👤 Ryan A Denu

The dual specificity mitogen-activated protein kinase phosphatase MKP3 downregulates mitogenic signaling through dephosphorylation of extracellular signal-regulated kinase (ERK). Like other MKPs, MKP3 consists of a noncatalytic N-terminal domain and a catalytic C-terminal domain. ERK binding to the N-terminal noncatalytic domain of MKP3 has been shown to increase (up to 100-fold) the catalytic activity of MKP3 toward small artificial substrates. Here, we address the function of the N-terminal domain of MKP3 in either inter- or intramolecular dephosphorylation of pERK (phosphorylated ERK) and the stoichiometry of the MKP3/pERK Michaelis complex. These are important mechanistic distinctions given the observation that ERK exists in a monomer/dimer equilibrium that is shifted toward the dimer when phosphorylated and given that MKP3 undergoes catalytic activation toward other substrates when bound to ERK. Wild-type and engineered mutants of ERK and MKP3, binding analyses, reaction kinetics, and chemical cross-linking studies were used to demonstrate that the monomer of MKP3 binds to the monomeric form of pERK and that MKP3 within the resulting heterodimer performs intramolecular dephosphorylation of pERK. This study provides the first direct evidence that MKP3 utilizes intramolecular dephosphorylation between a complex consisting of one molecule each of MKP3 and ERK. Catalytic activation and substrate tethering by MKP3 lead to a >or=4000-fold rate enhancement (k(cat)/K(m)) for dephosphorylation of pERK. Show less

The JNK group (for c-Jun N-terminal kinase) of mitogen-activated protein kinases (MAP kinases) is activated in cells in response to environmental stress and cytokines. Activation of JNK is the result of dual phosphorylation by specific upstream kinases which phosphorylate the TxY motif. Much less is known concerning the down-regulation by protein phosphatases. Here, we demonstrate that the tyrosine-specific and constitutively-expressed phosphatase VHR (for VH1-Related) down-regulates the JNK signaling pathway at the level of JNK dephosphorylation. VHR was shown to efficiently dephosphorylate JNK and to form a tight complex with activated JNK when the catalytically-inactive C124S VHR mutant was employed as an in vivo substrate trap. Utilizing an in vitro assay, the transcription factor c-Jun specifically inhibited the ability of VHR to dephosphorylate JNK, likely by sterically blocking access to the phosphorylation sites when JNK and c-Jun form a complex. c-Jun has no effect on the ability of VHR to inactivate the ERK MAP kinases or to hydrolyze artificial substrates. The c-Jun inhibition results are discussed in terms of the resistant-nature of JNK dephosphorylation in cellular extracts and in terms of a general model in which VHR may be a general MAP kinase phosphatase whose specificity and activity are dictated by the presence of MAP kinase-associated proteins that inhibit dephosphorylation. Show less