Many Gram-negative bacteria deliver their virulence factors into host cells through a secretion system. Those factors, called effector proteins, are involved in the pathogenicity in host cells by interfering with various cellular events. The phytopathogen Xanthomonas oryzae pv. oryzae uses a type III secretion system to inject its effectors, but the functional roles of these proteins remain largely uncharacterized. Here, we determined a crystal structure of XOO4466, an effector from X. oryzae pv. oryzae, and performed a functional analysis. We determined that XOO4466 is similar in sequence to Xanthomonas outer protein Q, a putative nucleoside hydrolase (NH). The overall structure of XOO4466 is homologous to that of NHs, including a metal-binding site, but differs in its oligomeric state and active site topology. Further analysis indicated that antiparallel beta-strands commonly found in NHs adjacent to the active site loop are replaced in XOO4466 with a short loop, causing the active site loop to adopt a conformation distinct from that of NHs. Thus, the catalytic residues emanating from the respective active site loop of NHs are absent in the putative active site of XOO4466. Consistent with these structural features, a functional assay indicated that XOO4466 does not exhibit NH activity and possibly catalyzes yet unknown reactions. (C) 2013 Elsevier Inc. All rights reserved.