Novel phosphorylation-dependent regulation in an unstructured protein

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Dynamics

Background:

• Inhibitor-2 (I2) regulates protein phosphatase-1 (PP1) activity.
• Free I2 is unstructured, but adopts structure upon binding PP1, with an i-helix blocking the active site.

Purpose of the Study:

• To investigate how phosphorylation of I2's unstructured region regulates PP1 activity.
• To explore the structural basis of phosphorylation-dependent PP1-I2 activation.

Main Methods:

• Molecular dynamics (MD) simulations in explicit solvent.
• Analysis of I2 orthologs from diverse species (human, yeast, worm, protozoa).

Main Results:

• I2 phosphorylation activates PP1 activity without dissociation, promoting early activation steps.
• Phosphorylation alters conformations around Thr74, specifically uncurling the Glu71-Tyr76 region.
• This conformational change, not electrostatics, drives i-helix displacement and PP1 activation.

Conclusions:

• Phosphorylation-dependent activation of PP1 by I2 is mediated by conformational changes in I2's unstructured domain.
• The mechanism is conserved across species, emphasizing the role of specific residue distances in regulating phosphatase activity.