Life, Death, and Ubiquitin: Taming the Mule

Ubiquitin-mediated protein degradation is an efficient way for the cell to get rid of unwanted proteins. A key player in this process is the E3 ubiquitin ligase. In this issue of Cell, Chen et al. (2005) and Zhong et al. (2005) describe a new E3 ligase, ARF-BP1/Mule, which targets two very different substrates, p53 and Mcl-1, with completely different cellular outcomes