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Akt regulates growth by directly phosphorylating Tsc2

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.

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Figure 1: Akt alters the localization of Tsc1 and Tsc2.
Figure 2: Insulin/Akt inhibits the stability of the Tsc1–Tsc2 complex.
Figure 3: Akt phosphorylates Tsc2 at Ser 924 and Thr 1518 in vitro.
Figure 4: Mutation of Ser 924 and Thr 1518 in Tsc2 results in an activated Tsc2 protein.
Figure 5: Mutation of Ser 924 and Thr 1518 in Tsc2 inhibits Akt-stimulated growth.
Figure 6: Akt-stimulated growth in the wing can be abolished by Tsc2ΔAkt-P.
Figure 7: S6K suppresses Tsc1–Tsc2ΔAkt-P-mediated growth reduction.
Figure 8: Akt regulates growth by phosphorylating and inhibiting the Tsc1–Tsc2 complex.

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Acknowledgements

We thank K. Wehner for helpful discussions, M. Birnbaum for Dakt1 reagents, the Developmental Studies Hybridoma bank for anti-gigas antibodies and X. Fei for injections. This work was supported in part by a National Institutes of Health grant (CA69408) and the Rothberg Courage Award, TS Alliance. T.X. is an investigator of the Howard Hughes Medical Institute. C.J.P. was a predoctoral candidate in the Department of Genetics.

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Figure S1. Akt expression does not enhance the Tsc1 mutant phenotype. (PDF 170 kb)

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Potter, C., Pedraza, L. & Xu, T. Akt regulates growth by directly phosphorylating Tsc2. Nat Cell Biol 4, 658–665 (2002). https://doi.org/10.1038/ncb840

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