Updated project metadata. HDX-MS of R15B and it's interactions with trimeric eIF2 substrate. Here we integrated diverse approaches to elucidate how the PP1 non-catalytic subu-nit PPP1R15B (R15B) captures its full trimeric eIF2 substrate. We found that the substrate recruitment module of R15B is largely disordered with three short helical elements, H1, H2 and H3. H1 and H2 form a clamp that grasps the substrate in a re-gion remote from the phosphorylated residue. A homozygous N423D variant, adja-cent to H1, reducing substrate binding and dephosphorylation was discovered in a ra-re syndrome with microcephaly, development delay and intellectual disability. These findings explain how R15B captures its 125 kDa substrate by binding the far end of the complex relative to the phospho-site to present it for dephosphorylation by PP1, a paradigm of broad relevance.