Non-traditional functions of ubiquitin and ubiquitin-binding proteins.

Until recently, proteasome-mediated proteolysis dominated discussion about the function of ubiquitin, a highly conserved 76-amino acid polypeptide. However, a spate of new discoveries has opened up research into the functions of ubiquitin in regulating proteins by proteasome-independent processes. These investigations have led to a deeper understanding of the different types of ubiquitin modifications and of proteins that bind to monoubiquitin and polyubiquitin chains. In this review we discuss proteasome-independent functions of ubiquitin with emphasis on how monoubiquitin and ubiquitin-binding proteins signal changes in protein location, activity, and interactions with binding partners.