Molecular Mechanism of PP2A/B55α Phosphatase Inhibition by IER5

Area of Science:

• Biochemistry
• Structural Biology
• Cell Biology

Background:

• Protein phosphatase 2A (PP2A) are crucial serine/threonine phosphatases regulating numerous physiological processes.
• PP2A activity is fine-tuned by regulatory subunits and associated proteins, including IER5.
• Dysregulation of PP2A is implicated in various diseases, highlighting the need for understanding its regulation.

Purpose of the Study:

• To elucidate the structural basis of PP2A/B55α inhibition by the IER5 protein.
• To investigate the functional consequences of IER5-PP2A/B55α interaction on cellular processes.
• To identify potential therapeutic strategies for modulating PP2A/B55α activity.

Main Methods:

• Cryoelectron microscopy (cryo-EM) to determine the structure of the PP2A/B55α-IER5 complex.
• Biochemical assays to assess PP2A/B55α phosphatase activity.
• Site-directed mutagenesis and co-immunoprecipitation to study protein-protein interactions.
• Cellular assays to evaluate the impact on gene expression (e.g., KRT1).

Main Results:

• The cryo-EM structure reveals that the N-terminal region of IER5 (IER5-N50) directly binds to B55α, occluding a key substrate-binding surface.
• IER5-N50 inhibits PP2A/B55α-mediated dephosphorylation of pTau in vitro.
• Mutations disrupting the IER5-PP2A/B55α interface impair complex formation and suppress IER5-dependent regulation of KRT1 expression.
• Structural homologs of IER5, such as SERTA domain proteins, were identified as potential PP2A/B55α binders.

Conclusions:

• IER5 inhibits PP2A/B55α by physically blocking substrate access.
• The IER5-PP2A/B55α interaction is critical for regulating specific cellular functions, including gene expression.
• These findings provide a molecular framework for understanding PP2A/B55α regulation and offer a basis for developing targeted therapeutics.