akt/PKB signalling, increasing the stability of the Tsc1-Tsc2 complex within the cell. Stimulating Akt/PKB signalling in vivo markedly increases cell growth/size, disrupts the Tsc1-Tsc2 complex and disturbs the distinct subcellular localization of Tsc1 and Tsc2. Furthermore, all Akt/PKB growth signals are blocked by expression of a Tsc2 mutant lacking Akt phosphorylation sites. Thus, Tsc2 seems to be the critical target of Akt in mediating growth signals for the insulin signalling pathway.
Insights
The serine/threonine kinase Akt stimulates cell growth by phosphorylating Tsc2, a tumor suppressor. This action inhibits the Tsc1-Tsc2 complex, revealing Akt
Area of Science:
- Cell Biology
- Molecular Biology
- Genetics
Background:
- The precise mechanism of cell growth regulation by serine/threonine kinase Akt (also known as protein kinase B, PKB) remains unclear.
- Understanding Akt's role is crucial for deciphering cellular growth control pathways.
Purpose of the Study:
- To elucidate the direct mechanism by which Akt/PKB regulates cell growth.
- To identify the specific target of Akt/PKB in mediating growth signals within the insulin signaling pathway.
Main Methods:
- In vitro phosphorylation assays using Drosophila melanogaster Akt/PKB and Tuberous Sclerosis Complex 2 (Tsc2).
- Site-directed mutagenesis of conserved phosphorylation residues (Ser 924 and Thr 1518) in Tsc2.
- In vivo studies in Drosophila to assess the effects of Akt/PKB signaling on cell growth, Tsc1-Tsc2 complex stability, and subcellular localization.
Main Results:
- Drosophila Akt/PKB directly phosphorylates Tsc2 at Ser 924 and Thr 1518.
- Mutations rendering Tsc2 insensitive to Akt/PKB phosphorylation stabilize the Tsc1-Tsc2 complex.
- In vivo activation of Akt/PKB increases cell size, disrupts the Tsc1-Tsc2 complex, and alters Tsc1/Tsc2 localization; these effects are blocked by non-phosphorylatable Tsc2 mutants.
Conclusions:
- Tuberous Sclerosis Complex 2 (Tsc2) is a critical direct target of Akt/PKB in regulating cell growth.
- Akt/PKB-mediated phosphorylation of Tsc2 inhibits Tsc1-Tsc2 complex formation, thereby promoting cell growth.
- This study clarifies a key mechanism in the insulin signaling pathway's control of cellular proliferation.