The 26S proteasome is responsible for the degradation of polyubiquitinated proteins. During this process the polyubiquitin chain is removed. The identity of the proteasomal component that is responsible for this activity has not been clear, as it contains no subunits that resemble known deubiquitinating enzymes. The Jab1/MPN domain is a widespread 120 amino acid protein module found in archaea, bacteria, and eukaryotes. In eukaryotes the Jab1/MPN domain is found in subunits of several multiprotein complexes including the proteasome. Recently it has been proposed that the Jab1/MPN domain of the proteasomal subunit Rpn11 is responsible for the removal of the polyubiquitin chain from substrate proteins. Here we report the crystal structure and characterization of AF2198, a Jab1/MPN domain protein from Archaeoglobolus fulgidus. The structure reveals a fold that resembles that of cytidine deaminase and places the Jab1/MPN domain in a superfamily of metal dependent hydrolases.