泛素连接酶，又称为E3泛素连接酶，是一个能够将泛素分子连接到目的蛋白质的某个赖氨酸上的酶。通常情况下，泛素连接酶可以将目的蛋白质多泛素化，即加上多个泛素分子，形成多泛素链；而带有多泛素链的蛋白质就可以为蛋白酶体所识别，而被降解。但在一些情况下，泛素连接酶只在目的蛋白质上连接一个泛素分子；这种单泛素化的蛋白质不会被蛋白酶体所降解，但其在细胞中的定位或功能却可能发生改变；例如，单泛素化的蛋白质可以通过结合其他具有泛素结合结构域的蛋白质来改变自身位置。

A ubiquitin ligase (also called an E3 ubiquitin ligase) is a protein that recruits an E2 ubiquitin-conjugating enzyme that has been loaded with ubiquitin, recognizes a protein substrate, and assists or directly catalyzes the transfer of ubiquitin from the E2 to the protein substrate. The ubiquitin is attached to a lysine on the target protein by an isopeptide bond. E3 ligases interact with both the target protein and the E2 enzyme, and so impart substrate specificity to the E2. Commonly, E3s polyubiquitinate their substrate with Lys48-linked chains of ubiquitin, targeting the substrate for destruction by the proteasome. However, many other types of linkages are possible and alter a protein's activity, interactions, or localization. Ubiquitination by E3 ligases regulates diverse areas such as cell trafficking, DNA repair, and signaling and is of profound importance in cell biology. E3 ligases are also key players in cell cycle control, mediating the degradation of cyclins, as well as cyclin dependent kinase inhibitor proteins. The human genome encodes over 600 putative E3 ligases, allowing for tremendous diversity in substrates.