Plays a role in the DNA damage response (DDR) pathway by regulating postreplication repair of UV-damaged DNA and genomic stability maintenance. The SLF1-SLF2 complex acts to link RAD18 with the SMC5-SMC6 complex at replication-coupled interstrand cross-links (ICL) and DNA double-strand breaks (DSBs) sites on chromatin during DNA repair in response to stalled replication forks. Promotes the recruitment of SLF2 and the SMC5-SMC6 complex to DNA lesions. Interacts (via N-terminus) with SLF2; this interaction links RAD18 to the SMC5-SMC6 complex. Interacts (via BRCT domains) with RAD18; this interaction occurs in a SLF2-independent manner. Interacts with SMC6. Interacts (via BRCT domains) with RAD18 (via C-terminus and phosphorylated form); this interaction is required for efficient repair of UV-induced DNA damage. Relocalizes with RAD18 to nuclear foci in response to DNA damage. Colocalizes with RAD18 in the nucleus and to centrosomes. Associates with chromatin. Accumulates with RAD18 and the SMC5-SMC6 complex at replication-coupled DNA interstrand repair and DNA double-strand breaks (DSBs) sites on chromatin in a ubiquitin-dependent manner. BRCT domains are necessary for its targeting to ionizing radiation-induced nuclear foci.