Protein kinase N controls a lysosomal lipid switch to facilitate nutrient signalling via mTORC1.

Mechanistic target of rapamycin (mTOR) kinase functions in two multiprotein complexes: lysosomal mTOR complex 1 (mTORC1) and mTORC2 at the plasma membrane. mTORC1 modulates the cell response to growth factors and nutrients by increasing protein synthesis and cell growth, and repressing the autophagy-lysosomal pathway; however, dysfunction in mTORC1 is implicated in various diseases. mTORC1 activity is regulated by phosphoinositide lipids. Class I phosphatidylinositol-3-kinase (PI3K)-mediated production of phosphatidylinositol-3,4,5-trisphosphate at the plasma membrane stimulates mTORC1 signalling, while local synthesis of phosphatidylinositol-3,4-bisphosphate by starvation-induced recruitment of class II PI3K-β (PI3KC2-β) to lysosomes represses mTORC1 activity. How the localization and activity of PI3KC2-β are regulated by mitogens is unknown. We demonstrate that protein kinase N (PKN) facilitates mTORC1 signalling by repressing PI3KC2-β-mediated phosphatidylinositol-3,4-bisphosphate synthesis downstream of mTORC2. Active PKN2 phosphorylates PI3KC2-β to trigger PI3KC2-β complex formation with inhibitory 14-3-3 proteins. Conversely, loss of PKN2 or inactivation of its target phosphorylation site in PI3KC2-β represses nutrient signalling via mTORC1. These results uncover a mechanism that couples mTORC2-dependent activation of PKN2 to the regulation of mTORC1-mediated nutrient signalling by local lipid signals.