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 (PubMed:21549715). Interacts with UBE2A and UBE2B, one homodimer binding one molecule of UBE2B. Interacts with SHPRH (PubMed:17130289, PubMed:17108083). Interacts with HLTF (PubMed:18316726, PubMed:18719106). Interacts with SPRTN; leading to enhance chromatin association of RAD18 and RAD18-mediated PCNA ubiquitination and translesion DNA synthesis (PubMed:22681887). Interacts (via C-terminus and phosphorylated form) with SLF1 (via BRCT domains); this interaction is required for efficient repair of UV-induced DNA damage (PubMed:15632077, PubMed:22036607, PubMed:25931565). Interacts with SLF2 (PubMed:25931565). Interacts with SMC5; this interaction is increased in a SLF1 or SLF2-dependent manner (PubMed:25931565). Associates with chromatin (PubMed:25931565). Colocalizes with SLF1 in the nucleus and to centrosomes (PubMed:15632077). Relocalizes with SLF1 to nuclear foci in response to DNA damage (PubMed:22036607). 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 (PubMed:25931565). Belongs to the RAD18 family.