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. The UBC2-RAD18 complex cooperates with RAD5 and the UBC13-MMS2 dimer to attach mono-ubiquitin chains on 'Lys-164' of POL30, which is necessary for PRR. The UBC2-RAD18 complex is also involved in prevention of spontaneous mutations caused by 7,8-dihydro-8-oxoguanine. 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 E2 UBC2, forming a complex with ubiquitin ligase activity. The UBC2-RAD18 complex interacts itself with the UBC13-MMS2 ubiquitin ligase complex through direct interactions of both RAD18 and UBC13 with RAD5. Interacts also with UBC9. Binds single strand DNA. Present with 206 molecules/cell in log phase SD medium. Belongs to the RAD18 family.