Recruitment of trimeric eIF2 by phosphatase non-catalytic subunit PPP1R15B

Area of Science:

• Molecular Biology
• Cellular Biology
• Biochemistry

Background:

• Protein phosphorylation is a key regulator of cellular functions.
• Protein phosphatase 1 (PP1) holoenzymes dephosphorylate serine/threonine residues.
• Substrate recruitment mechanisms for PP1 holoenzymes remain largely uncharacterized.

Purpose of the Study:

• To elucidate the mechanism by which the PP1 non-catalytic subunit PPP1R15B (R15B) captures its trimeric eIF2 substrate.
• To understand the structural basis of substrate recognition by R15B.

Main Methods:

• Integrated diverse biochemical and structural approaches.
• Characterized the substrate-recruitment module of R15B.
• Investigated the impact of a patient-derived variant (N423D).

Main Results:

• The substrate-recruitment module of R15B is largely intrinsically disordered, featuring three short helical elements (H1, H2, H3).
• Helices H1 and H2 form a clamp that binds the eIF2 substrate at a region distant from the phosphorylation site.
• A homozygous N423D variant near H1 impairs substrate binding and dephosphorylation, correlating with a rare syndrome.

Conclusions:

• R15B employs a unique clamp mechanism to capture its large 125 kDa eIF2 substrate by binding its distal end.
• This interaction positions the substrate for dephosphorylation by PP1, revealing a novel paradigm for enzyme-substrate interaction.
• The findings provide insight into the molecular basis of a rare developmental disorder linked to R15B dysfunction.