Protein regulation by monoubiquitin

Multi-ubiquitin chains at least four subunits long are required for efficie nt recognition and degradation of ubiquitylated proteins by the proteasome, but other functions of ubiquitin have been discovered that do not involve the proteasome. Some proteins are modified by a single ubiquitin or short u biquitin chains. Instead of sending proteins to their death through the pro teasome, monoubiquitylation regulates processes that range from membrane tr ansport to transcriptional regulation.