Accepts the ubiquitin from the E1 complex and catalyzes its covalent attachment to other proteins (By similarity). Mediates PHO1 degradation through multivesicular body-mediated vacuolar proteolysis in response to inorganic phosphate (Pi) availability (PubMed:22634761). Negatively regulates the protein abundance of PHF1 and PHT1s under Pi-sufficient conditions by facilitating the degradation of PHT1 proteins at the endomembrane (PubMed:24122829). 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 PHO1. Expressed in the vascular tissues of cotyledons, leaves, roots, sepals, filaments, anthers and junctions between the inflorescence stems and siliques. Up-regulated in senescing leaves and maturating seeds. Down-regulated by phosphate deprivation (PubMed:16679424). Systemically regulated by microRNA399 (miR399) (PubMed:16679424, PubMed:18390805). Plants are unable to regulate the amount of phosphate accumulated into shoots. MicroRNA399 (miR399) can be sequestered by IPS1, a non-protein coding RNA containing a motif with sequence complementarity to miR399, but with a mismatched loop at the expected miRNA cleavage site. Thus IPS1 mimics the target of miR399 to block the cleavage of UBC24/PHO2 under Pi-deficient conditions. Belongs to the ubiquitin-conjugating enzyme family.