Catalyzes the covalent attachment of ubiquitin to other proteins. Seems to function in the selective degradation of misfolded membrane proteins from the endoplasmic reticulum (ERAD). In case of infection by the murid herpesvirus 4, its association with the viral E3 ligase K3 mediates ubiquitination of host surface class I (MHC-I) H-2D(b)/H2-D1 and H-2K(b)/H2-K1 molecules before they exit the endoplasmic reticulum, leading to their degradation by the ERAD system, thus blocking the immune detection of virus-infected cells. The complex formed with the murid herpesvirus 4 protein K3 mediates ubiquitination of lysine, as well as serine and threonine residues present in the cytoplasmic tail of surface class I molecules and promotes ubiquitination of hydroxylated serine or threonine residues via ester bonds instead of the classical isopeptide linkage. S-ubiquitinyl-[E1 ubiquitin-activating enzyme]-L-cysteine + [E2 ubiquitin-conjugating enzyme]-L-cysteine = [E1 ubiquitin-activating enzyme]-L-cysteine + S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine. Protein modification; protein ubiquitination. Interacts with murid herpesvirus 4 protein K3 (mK3). Belongs to the ubiquitin-conjugating enzyme family.