Inositol phosphates dynamically enhance stability, solubility and catalytic activity of mTOR.

Mechanistic Target of Rapamycin (mTOR) binds the small metabolite inositol hexakisphosphate (IP) as shown in structures of mTOR, however it remains unclear if IP, or any other inositol phosphate species, function as an integral structural element(s) or catalytic regulator(s) of mTOR. Here, we show that multiple, exogenously added inositol phosphate species can enhance the ability of mTOR and mTORC1 to phosphorylate itself and peptide substrates in in vitro kinase reactions, with the higher order phosphorylated species being more potent (IP=IP>IP>>IP). IP increased the V and decreased the apparent K of mTOR for ATP.

Design, synthesis and cellular characterization of a new class of IPMK kinase inhibitors.

Many genetic studies have established the kinase activity of inositol phosphate multikinase (IPMK) is required for the synthesis of higher-order inositol phosphate signaling molecules, the regulation of gene expression and control of the cell cycle. These genetic studies await orthogonal validation by specific IPMK inhibitors, but no such inhibitors have been synthesized. Here, we report complete chemical synthesis, cellular characterization, structure-activity relationships and rodent pharmacokinetics of a novel series of highly potent IPMK inhibitors.

IPMK regulates HDAC3 activity and histone H4 acetylation in human cells.

Histone deacetylases (HDACs) repress transcription by catalyzing the removal of acetyl groups from histones. Class 1 HDACs are activated by inositol phosphate signaling molecules , but it is unclear if this regulation occurs in human cells. Inositol Polyphosphate Multikinase (IPMK) is required for production of inositol hexakisphosphate (IP6), pentakisphosphate (IP5) and certain tetrakisphosphate (IP4) species, all known activators of Class 1 HDACs .

Multiple inositol phosphate species enhance stability of active mTOR.

Mechanistic Target of Rapamycin (mTOR) binds the small metabolite inositol hexakisphosphate (IP) as shown in structures of mTOR, however it remains unclear if IP, or any other inositol phosphate species, can activate mTOR kinase activity. Here, we show that multiple, exogenously added inositol phosphate species (IP, IP, IP and IP) can all enhance the ability of mTOR and mTORC1 to auto-phosphorylate and incorporate radiolabeled phosphate into peptide substrates in kinase reactions. Although IP did not affect the apparent K of mTORC1 for ATP, monitoring kinase activity over longer reaction times showed increased product formation, suggesting inositol phosphates stabilize an active form of mTORC1 .

25 Years of PI5P.

The accidental discovery of PI5P (phosphatidylinositol-5-phosphate) was published 25 years ago, when PIP5K type II (phosphoinositide-4-phosphate 5-kinase) was shown to actually be a 4-kinase that uses PI5P as a substrate to generate PI(4,5)P. Consequently, PIP5K type II was renamed to PI5P4K, or PIP4K for short, and PI5P became the last of the 7 signaling phosphoinositides to be discovered. Much of what we know about PI5P comes from genetic studies of PIP4K, as the pathways for PI5P synthesis, the downstream targets of PI5P and how PI5P affects cellular function all remain largely enigmatic.