Ubiquitin mediates the physical and functional interaction between human DNA polymerases η and ι

Human DNA polymerases eta and iota are best characterized for their ability to facilitate translesion DNA synthesis (TLS). Both polymerases (pols) co-localize in 'replication factories' in vivo after cells are exposed to ultraviolet light and this co-localization is mediated through a physical interaction between the two TLS pols. We have mapped the pol eta-iota interacting region to their respective ubiquitin-binding domains (UBZ in pol mu and UBM1 and UBM2 in pol iota), and demonstrate that ubiquitination of either TLS polymerase is a prerequisite for their physical and functional interaction. Importantly, while monoubiquitination of pol eta precludes its ability to interact with proliferating cell nuclear antigen (PCNA), it enhances its interaction with poli. Furthermore, a pol iota-ubiquitin chimera interacts avidly with both pol eta and PCNA. Thus, the ubiquitination status of pol eta, or poli plays a key regulatory function in controlling the protein partners with which each polymerase interacts, and in doing so, determines the efficiency of targeting the respective polymerase to stalled replication forks where they facilitate TLS.