mTOR Regulation of AGC Kinases: New Twist to an Old Tail

The family of AGC kinases not only regulates cellular biology by phosphorylating substrates but is itself controlled by phosphorylation. Phosphorylation generally occurs at two conserved regions in these kinases: a loop near the entrance to the active site, termed the activation loop, that correctly aligns residues for catalysis, and a C-terminal tail whose phosphorylation at a site termed the hydrophobic motif stabilizes the active conformation. Whereas phosphorylation of the activation loop is well established to be catalyzed by the phosphoinositide-dependent kinase 1, the mechanism of phosphorylation of the C-tail hydrophobic motif has been controversial. For a subset of AGC kinases, which include most protein kinase C (PKC) isozymes and Akt, phosphorylation of the hydrophobic motif in cells was shown to depend on mTORC2 over 15 years ago, yet whether this was by direct phosphorylation or by another mechanism has remained elusive. The recent identification of a novel and evolutionarily conserved phosphorylation site on the C-tail, termed the TOR interaction motif (TIM), has finally unraveled the mystery of how mTORC2 regulates its client kinases. mTORC2 does not directly phosphorylate the hydrophobic motif; instead, it converts kinases such as PKC and Akt into a conformation that can ultimately autophosphorylate at the hydrophobic motif. Identification of the direct mTOR phosphorylation that facilitates autoregulation of the C-tail hydrophobic motif revises the activation mechanisms of mTOR-regulated AGC kinases. This new twist to an old tail opens avenues for therapeutic intervention. SIGNIFICANCE STATEMENT: The enzyme mTORC2 has been an enigmatic regulator of AGC kinases such as protein kinase C (PKC) and Akt. The recent discovery of a motif named the TOR interaction motif in the C-tail of these kinases solves the mystery: mTORC2 marks these kinases for maturity by, ultimately, facilitating autophosphorylation of another C-tail site, the hydrophobic motif.