Ubiquitin does more than mark proteins for degradation. In 2004, Ingrid Wertz, a University of California, Davis graduate student, and advisor Vishva Dixit of Genentech, demonstrated that the protein A20, which was known to downregulate tumor necrosis factor á (TNFá) signaling, has both an ubiquitin ligase domain and a deubiquitinating domain.1 Wertz, now at the University of Washington, St. Louis, says she found the presence of these opposing functions disconcerting: "I was thinking, how could a deubiquitinase downregulate TNF receptor signaling? And then it all came together very clearly when I realized that our antibodies were not distinguishing between the K48 polyubiquitination versus the K63 polyubiquitination."

Indeed, A20 delivers a one-two punch to the regulatory protein RIP, first removing the activating K63 ubiquitination, then polyubiquitinating K48, marking the protein for degradation, effectively attenuating TNFá-induced NFêB signaling. "[The paper] identified a major step in NFêB signaling; it revealed a role...

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