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Two RING finger proteins mediate cooperation between ubiquitin‐conjugating enzymes in DNA repair

Two ubiquitin‐conjugating enzymes, RAD6 and the heteromeric UBC13–MMS2 complex, have been implicated in post‐replicative DNA damage repair in yeast. Here we provide a mechanistic basis for cooperation between the two enzymes. We show that two chromatin‐associated RING finger proteins, RAD18 and RAD5, play a central role in mediating physical contacts between the members of the RAD6 pathway. RAD5 recruits the UBC13–MMS2 complex to DNA by means of its RING finger domain. Moreover, RAD5 association with RAD18 brings UBC13–MMS2 into contact with the RAD6–RAD18 complex. Interaction between the two RING finger proteins thus promotes the formation of a heteromeric complex in which the two distinct ubiquitin‐conjugating activities of RAD6 and UBC13–MMS2 can be closely coordinated. Surprisingly, UBC13 and MMS2 are largely cytosolic proteins, but DNA damage triggers their redistribution to the nucleus. These findings suggest a mechanism by which the activity of this DNA repair pathway could be regulated.