Signal transduction pathways use protein kinases for the modification of protein function by phosphorylation. A major question in the field is how protein kinases achieve the specificity required to regulate multiple cellular functions. Here we review recent studies that illuminate the mechanisms used by three families of Ser/Thr protein kinases to achieve substrate specificity. These kinases rely on direct docking interactions with substrates, using sites distinct from the phospho-acceptor sequences. Docking interactions also contribute to the specificity and regulation of protein kinase activities. Mitogen-activated protein kinase (MAPK) family members can associate with and phosphorylate specific substrates by virtue of minor variations in their docking sequences. Interestingly, the same MAPK docking pocket that binds substrates also binds docking sequences of positive and negative MAPK regulators. In the case of glycogen synthase kinase 3 (GSK3), the presence of a phosphate-binding site allows docking of previously phosphorylated (primed) substrates; this docking site is also required for the mechanism of GSK3 inhibition by phosphorylation. In contrast, non-primed substrates interact with a different region of GSK3. Phosphoinositide-dependent protein kinase-1 (PDK1) contains a hydrophobic pocket that interacts with a hydrophobic motif present in all known substrates, enabling their efficient phosphorylation. Binding of the substrate hydrophobic motifs to the pocket in the kinase domain activates PDK1 and other members of the AGC family of protein kinases. Finally, the analysis of protein kinase structures indicates that the sites used for docking substrates can also bind N- and C-terminal extensions to the kinase catalytic core and participate in the regulation of its activity.
- glycogen synthase kinase 3 (GSK3)
- mitogen-activated protein kinase (MAPK)
- phosphoinositide-dependent protein kinase-1 (PDK1)
- protein kinase
↵1 Present address: PhosphoSites, Starterzentrum 3, 66421, Homburg, Germany.
Abbreviations used: CD domain, common docking domain; CDK, cyclin-dependent kinase; D-domain, docking domain; DEF domain, domain characterized by sequence Phe-Xaa-Phe-Pro; ERK, extracellular-signal-regulated kinase; FRAT, frequently rearranged in advanced T-cell lymphomas; FRATide, FRAT residues 188–226; GSK3, glycogen synthase kinase 3; JNK, c-Jun N-terminal kinase; MAPK, mitogen-activated protein kinase; MAPKAPK, MAPK-activated protein kinase; MAPKK, MAPK kinase; MKP, MAPK phosphatase; MEF, myocyte enhancer factor; PAK, p21-activated kinase; MEK, MAPK/ERK kinase; MKK, MAP kinase kinase; MNK, MAPK-interacting kinase; MSK, mitogen- and stress-activated protein kinase; PDE, phosphodiesterase; PDK1, phosphoinositide-dependent protein kinase-1; PH, pleckstrin homology; PI3K, phosphatidylinositol 3-kinase; PIF, PDK1-interacting fragment; PIP3, phosphatidylinositol 3,4,5-trisphosphate; PKA, cAMP-dependent protein kinase; PKB, protein kinase B; PKC, protein kinase C; PRK2, PKC-related protein kinase 2; PTP, protein tyrosine phosphatase; RSK, p90 ribosomal S6 kinase; S6K, p70 S6 kinase; STEP, striatum-enriched phosphatase.
- The Biochemical Society, London ©2003