Mammalian Target of Rapamycin Complex 1 (mTORC1)-mediated Phosphorylation Stabilizes ISCU Protein: IMPLICATIONS FOR IRON METABOLISM

Ping La,Guang Yang,Phyllis A Dennery

doi:10.1074/jbc.m112.424499

Abstract

The scaffold protein ISCU facilitates the assembly of iron-sulfur clusters (ISCs), which are essential cofactors for many vital metabolic processes. The mTOR pathways are central to nutrient and energy-sensing networks. Here, we demonstrate that mTORC1 associates with ISCU and phosphorylates ISCU at serine 14. This phosphorylation stabilized ISCU protein. Insufficiency of ISCU triggered by mTORC1 inhibition prevented ISC assembly. Sustained ISCU protein levels enhanced by mTORC1 sensitized TSC2-null cells to iron deprivation due to constitutive ISC biogenesis-triggered iron demand, which outstrips supply. We conclude that the mTORC1 pathway serves to modulate iron metabolism and homeostasis, and we speculate that iron deprivation may be an adjunct in the treatment of cancers characterized by constitutive mTORC1 activation.

Highlights

ISCU facilitates assembly of iron-sulfur clusters (ISCs), the essential metabolic cofactors
We demonstrate that ISCU, a critical scaffold protein for ISC assembly, is an mTOR kinase target and that mTORC1 Regulates ISCU Protein phosphorylation of ISCU by mTORC1 prevents its degradation and increases ISC biogenesis
Given that the upstream stimuli of mTORC1 are nutrients, growth factors, and energy, our findings demonstrate that metabolic cues play an important role in ISCU gene expression and function and broaden our knowledge of iron homeostasis mediated by metabolism

Summary

Background

ISCU facilitates assembly of iron-sulfur clusters (ISCs), the essential metabolic cofactors. The mammalian target of rapamycin, mTOR, a serine-threonine kinase, plays a central role in nutrient-sensing networks ensuring cell survival and growth. It can form two distinct complexes with other protein partners, mTORC1 (mTOR complex 1) and mTORC2 (mTOR complex 2) [8]. We establish a novel role of mTORC1 by demonstrating that mTORC1 phosphorylates ISCU at serine 14 (Ser-14), stabilizing ISCU protein in the cytosol and increasing its abundance in the mitochondria and function in ISC assembly. We demonstrate that unrestrained mTORC1-mediated stabilization of ISCU protein sensitizes cells to iron deprivation, due to constitutive ISC biogenesistriggered iron demand that outstrips supply

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION