The ongoing outbreak of severe acute respiratory syndrome (SARS) coronavirus 2 (SARS-CoV-2) demonstrates the continuous threat of emerging coronaviruses (CoVs) to public health . SARS-CoV-2 and SARS-CoV share an otherwise non-conserved part of non-structural protein 3 (Nsp3), therefore named as``SARS-unique domain"( SUD). We previously found a yeast-2-hybrid screen interaction of the SARS-CoV SUD with human poly (A) -binding protein (PABP) -interacting protein 1 (Paip1), a stimulator of protein translation . Here, we validate SARS-CoV SUD: Paip1 interaction by size-exclusion chromatography, split-yellow fluorescent protein, and co-immunoprecipitation assays, and confirm such interaction also between the corresponding domain of SARS-CoV-2 and Paip1 . The three-dimensional structure of the N-terminal domain of SARS-CoV SUD (`` macrodomain II", Mac2) in complex with the middle domain of Paip1, determined by X-ray crystallography and small-angle X-ray scattering, provides insights into the structural determinants of the complex formation . In cellulo, SUD enhances synthesis of viral but not host proteins via binding to Paip1 in pBAC-SARS-CoV replicon-transfected cells . We propose a possible mechanism for stimulation of viral translation by the SUD of SARS-CoV and SARS-CoV-2.