In response to a stalled replication fork, HLTF polyubiquitinates lysine-164 of PCNA that has already been monoubiquitinated on lysine-164 by RAD18:UBE2B (RAD18:RAD6) (Unk et al. 2008, Motegi et al. 2008, MacKay et al. 2009, Achar et al. 2015). The ubiquitin donor is the E2 complex UBE2N:UBE2V2 (UBC13:MMS2) containing ubiquitin conjugated to UBE2N. The resulting polyubiquitin chain contains lysine-63 (K63) linkages and appears to change the repair process from translesion synthesis (TLS) to template switching (TS). HLTF interacts directly with PCNA, RAD18:UBE2B, and UBE2N:UBE2V2. HLTF and SHPRH are not completely redundant: HLTF is involved in repair of DNA lesions created by ultraviolet light while SHPRH is involved in repair of lesions created by methylmethane sulfonate (Lin et al. 2011). Despite the polyubiquitination activity of HLTF, in vivo HLTF appears to increase monoubiquitination of PCNA (Lin et al. 2011).
ubiquitin protein ligase activity of HLTF:monoUb:K164-PCNA:RAD18:UBE2B:Ub:UBE2N:UBE2V2 [nucleoplasm]