Large FK506-binding proteins shape the pharmacology of rapamycin

Area of Science:

• Biochemistry
• Molecular Biology
• Pharmacology

Background:

• Rapamycin is an immunosuppressant and anticancer drug that inhibits the mTOR kinase.
• FKBP12 is considered the essential partner for rapamycin to bind mTOR.
• mTOR regulates cell growth and proliferation.

Purpose of the Study:

• To investigate if larger FKBP family members can also bind mTOR with rapamycin.
• To elucidate the molecular mechanism of these alternative ternary complexes.
• To assess the functional implications of using larger FKBPs in cellular systems.

Main Methods:

• Cocrystallography to determine the structure of rapamycin-FKBP-mTOR complexes.
• Cellular model systems to evaluate FKBP functional replacement and mTOR inhibition.
• Western blotting to assess S6K and Akt phosphorylation levels.

Main Results:

• Larger FKBPs (FKBP51, FKBP52) form potent inhibitory complexes with rapamycin and mTOR.
• Cocrystal structures reveal the detailed binding modes of these alternative complexes.
• FKBP12 can be functionally replaced by larger FKBPs in cellular assays.
• FKBP51 overexpression enhances mTOR inhibition of S6K phosphorylation under limiting rapamycin conditions.
• FKBP51 enables rapamycin-induced Akt hyperphosphorylation, which requires higher FKBP levels.

Conclusions:

• The study identifies larger FKBP homologs as functional partners for rapamycin in mTOR inhibition.
• Structural and cellular data provide a mechanistic basis for alternative FKBP usage.
• These findings suggest a strategy for selective mTOR inhibition in specific cell or tissue types by utilizing different FKBP homologs.