Protein kinase C: release from quarantine by mTORC2

Newly synthesized protein kinase C (PKC) becomes phosphorylated at multiple sites necessary to produce a mature kinase in a signaling-competent and 'ready-to-go' conformation that can rapidly respond to second messengers. The upstream kinase phosphoinositide-dependent kinase 1 (PDK1) has long been known to catalyze phosphorylation of the activation loop to stabilize the active site for catalysis, in turn triggering autophosphorylation at a C-terminal site known as the hydrophobic motif. Why the kinase mechanistic target of rapamycin complex 2 (mTORC2) is also necessary for some PKC isozymes to 'mature' into this signaling-competent species has remained elusive. The identification of an evolutionarily conserved phosphorylation motif (F-X3-F-pT) that is present in all mTORC2-regulated AGC kinases – the TOR interaction motif (TIM) – provided the key to unlock the puzzle. A new model has emerged in which nascent PKC is a stable homodimer mediated by the TIM; phosphorylation by mTORC2 dissociates the dimer to facilitate PDK1 binding, activation loop phosphorylation, and ultimately hydrophobic motif autophosphorylation. Protein kinase C (PKC) isozymes are maintained in a 'ready-to-go' but 'safe' autoinhibited conformation until second messenger binding unleashes an autoinhibitory pseudosubstrate to allow substrate phosphorylation. However, to gain this 'ready-to-go' conformation, PKC must be processed by a series of complex priming phosphorylations, the mechanism of which was enigmatic until now. Recent findings snapped the pieces of the phosphorylation puzzle into place to unveil a process that involves a newly described motif (TOR interaction motif, TIM), a well-described kinase [mechanistic target of rapamycin complex 2 (mTORC2)], and an often-used mechanism (autophosphorylation) to prime PKC to signal. This review highlights new insights into how phosphorylation controls PKC and discusses them in the context of common mechanisms for AGC kinase regulation by phosphorylation and autophosphorylation. Protein kinase C (PKC) isozymes are maintained in a 'ready-to-go' but 'safe' autoinhibited conformation until second messenger binding unleashes an autoinhibitory pseudosubstrate to allow substrate phosphorylation. However, to gain this 'ready-to-go' conformation, PKC must be processed by a series of complex priming phosphorylations, the mechanism of which was enigmatic until now. Recent findings snapped the pieces of the phosphorylation puzzle into place to unveil a process that involves a newly described motif (TOR interaction motif, TIM), a well-described kinase [mechanistic target of rapamycin complex 2 (mTORC2)], and an often-used mechanism (autophosphorylation) to prime PKC to signal. This review highlights new insights into how phosphorylation controls PKC and discusses them in the context of common mechanisms for AGC kinase regulation by phosphorylation and autophosphorylation. a flexible segment near the entrance to the active site that regulates activity, often in a phosphorylation-dependent manner. The activation loop begins with a conserved Asp-Phe-Gly (DFG) motif and extends 20–30 residues to a conserved Ala-Pro-Glu (APE) motif. Phosphorylation of this segment is crucial for orientating the DFG Asp that coordinates Mg2+ to facilitate ATP binding, as well as for proper alignment of a separate His-Arg-Asp (HRD) motif in which the Asp acts a catalytic base in the phosphorylation reaction. reduction in steady-state levels of a protein that can result from decreased transcription, destabilizing mutations, or altered post-translational modification leading to increased protein degradation. With reference to PKC, downregulation signifies loss of protein following prolonged treatment with activators such as phorbol esters. a mutation that results in increased function of a protein or the gain of a new molecular activity. These mutations are generally missense mutations that eliminate mechanisms of inhibition, resulting in constitutive activation of the protein. inherited mutation that occurs in germ cells and is present in the majority of cells in the human body. a conserved phosphorylation site on the C-tail of AGC kinases (named from the protein kinases A, G, and C) such as PKC and Akt that is flanked by hydrophobic residues. a mutation that results in decreased protein expression or compromised protein function. LOF mutations can be missense mutations that result in a change in a single amino acid but are more commonly indels (insertions or deletions) or nonsense mutations that introduce a stop codon early in the gene. the protein product of an oncogene, a gene that harbors frequent gain-of-function mutations or has increased expression in cancer that contribute to cancer-associated phenotypes such as increased proliferation or survival. an AGC family member composed of an N-terminal kinase domain and C-terminal pleckstrin homology (PH) domain that phosphorylates the activation loop of other members of this family, such as PKA, PKC and Akt, to structure the active site for catalysis. a mutation acquired in a cell from damage to DNA that is then passed on through cell division. a newly identified sequence (F-X3-F-pT) preceding the turn motif that is present on all known AGC kinases regulated by mTOR, where the Thr is directly phosphorylated by mTOR. a conserved phosphorylation site on the C-tail of AGC kinases such as PKC and Akt. a gene that harbors frequent LOF mutations or deletions in cancer; it generally acts to suppress cell proliferation and growth or promotes cell death under physiological conditions.