FBXL2- and PTPL1-mediated degradation of p110-free p85β regulatory subunit controls the PI(3)K signalling cascade

Abstract

F-box proteins are the substrate-recognition subunits of SCF (Skp1/Cul1/F-box protein) ubiquitin ligase complexes. Purification of the F-box protein FBXL2 identified the PI(3)K regulatory subunit p85β and tyrosine phosphatase PTPL1 as interacting proteins. FBXL2 interacts with the pool of p85β that is free of p110 PI(3)K catalytic subunits and targets this pool for ubiquitylation and subsequent proteasomal degradation. FBXL2-mediated degradation of p85β is dependent on the integrity of its CaaX motif. Whereas most SCF substrates require phosphorylation to interact with their F-box proteins, phosphorylation of p85β on Tyr 655, which is adjacent to the degron, inhibits p85β binding to FBXL2. Dephosphorylation of phospho-Tyr-655 by PTPL1 stimulates p85β binding to and degradation through FBXL2. Finally, defects in the FBXL2-mediated degradation of p85β inhibit the binding of p110 subunits to IRS1, attenuate the PI(3)K signalling cascade and promote autophagy. We propose that FBXL2 and PTPL1 suppress p85β levels, preventing the inhibition of PI(3)K by an excess of free p85 that could compete with p85–p110 heterodimers for IRS1.

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Figure 1: FBXL2 binds p110-free p85 regulatory subunits.
Figure 2: p85β is targeted for ubiquitylation and degradation by SCFFBXL2.
Figure 3: Identification of p85β degron.
Figure 4: p85β interaction with FBXL2 is negatively regulated by Tyr phosphorylation.
Figure 5: PTPL1 dephosphorylates p85β, promoting its binding to FBXL2 and degradation.
Figure 6: Failure to degrade p85β results in PI(3)K activation defects.
Figure 7: p85β degradation regulates cell autophagy.
Figure 8: A model of the FBXL2- and PTPL1-dependent regulation of the PI(3)K pathway.

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Acknowledgements

The authors thank J. Backer, J. H. Lee and R. K. Mallampalli for reagents, and J. Backer, J. R. Skaar and E. Skolnik for critical reading of the manuscript. M.P. is grateful to T. M. Thor for continuous support. This work was financially supported by grants from the National Institutes of Health (R01-GM057587, R37-CA076584 and R21-CA161108) to M.P. and a grant from Susan G. Komen for the Cure to S.D. A.S., L.F. and M.P.W. are supported by the Stowers Institute for Medical Research. M.P. is an Investigator with the Howard Hughes Medical Institute.

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S.K. planned and performed most experiments and helped to write the manuscript. M.P. coordinated the study, oversaw the results, and wrote the manuscript. S.D., E.S. and A.P. helped with some experiments. A.S., L.F. and M.P.W. performed the mass spectrometry analysis of the FBXL2 complex purified by S.K. All authors discussed the results and commented on the manuscript.

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Correspondence to Michele Pagano.

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Kuchay, S., Duan, S., Schenkein, E. et al. FBXL2- and PTPL1-mediated degradation of p110-free p85β regulatory subunit controls the PI(3)K signalling cascade. Nat Cell Biol 15, 472–480 (2013). https://doi.org/10.1038/ncb2731

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