Both rad6 and ubcls are ubiquitin - conjugating enzymes , and mms2 is a ubc - like protein . radls and rad5 are both single strand binding ring linger proteins . rad6 binds to radls and mms2 - ubcls binds to rad5 在泛素系統(tǒng)中起主要作用的是三種酶:泛素激活酶e1 ;泛素綴合酶e2 / ubc和泛素連接酶e3 。