The crystal structure of a conserved domain of nonstructural protein 3 (nsP3) from severe acute respiratory syndrome coronavirus (SARS-CoV) has been solved by single-wavelength anomalous dispersion to 1.4 ร… resolution. The structure of this "X" domain, seen in many single-stranded RNA viruses, reveals a three-layered ฮฑ/ฮฒ/ฮฑ core with a macro-H2A-like fold. The putative active site is a solvent-exposed cleft that is conserved in its three structural homologs, yeast Ymx7, Archeoglobus fulgidus AF1521, and Er58 from E. coli. Its sequence is similar to yeast YBR022W (also known as Poa1P), a known phosphatase that acts on ADP-ribose-1โ€ณ-phosphate (Appr-1โ€ณ-p). The SARS nsP3 domain readily removes the 1โ€ณ phosphate group from Appr-1โ€ณ-p in in vitro assays, confirming its phosphatase activity. Sequence and structure comparison of all known macro-H2A domains combined with available functional data suggests that proteins of this superfamily form an emerging group of nucleotide phosphatases that dephosphorylate Appr-1โ€ณ-p. ยฉ2005 Elsevier Ltd.