Higher order phosphatase-substrate contacts terminate the Integrated Stress Response

Many regulatory PPP1R subunits join few catalytic PP1c subunits to mediate phosphoserine and phosphothreonine dephosphorylation in metazoans. Regulatory subunits are known to engage PP1cs surface, locally affecting flexible phosphopeptides access to the active site. However, catalytic efficiency of holophosphatases towards their natively-folded phosphoprotein substrates is largely unexplained. Here we present a Cryo-EM structure of the tripartite PP1c/PPP1R15A/G-actin holophosphatase that terminates signalling in the Integrated Stress Response (ISR) in pre-dephosphorylation complex with its substrate, translation initiation factor 2 (eIF2). G-actins role in eIF2 dephosphorylation is supported crystallographically by the structure of the binary PPP1R15A-G-actin complex, and by biochemical and genetic confirmation of the essential role of PPP1R15A-G-actin contacts to eIF2P dephosphorylation. In the pre-dephosphorylation CryoEM complex, G-actin aligns the catalytic and regulatory subunits, creating a composite surface that engages eIF2s N-terminal domain to position the distant phosphoserine-51 at the active site. eIF2 residues specifying affinity for the holophosphatase are confirmed here to make critical contacts with the eIF2 kinase PERK. Thus, a convergent process of higher-order substrate recognition specifies functionally-antagonistic phosphorylation and dephosphorylation in the ISR.