UC San Diego

Protein kinase C (PKC) isozymes transduce the myriad of signals downstream of phospholipid hydrolysis that potentiate an array of cellular processes including proliferation, differentiation, migration, and memory. PKC function is dysregulated in a variety of pathological states, including cancer and neurodegenerative disease. To maintain signaling fidelity, PKCs rely upon precise regulatory mechanisms that orchestrate the phosphorylations and conformational transitions that specify their signaling output. This thesis describes the molecular mechanisms by which PKC phosphorylation and autoinhibition depends upon the kinases PDK1 and mTORC2, and is opposed by PHLPP phosphatases, to produce a primed enzyme that is appropriately tuned to respond to activating signals. Specifically, we uncover the molecular basis for the controversial role of mTORC2 in AGC kinase activation by identifying a novel and conserved mTOR phosphorylation site in the C-terminal tail. Phosphorylation of this, which we term the TOR-Interaction Motif (TIM), promotes PDK1 phosphorylation of the activation loop and intramolecular autophosphorylation of the hydrophobic motif to control activation of PKC and related AGC kianse Akt. Examination of the interrelated processes of phosphorylation and autoinhibition unveils a critical role for the pseudosubstrate in protecting PKC from dephosphorylation by phosphatase PHLPP1, which selectively promotes the dephosphorylation and degradation of aberrantly active PKCs to provide a PKC quality control mechanism. High-throughput protein-level analysis from patient samples reveals that PKC quality control is a critical signaling node that sets PKC expression levels and serves as a prominent loss-of-function mechanism to impair PKC tumor-suppressive function in cancer. Critically, diseases driven by PKC dysregulation rely upon impaired PKC quality control. LOF PKC mutations in chordoid glioma act in a dominant-negative fashion to globally suppress PKC output; whereas, GOF PKC mutations in spinocerebellar ataxia drive phosphoproteome-wide changes in the cerebellum. Taken together, this thesis expands upon biochemical mechanisms of PKC maturation to identify the structural and molecular determinants of PKC phosphorylation and implicates PHLPP1 as the master regulator of PKC signaling fidelity through PKC quality control. This work is not only relevant to the pathology of disease-associated mutations in cancer and neurodegenerative disease, but also to the development of therapeutics that attempt to modulate PKC activity by targeting these regulatory mechanisms.