Fig. 2.

The ubiquitin (Ub)-proteasome Pathway. The process of ubiquitination is controlled by the Ub-activating enzymes (E1), Ub-conjugating enzymes (E2), and Ub-protein ligases (E3). Both MuRF1 and MAFbx are E3 Ub ligases that control the ubiquitination of specific substrates. Ub can be added to a substrate as a single monomer on one (monoubiquitination) or more lysines (multiubiquitination) or as a chain of ubiquitins of variable length on a single lysine (polyubiquitination). The model of ubiquitination can lead to different substrate fates. Polyubiquitination of a substrate with a ubiquitin chain using K11 or K48 linkages generally results in proteasomal degradation, whereas Ub chains using K63 linkages can lead to alterations in signaling or endocytosis. RBX, RING finger protein.