Crystal Structure of the VP4 Protease from Infectious Pancreatic Necrosis Virus Reveals the Acyl-Enzyme Complex

Infectious pancreatic necrosis virus (IPNV), an aquatic birna-virus that infects salmonid fish, encodes a large polyprotein (NH2-pVP2-VP4-VP3-COOH) that is processed through the proteolytic activity of its own protease, VP4, to release the pro-teins pVP2 andVP3. pVP2 is further processed to give rise to the capsid protein VP2 and three peptides that are incorporated into the virion. Reported here are two crystal structures of the IPNV VP4 protease solved from two different crystal symme-tries. The electrondensity at the active site in the triclinic crystal form, refined to 2.2-A ̊ resolution, reveals the acyl-enzyme com-plex formedwith an internalVP4cleavage site.The complexwas generated using a truncated enzyme in which the general base lysine was substituted. Inside the complex, the nucleophilic Ser633O forms an ester bond with the main-chain carbonyl of the C-terminal residue, Ala716, of a neighboring VP4. The struc