Molecular mechanism of ERK dephosphorylation by striatal-enriched protein tyrosine phosphatase.

Striatal-enriched tyrosine phosphatase (STEP) is an important regulator of neuronal synaptic plasticity, and its abnormal level or activity contributes to cognitive disorders. One crucial downstream effector and direct substrate of STEP is extracellular signal-regulated protein kinase (ERK), which has important functions in spine stabilisation and action potential transmission. The inhibition of STEP activity toward phospho-ERK has the potential to treat neuronal diseases, but the detailed mechanism underlying the dephosphorylation of phospho-ERK by STEP is not known. Therefore, we examined STEP activity toward para-nitrophenyl phosphate, phospho-tyrosine-containing peptides, and the full-length phospho-ERK protein using STEP mutants with different structural features. STEP was found to be a highly efficient ERK tyrosine phosphatase that required both its N-terminal regulatory region and key residues in its active site. Specifically, both kinase interaction motif (KIM) and kinase-specific sequence of STEP were required for ERK interaction. In addition to the N-terminal kinase-specific sequence region, S245, hydrophobic residues L249/L251, and basic residues R242/R243 located in the KIM region were important in controlling STEP activity toward phospho-ERK. Further kinetic experiments revealed subtle structural differences between STEP and HePTP that affected the interactions of their KIMs with ERK. Moreover, STEP recognised specific positions of a phospho-ERK peptide sequence through its active site, and the contact of STEP F311 with phospho-ERK V205 and T207 were crucial interactions. Taken together, our results not only provide the information for interactions between ERK and STEP, but will also help in the development of specific strategies to target STEP-ERK recognition, which could serve as a potential therapy for neurological disorders. Regulation of phospho-ERK by STEP underlies important neuronal activities. A detailed enzymologic characterisation and cellular studies of STEP revealed that specific residues in KIM and active site mediated ERK recognition. Structural differences between the KIM-ERK interfaces and the active site among different ERK phosphatases could be targeted to develop specific STEP inhibitor, which has therapeutic potential for neurological disorders. PKA, protein kinase A & NGF, nerve growth factor.

Metal-dependent protein phosphatase 1A functions as an extracellular signal-regulated kinase phosphatase.

Protein phosphorylation is an important post-translational modification that regulates almost every aspect of signal transduction in cells. Activation of the mitogen-activated protein kinase (MAPK) family kinase extracellular signal-regulated kinase (ERK) is a point of convergence for many cellular activities in response to external stimulation. With stimuli, ERK activity is significantly increased by the phosphorylation of Thr202 and Tyr204 at its activation loop. Downregulation of ERK phosphorylation at these two sites by several phosphatases, such as protein phosphatase 2A, HePTP and MAPK phosphatase 3, is essential for maintaining appropriate ERK function in different cellular processes. However, it is unknown whether metal-dependent protein phosphatase (PPM) family phosphatases directly dephosphorylate ERK. In this study, we found that PPM1A negatively regulated ERK by directly dephosphorylating its pThr202 position early in EGF stimulation. Additional kinetic studies revealed that key residues participated in phospho-ERK recognition by PPM1A. Importantly, PPM1A preferred the phospho-ERK peptide sequence over a panel of other phosphopeptides through the interactions of basic residues in the active site of PPM1A with the pThr-Glu-pTyr motif of ERK. Whereas Lys165 and Arg33 were required for efficient catalysis or phosphosubstrate binding of PPM1A, Gln185 and Arg186 were determinants of PPM1A substrate specificity. The interaction between Arg186 of PPM1A and Glu203 and pTyr204 of phospho-ERK was identified as a hot-spot for phospho-ERK-PPM1A interaction.

Cadmium is a potent inhibitor of PPM phosphatases and targets the M1 binding site.

The heavy metal cadmium is a non-degradable pollutant. By screening the effects of a panel of metal ions on the phosphatase activity, we unexpectedly identified cadmium as a potent inhibitor of PPM1A and PPM1G. In contrast, low micromolar concentrations of cadmium did not inhibit PP1 or tyrosine phosphatases. Kinetic studies revealed that cadmium inhibits PPM phosphatases through the M1 metal ion binding site. In particular, the negative charged D441 in PPM1G specific recognized cadmium. Our results suggest that cadmium is likely a potent inhibitor of most PPM family members except for PHLPPs. Furthermore, we demonstrated that cadmium inhibits PPM1A-regulated MAPK signaling and PPM1G-regulated AKT signaling potently in vivo. Cadmium reversed PPM1A-induced cell cycle arrest and cadmium insensitive PPM1A mutant rescued cadmium induced cell death. Taken together, these findings provide a better understanding of the effects of the toxicity of cadmium in the contexts of human physiology and pathology.