CELL SIGNALING: Anchors Away for Ubiquitin Chains

It has become increasingly clear that cellular proteins can become conjugated with chains of the small proteinubiquitin without becoming degraded by the proteasome. This "noncanonical" process allows ubiquitinated proteins to regulate numerous processes including DNA repair and cell signaling. However, determining the mechanisms by which polyubiquitin chains affect such processes has been challenging, in part because of the unavailability of genetically engineered mice that lack ubiquitin and the partial redundancies of molecules that support ubiquitin chain assembly. Recent work now shows that rather than being assembled onto target proteins by the cell's ubiquitinconjugation machinery, ubiquitin is also present in cells as free, unanchored chains that can bind directly to a signaling protein involved in the immune response to viral infection (1). This raises the question of whether unanchored ubiquitin chains generally regulate other physiological functions.