E3 ubiquitin-protein ligase involved in postreplication repair of UV-damaged DNA. Postreplication repair functions in gap-filling of a daughter strand on replication of damaged DNA. Associates to the E2 ubiquitin conjugating enzyme UBE2B to form the UBE2B-RAD18 ubiquitin ligase complex involved in mono-ubiquitination of DNA-associated PCNA on 'Lys-164'. Has ssDNA binding activity. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Homodimer. Interacts with UBE2A and UBE2B, one homodimer binding one molecule of UBE2B. Interacts with HLTF. Interacts with SHPRH. Interacts with SPRTN; leading to enhance chromatin association of RAD18 and RAD18-mediated PCNA ubiquitination and translesion DNA synthesis. Interacts (via C-terminus and phosphorylated form) with SLF1 (via BRCT domains); this interaction is required for efficient repair of UV-induced DNA damage. Interacts with SLF2. Interacts with SMC5; this interaction is increased in a SLF1 or SLF2-dependent manner. Associates with chromatin. Colocalizes with SLF1 in the nucleus and to centrosomes. Relocalizes with SLF1 to nuclear foci in response to DNA damage. Accumulates with the SLF1-SLF2 and SMC5-SMC6 complexes at replication-coupled DNA interstrand repair and DNA double-strand breaks (DSBs) sites on chromatin in a ubiquitin-dependent manner. Expressed in thymus, spleen, brain, and ovary. Belongs to the RAD18 family.