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MO25 is a master regulator of SPAK/OSR1 and MST3/MST4/YSK1 protein kinases

Beatrice M Filippi1, Paola de los Heros, Youcef Mehellou

  • 1MRC Protein Phosphorylation Unit, College of Life Sciences, University of Dundee, Dundee, UK. bfilippi@uhnresearch.ca

The EMBO Journal
|March 23, 2011
PubMed

Insights

Mouse protein-25 (MO25) regulates STE20 kinases, including LKB1, SPAK/OSR1, and MST3/4/YSK1. MO25 binding activates these kinases, impacting ion homeostasis, blood pressure, development, and morphogenesis.

Area of Science:

  • Biochemistry
  • Molecular Biology
  • Cell Signaling

Background:

  • Mouse protein-25 (MO25) is known to bind STRAD pseudokinases, stabilizing them to activate the LKB1 tumor suppressor kinase.
  • The broader regulatory roles of MO25 beyond LKB1 activation were not fully understood.

Purpose of the Study:

  • To investigate novel targets of MO25 isoforms and elucidate their functions.
  • To characterize the mechanism by which MO25 regulates STE20 family kinases.

Main Methods:

  • Biochemical assays to measure kinase activity.
  • Co-immunoprecipitation to assess protein-protein interactions.
  • siRNA-mediated gene silencing in mammalian cells.
  • Mass spectrometry to identify phosphorylation sites.

Main Results:

  • MO25 isoforms were found to bind and activate SPAK/OSR1 kinases (regulators of ion homeostasis and blood pressure) and MST3/MST4/YSK1 kinases (involved in development and morphogenesis).
  • MO25 binding induced a ~100-fold activation of SPAK/OSR1, enhancing phosphorylation of ion cotransporters NKCC1, NKCC2, and NCC, and identified new phosphorylation sites.
  • Knockdown of MO25 in cells reduced phosphorylation of endogenous NKCC1, which was rescued by MO25 re-expression.
  • MO25 binding also stimulated MST3/MST4/YSK1 activity by 3-4 fold.

Conclusions:

  • MO25 acts as a key regulator for a group of STE20 kinases, extending its known function beyond LKB1.
  • MO25 may employ an ancestral mechanism to regulate pseudokinase conformation and activate catalytically competent protein kinases.
  • These findings reveal new roles for MO25 in ion homeostasis, blood pressure regulation, development, and morphogenesis.

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