Large FK506-Binding Proteins Shape the Pharmacology of Rapamycin

Abstract

The immunosuppressant and anticancer drug rapamycin works by inducing inhibitory protein complexes with the kinase mTOR, an important regulator of growth and proliferation. The obligatory accessory partner of rapamycin is believed to be FK506-binding protein 12 (FKBP12). Here we show that rapamycin complexes of larger FKBP family members can tightly bind to mTOR and potently inhibit its kinase activity. Cocrystal structures with FKBP51 and FKBP52 reveal the modified molecular binding mode of these alternative ternary complexes in detail. In cellular model systems, FKBP12 can be functionally replaced by larger FKBPs. When the rapamycin dosage is limiting, mTOR inhibition of S6K phosphorylation can be enhanced by FKBP51 overexpression in mammalian cells, whereas FKBP12 is dispensable. FKBP51 could also enable the rapamycin-induced hyperphosphorylation of Akt, which depended on higher FKBP levels than rapamycin-induced inhibition of S6K phosphorylation. These insights provide a mechanistic rationale for preferential mTOR inhibition in specific cell or tissue types by engaging specific FKBP homologs.

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SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://dx.doi.org/10.1128/MCB.00678-12.

ACKNOWLEDGMENTS

Initial crystal screening was performed at the crystallization facility of the Max Planck Institute of Biochemistry. The generous support during data collection by the Joint Structural Biology Group at the European Synchrotron Radiation Facility in Grenoble, France, is gratefully acknowledged. We are indebted to F. Holsboer (Max Planck Institute of Psychiatry, Munich, Germany) for continuous financial support.

We thank D. M. Sabatini and N. S. Gray for a sample of torin-1 and M. Gerard and V. Blaekelandt for a sample of shFKBP12-SH-SY5Y cells. We thank J. Chen (UCLA, Riverside, CA) for providing FLAG-mTOR wt/S2035T expression plasmids; T. Rein (Max Planck Institute of Psychiatry, Munich, Germany) for FKBP12, FKBP51, and FKBP52 cDNAs; and G. Fischer (Max Planck Gesellschaft, Halle, Germany) for FKBP12.6, FKBP13, and FKBP25 cDNAs.