Synoviolin is a RING type E3 ubiquitin ligase. Top, schematic diagram of the primary structure of Synoviolin. a.a., amino acid residues; PR, proline-rich domain; RING, RING finger domain; TM, transmembrane domain. Bottom, consensus sequence of the RING finger domain. Mutations in consensus cysteine or histidine abolish the enzymatic activity of Synoviolin.