E3 RING-finger protein, member of the UBC2/RAD6 epistasis group. Associates to the E2 ubiquitin conjugating enzyme ubc2/rad6 to form the ubc2-rad18 ubiquitin ligase complex involved in postreplicative repair (PRR) of damaged DNA (By similarity). 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. Interacts with E2 ubc2, forming a complex with ubiquitin ligase activity. Belongs to the RAD18 family.