Accepts ubiquitin from the E1 complex and catalyzes its covalent attachment to other proteins. In vitro, in the presence or in the absence of BRCA1-BARD1 E3 ubiquitin-protein ligase complex, catalyzes the synthesis of 'Lys-48'-linked polyubiquitin chains. Does not transfer ubiquitin directly to but elongates monoubiquitinated substrate protein. Mediates the selective degradation of short-lived and abnormal proteins, such as the endoplasmic reticulum-associated degradation (ERAD) of misfolded lumenal proteins. Ubiquitinates huntingtin. May mediate foam cell formation by the suppression of apoptosis of lipid-bearing macrophages through ubiquitination and subsequence degradation of p53/TP53. Proposed to be involved in ubiquitination and proteolytic processing of NF-kappa-B; in vitro supports ubiquitination of NFKB1. In case of infection by cytomegaloviruses may be involved in the US11-dependent degradation of MHC class I heavy chains following their export from the ER to the cytosol. In case of viral infections may be involved in the HPV E7 protein-dependent degradation of RB1. {ECO:0000269|PubMed:10634809, ECO:0000269|PubMed:10675012, ECO:0000269|PubMed:16714285, ECO:0000269|PubMed:16868077, ECO:0000269|PubMed:17873885, ECO:0000269|PubMed:19906396, ECO:0000269|PubMed:20061386, ECO:0000269|PubMed:8702625}. This is the function of UBE2K (ubiquitin conjugating enzyme E2 K, Ensembl gene identifier ENSG00000078140).