[Ubiquitin-dependent degradation and modification of proteins].

A large part of cellular proteins is in a dynamic state of turnover. Protein breakdown is responsible for essential cellular functions like modulation of key enzyme levels or removal of abnormal proteins. A major pathway for this selective proteolysis is mediated by the ubiquitin system, in which proteins are committed to degradation by their ligation to ubiquitin, a highly conserved 76 amino acid polypeptide. Recent evidence indicates that ubiquitination serves other functions besides marking proteins for destruction. As originally described for histones, the activities of several cellular proteins are reversibly regulated by ubiquitination and a successive de-ubiquitination step mediated by the activity of one or more isopeptidases.