The COP9 signalosome (CSN) is a protein complex whose eight subunits display significant sequence similarity to those of the 26S proteasome LID— a component of the 26S proteasome complex that degrades ubiquitin-conjugated proteins. What is the function of this LID-like structure? Wolfgang Dubiel and co-workers examine studies suggesting that it functions as a platform that connects signalling with proteolysis. First identified as being important for proper plant photomorphogenesis, the CSN appears to control many aspects of plant development. It is also implicated in regulation of the yeast cell cycle and fruitfly development. Recent work suggests that the CSN cooperates with the E3 ubiquitin ligase SCF to control stability of key cell cycle and developmental regulators. Furthermore, one of its subunits, CSN5, has been shown to interact with a variety of cellular regulators, including the CDK inhibitor p27Kip1 and the transcription factor JUN. Indeed, such molecules might be targets of a CSN-associated kinase that targets them for degradation by the ubiquitin system.