Unique fold and active site in cytomegalovirus protease.

HUMAN herpesviruses are responsible for a variety of diseases. They are divided into three subfamilies: alpha includes herpes simplex viruses (HSV-1 and HSV-2) and varicella-zoster virus (VZV); beta includes cytomegalovirus (CMV) and human herpes-virus-6 (HHV-6); and gamma includes Epstein–Barr virus (EBV).Each virus encodes a serine protease that is essential for its replication1–14and is a potential target for therapeutic intervention. Human CMV is a ubiquitous opportunistic pathogen that can result in life-threatening infections in congenitally infected infants, immunocompromised individuals and immuno-suppressed cancer or transplant patients15. Here we report the crystal structure of human CMV protease at 2.5 A resolution. The structure reveals a fold that has not been reported for any other serine protease, and an active site consisting of a novel catalytic triad in which the third member is a histidine instead of an aspartic acid, or possibly a catalytic tetrad consisting of a serine, two histidines and an aspartic acid. An unusual dimer interface that is important to the protease activity has also been identified.