Substrates that are destined for the proteasome are covalently modified with ubiquitin by a cascade of enzymes,including E1,E2 and E3 enzymes,the last of which determines substrate specificity.

• 泛素化是单个或多个泛素在泛素激活酶、泛素结合酶及泛素蛋白连接酶的作用下共价修饰底物蛋白质的过程。