Abstract

This entry covers an auto-activating linear motif of AGC group kinases. Several variants of the motif exist, and for many kinases, the motif has been shown to operate in trans to bind and activate the upstream activating kinase PDK1. To make matters more complicated, some variants are regulated by phosphorylation.

The AGC kinases regulate critical processes including metabolism, cell growth, proliferation, survival and differentiation, hence deregulation of these enzymes is a causative factor in different diseases such as cancer and diabetes. Solved structures of AGC kinases show the typical bilobal kinase fold of the kinase domain, consisting of a small N-terminal lobe (N-lobe) and a larger C-terminal lobe (C-lobe). Regulation of kinase activity is mainly achieved through phosphorylation of the activation or T-loop, located in the C-lobe and connected to the N-lobe through the alpha-C helix. This modification results in conformational changes, mainly in the alpha-C helix, that reposition key catalytic and substrate binding residues. Sandwiched between the N- and C-lobe is an ATP-binding site that provides the phosphate-donor during phosphorylation. Repositioning of the alpha-C helix upon kinase activation allows formation of a salt bridge between an alpha-C helix glutamate and a conserved lysine residue within the beta-3 strand that interacts with the alpha and beta phosphates of ATP.

The non-catalytic C-terminal tail of the kinase is also involved in repositioning of the alpha-C helix. The alpha-C helix is part of a hydrophobic pocket and an adjacent phosphate-binding site in the N-lobe, called the PIF pocket, which interacts with the AGCK docking motif (PDK1 Interacting Fragment (PIF) / hydrophobic motif (HM)) that is present in the C-tail of AGC kinases. This interaction stabilizes the active conformation of the alpha-C helix through an allosteric mechanism. Both the PIF pocket and the C-terminal region are conserved in Eukaryotic AGC kinases, except for PDK1, which lacks the C-terminal part. The AGCK docking motif mediates intramolecular interactions to the PIF pocket, serving as a cis-activating module, together with other regulatory sequences present in the C-tail of the kinase. However, in some kinases it also serves as a PDK1 docking site that trans-activates PDK1, which itself does not possess this regulatory region. Activated PDK1 in turn will phosphorylate and activate the docked AGC kinase.

Several AGC kinases are involved in mediating signaling downstream of phosphatidyl-inositol-4,5-bisphosphate 3-kinase (PI3K) in response to a wide range of stimuli such as growth factors and hormones. PDK1 functions as a common upstream activator by phosphorylating the other AGC kinases at their activation loop. The PDK1 PIF pocket serves both as an allosteric regulatory site for PDK1 activity and as a docking site for the AGC kinases it phosphorylates, by binding to the AGCK docking motif that is present in the C-tail of its substrates. AGC kinases in their inactive state have an incompatible PIF pocket due to the alpha-C helix being disordered, meaning that their AGCK docking motif is available for binding to the PIF pocket of PDK1, which becomes trans-activated. After being phosphorylated by PDK1, the AGCK docking motif can engage in an intramolecular interaction with its functional PIF pocket, resulting in release from PDK1 and full activation of the kinase.

The AGCK docking motif generally appears as three aromatic residues, most often phenylalanines, surrounding a phosphorylatable serine or threonine residue (LIG_AGCK_PIF_1). Phosphorylation of this serine/threonine increases the affinity of the motif for the PIF pocket, which allows fine-tuning the cis and trans interactions of the motif. The mechanisms and kinases involved in phosphorylation of the AGCK docking motif differ for the different kinases. Some alternative patterns of the motif exist. In atypical PKC forms and PKC-like (PKN) kinases, the phosphorylatable serine or threonine residue is replaced by an acidic phosphomimetic aspartate or glutamate residue (LIG_AGCK_PIF_2). In other AGC kinases, including PKA, the motif is located at the very C-terminal and contains only the first two core aromatic residues (LIG_AGCK_PIF_3). In many cases, full activation of the AGC kinases is also dependent on additional signals that are specific for each kinase and that provide spatial and conformational regulation.