Catalyzes the covalent attachment of ubiquitin to other proteins. Capable, in vitro, to ubiquitinate histone H2A. Mediates the initiation of DNA replication (transition of G1 to S phase in cell cycle). Essential component of the E3 ubiquitin ligase complex SCF (SKP1-CUL1-F-box protein), which mediates the ubiquitination and subsequent proteasomal degradation of target proteins. Involved in the regulation of methionine biosynthesis genes and in the degradation of CDC6 together with CDC4 and CDC53. 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 CDC53. Component of the E3 ubiquitin ligase complexes SCF with CDC53, SKP1/CBF3D, HRT1 and some F-box proteins like MET30 and CDC4. The acidic C-terminal extension is essential for the cell cycle function. Present with 8170 molecules/cell in log phase SD medium. Belongs to the ubiquitin-conjugating enzyme family.