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Dephosphorylation and inactivation of the mitogen-activated protein kinase by a mitogen-induced Thr/Tyr protein
Abstract:
The activation of extracellular signal-regulated kinase (ERK) or mitogen-activated protein kinase (MAPK) by a dual specific kinase, MEK (MAPK or ERK kinase), is a critical event in the mitogenic signal transduction pathway. However, little is known about the mechanism of ERK inactivation, which occurs after stimulation. In this report, we demonstrated that a dual specific protein phosphatase, HVH1 (human VH1 phosphatase homolog) whose expression is induced by mitogenic growth factors, specifically inactivates ERK. When several phosphoproteins were tested for recombinant HVH1, only MEK-activated ERK1 was dephosphorylated. HVH1 selectively dephosphorylated threonine and tyrosine residues but not serine residues of the activated ERK1. Inactivation of ERK1 by HVH1 could be reversed by MEK, suggesting that HVH1 dephosphorylates the same residues that are recognized and phosphorylated by MEK. Our results suggest that mitogenic growth factors transiently activate ERK (peak at 5 min followed by a rapid decline) by temporally activating MEK (the on signal) and inducing the expression of HVH phosphatase (the off signal).
Insights
Mitogen-activated protein kinase (MAPK) pathway regulation is clarified. A novel phosphatase, HVH1 (human VH1 phosphatase homolog), inactivates extracellular signal-regulated kinase (ERK) by dephosphorylation, revealing a key "off signal" in cellular signaling.
Area of Science:
- Cellular signaling and signal transduction pathways
- Molecular biology and enzymology
Background:
- The mitogen-activated protein kinase (MAPK) pathway, particularly extracellular signal-regulated kinase (ERK) activation by MEK (MAPK or ERK kinase), is crucial for mitogenic responses.
- Mechanisms governing the inactivation of ERK after cellular stimulation remain largely unelucidated.
Purpose of the Study:
- To investigate the molecular mechanisms responsible for the inactivation of extracellular signal-regulated kinase (ERK).
- To identify and characterize novel phosphatases involved in the negative regulation of the MAPK signaling pathway.
Main Methods:
- Recombinant HVH1 (human VH1 phosphatase homolog) was used to test its activity against various phosphoproteins, including MEK-activated ERK1.
- Dephosphorylation assays were performed to determine the specificity of HVH1 activity on different residues (threonine, tyrosine, serine) of activated ERK1.
- Reversibility of HVH1-mediated inactivation by MEK was assessed to understand the interplay between phosphorylation and dephosphorylation sites.
Main Results:
- HVH1 was identified as a dual-specific protein phosphatase that specifically inactivates MEK-activated ERK1.
- HVH1 selectively dephosphorylated threonine and tyrosine residues on ERK1, but not serine residues.
- MEK could reverse the inactivation of ERK1 by HVH1, indicating that HVH1 targets the same residues phosphorylated by MEK.
Conclusions:
- Mitogenic growth factors induce transient ERK activation through the temporal coordination of MEK (the 'on signal') and HVH1 phosphatase induction (the 'off signal').
- HVH1 plays a critical role in the negative feedback loop of the MAPK pathway, ensuring the transient nature of mitogenic signaling.