Steady-State Levels of Phosphorylated Mitogen-Activated Protein Kinase Kinase 1/2 Determined by Mortalin/HSPA9 and Protein Phosphatase 1 Alpha in KRAS and BRAF Tumor Cells

Mol Cell Biol. 2017 Aug 28;37(18):e00061-17. doi: 10.1128/MCB.00061-17. Print 2017 Sep 15.


Although deregulation of MEK/extracellular signal-regulated kinase (ERK) activity is a key feature in cancer, high-magnitude MEK/ERK activity can paradoxically induce growth inhibition. Therefore, additional mechanisms may exist to modulate MEK/ERK activity in favor of tumor cell proliferation. We previously reported that mortalin/HSPA9 can facilitate proliferation of certain KRAS and BRAF tumor cells by modulating MEK/ERK activity. In this study, we demonstrated that mortalin can regulate MEK/ERK activity via protein phosphatase 1α (PP1α). We found that PP1α inhibition increases steady-state levels of phosphorylated MEK1/2 in various tumor cells expressing B-RafV600E or K-RasG12C/D Intriguingly, coimmunoprecipitation and in vitro binding assays revealed that mortalin facilitates PP1α-mediated MEK1/2 dephosphorylation by promoting PP1α-MEK1/2 interaction in an ATP-sensitive manner. The region spanning Val482 to Glu491 in the substrate-binding cavity and the substrate lid of mortalin were necessary for these physical interactions, which is consistent with conventional heat shock protein 70 (HSP70)-client interaction mechanisms. Nevertheless, mortalin depletion did not affect cellular PP1α levels or its regulatory phosphorylation, suggesting a nonconventional role for mortalin in promoting PP1α-MEK1/2 interaction. Of note, PP1α was upregulated in human melanoma and pancreatic cancer biopsy specimens in correlation with mortalin upregulation. PP1α may therefore have a role in tumorigenesis in concert with mortalin, which affects MEK/ERK activity in tumor cells.

Keywords: ERK1/2; MEK1/2; heat shock protein; mortalin; protein phosphatase 1.

MeSH terms

  • Cell Line, Tumor
  • Cell Proliferation
  • Cell Transformation, Neoplastic / pathology
  • Extracellular Signal-Regulated MAP Kinases / metabolism
  • HSP70 Heat-Shock Proteins / metabolism*
  • Humans
  • MAP Kinase Kinase 1 / metabolism*
  • MAP Kinase Kinase 2 / metabolism*
  • Melanoma / pathology*
  • Mitochondrial Proteins / metabolism*
  • Pancreatic Neoplasms / pathology*
  • Phosphorylation
  • Protein Phosphatase 1 / antagonists & inhibitors
  • Protein Phosphatase 1 / metabolism*
  • Proto-Oncogene Proteins B-raf / metabolism*
  • Proto-Oncogene Proteins p21(ras) / metabolism*
  • RNA Interference
  • RNA, Small Interfering / genetics


  • HSP70 Heat-Shock Proteins
  • HSPA9 protein, human
  • KRAS protein, human
  • Mitochondrial Proteins
  • RNA, Small Interfering
  • MAP2K2 protein, human
  • BRAF protein, human
  • Proto-Oncogene Proteins B-raf
  • Extracellular Signal-Regulated MAP Kinases
  • MAP Kinase Kinase 1
  • MAP Kinase Kinase 2
  • MAP2K1 protein, human
  • Protein Phosphatase 1
  • Proto-Oncogene Proteins p21(ras)