Abstract: | Substrate specificity in ubiquitylation is conferred by ubiquitin ligases (E3s). Now, several ways that E3s can interact to mediate ubiquitylation are illustrated for Ubr1 (a RING finger E3) and Ufd4 (a HECT domain E3), in Saccharomyces cerevisiae. These interactions and the related concept of E4 activity are discussed. |