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Par-4: a new activator of myosin phosphatase

Susanne Vetterkind1, Eunhee Lee, Eric Sundberg

  • 1Department of Health Sciences, Sargent College of Health and Rehabilitation Sciences, Boston University, Boston, MA 02215, USA.

Molecular Biology of the Cell
|February 5, 2010
PubMed

Insights

Prostate apoptosis response (Par)-4 activates myosin phosphatase (MP) by binding to MYPT1, enhancing smooth muscle function. This interaction blocks inhibitory phosphorylation, revealing a novel MP activation mechanism.

Area of Science:

  • Molecular Biology
  • Cellular Signaling
  • Biochemistry

Background:

  • Myosin phosphatase (MP) regulates myosin light chain (LC20) phosphorylation, critical for smooth muscle contractility, motility, and apoptosis.
  • MP inhibition is extensively studied, but mechanisms of MP activation remain largely unknown.
  • Prostate apoptosis response (Par)-4 has been identified as a modulator of vascular smooth muscle contractility.

Purpose of the Study:

  • To investigate the hypothesis that Par-4 directly regulates myosin phosphatase (MP) activity.
  • To elucidate the molecular mechanism by which Par-4 influences MP function and smooth muscle contractility.

Main Methods:

  • Proximity ligation assays, surface plasmon resonance, and coimmunoprecipitation to confirm Par-4 and MYPT1 interaction.
  • Overexpression and small interfering RNA (siRNA) knockdown studies to assess the impact of Par-4 on MP activity.
  • LC20 phosphorylation assays and analysis of zipper-interacting protein kinase (ZIPK)-mediated inhibition.

Main Results:

  • Par-4 directly interacts with MYPT1, the targeting subunit of MP, via leucine zippers.
  • Par-4 overexpression increases MP activity and reduces LC20 phosphorylation; Par-4 knockdown decreases MP activity.
  • Par-4 phosphorylation by ZIPK displaces it from MP, enabling inhibitory phosphorylation of MYPT1.

Conclusions:

  • Par-4 acts as an activator of MP through a 'padlock' mechanism, binding MYPT1 to prevent inhibitory phosphorylation.
  • Par-4 phosphorylation by ZIPK is necessary to 'unlock' MP, allowing for its inhibition and subsequent MYPT1 phosphorylation.
  • This study reveals a novel pathway for MP activation, crucial for understanding smooth muscle contractility regulation.

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