Negative Regulation of MAPKK by Phosphorylation of a Conserved Serine Residue Equivalent to Ser212 of MEK1

Abstract

The MAPKKs MEK1 and MEK2 are activated by phosphorylation, but little is known about how these enzymes are inactivated. Here Mitogen-activated protein kinase (MAPK) 1 pathways are evolutionarily conserved signaling modules by which cells transduce extracellular chemical and physical signals into intracellular responses (reviewed in Refs. 1-3). These modules are organized into an architecture of three sequentially acting protein kinases comprising a MAPK kinase kinase (MAPKKK or MEK kinase), a MAPK kinase (MAPKK or MEK), and the MAPK itself. The propagation of the signal through MAPK pathways is facilitated by specific protein-protein interactions between individual components of the pathway and scaffolding proteins (3, 4).The prototypical and most studied MAPK pathway is the ERK1/2 pathway, which controls cell proliferation, differentiation, and development (1). Stimulation of cells with growth and differentiation factors leads to the activation of the MAPKKK Raf by a complicated mechanism involving cellular relocalization and multiple phosphorylation events (5, 6). Activated Raf isoforms bind to and activate the MAPKKs MEK1 and MEK2 by phosphorylation of two serine residues (corresponding to Ser 218 and Ser 222 in MEK1) in their activation loop (7,8). Substitution of the two regulatory serines with acidic residues is sufficient to enhance the basal activity of MEK1/2 (7-12). The dual-specificity kinases MEK1 and MEK2 then catalyze the phosphorylation of the MAPKs ERK1 and ERK2 at threonine and tyrosine residues within the activation loop motif Thr-Glu-Tyr (13), causing a reorientation of the loop and activation of the enzyme (14). Both MEK1 and MEK2 stably associate with ERK1/2, and this association is required for efficient activation of the latter in cells (15, 16). The binding site for ERK1/2 is located at the N terminus of MEK1/2 and consists of a short basic region known as the D domain (16). MEK1 and MEK2 also contain a unique proline-rich insert between subdomains IX and X, which is required for full activation of ERK1/2 in intact cells (17,18).The magnitude and duration of MAPK activation are important determinants of the cellular response to extracellular signals (19,20). Therefore, a tightly regulated balance between activation and inactivation mechanisms must exist to control the cellular activity of ERK1/2. Inactivation of the ERK1/2 enzymes is mainly achieved by dephosphorylation of the activating threonine and tyrosine residues. Biochemical and genetic studies have implicated both tyrosine-specific phosphatases and dual-specificity MAPK phosphatases in the negative regulation of ERK1/2 and other MAPKs (21,22). Much less is known about the mechanisms that negatively regulate the pathway at the MAPKK level. The serine/threonine phosphatase protein phosphatase 2A was identified as the major phosphatase inactivating MEK1 in lysates of PC12 cells (23). Furthermore, overexpression of SV40 small t antigen, which binds to the A subunit of protein phosphatase 2A and inactivates the enzyme, was found to s...

Regulation of ERK Kinase by MEK1 Kinase Inhibition in the Brain

“…MEK1 has also been suggested to be phosphorylated at Thr-386 by ERK (48). Furthermore, MEK1 has been shown to undergo inhibitory phosphorylation in its activation loop at Ser-212 by an unknown protein kinase (72). The functions of these phosphorylations and the possibility of additional posttranslational modifications of MEK1 remain to be explored further.…”

Effect of merlin phosphorylation on neurofibromatosis 2 (NF2) gene function

“…analogues has been previously used to study the function and phosphorylation-dependent regulation of other proteins (Jabbur and Zhang, 2002;Ackerley et al, 2003;Gopalbhai et al, 2003). For example, mutations that mimic phosphorylation of the neurofilament heavy chain result in reduced neurofilament transport (Ackerley et al, 2003).…”

“…For example, mutations that mimic phosphorylation of the neurofilament heavy chain result in reduced neurofilament transport (Ackerley et al, 2003). Similarly, aspartic acid substitution at the phosphorylated amino-acid residue S212 of mitogenactivated protein kinase-1 (MEK-1) abolishes enzymatic activity, whereas alanine substitution enhances MEK activity (Gopalbhai et al, 2003). Lastly, mutations of the p53 phosphorylatable amino-acid residues T18 and S20 to aspartic acid results in reduced p53 binding to its inhibitory partner Mdm-2, and enhanced expression of p53 transactivation targets (Jabbur and Zhang, 2002).…”

MAP kinase signalling pathways in cancer

“…MEK is also phosphorylated at S298 by PAK1, an event that may facilitate its coupling to Raf (Frost et al, 1997;Coles and Shaw, 2002). In addition, an inhibitory phosphorylation site on MEK, S212 was recently reported (Gopalbhai et al, 2003).…”