E3 Ubiquitin Ligases as Regulators of Membrane Protein Trafficking and Degradation

Abstract

Ubiquitination is a regulated post-translational modification that conjugates ubiquitin (Ub) to lysine residues of target proteins and determines their intracellular fate. The canonical role of ubiquitination is to mediate degradation by the proteasome of short-lived cytoplasmic proteins that carry a single, polymeric chain of Ub on a specific lysine residue. However, protein modification by Ub has much broader and diverse functions involved in a myriad of cellular processes. Monoubiquitination, at one or multiple lysine residues of transmembrane proteins, influences their stability, protein-protein recognition, activity and intracellular localization. In these processes, Ub functions as an internalization signal that sends the modified substrate to the endocytic/sorting compartments, followed by recycling to the plasma membrane or degradation in the lysosome. E3 ligases play a pivotal role in ubiquitination, because they recognize the acceptor protein and hence dictate the high specificity of the reaction. The multitude of E3s present in nature suggests their nonredundant mode of action and the need for their controlled regulation. Here we give a short account of E3 ligases that specifically modify and regulate membrane proteins. We emphasize the intricate network of interacting proteins that contribute to the substrate-E3 recognition and determine the substrate's cellular fate. E3 LigasesThe covalent ligation of the 76-amino acid peptide Ubiquitin (Ub) to substrate proteins is a highly conserved process that involves a plethora of enzymes and accessory proteins that are usually homologous across species (1). The reaction occurs via the sequential action of three enzymes: a Ub-activating enzyme E1, a Ub-conjugating enzyme E2 and a Ub ligase E3 (1). Although there is only one isoform of E1 in eukaryotic cells, many different E2s and E3s work together to ensure the correct timing, localization and specificity of the ubiquitination reaction (1). The coexistence of many E3 ligases in the same cell type underscores the intricacy of the selection and activation of the appropriate E2 and the recruitment of the substrate. To date, E3s have been considered the only ubiquitination components subject to regulation, but the recent finding of an E2 post-translationally modified by the Ub-like protein SUMO suggests an additional layer of regulation at the E2 level (2).E3 ligases are modular, single proteins or multiprotein complexes that contribute different motifs or domains to the catalytic core, including protein-protein interaction domains for substrate recognition. Together with the E2 enzymes, E3s determine the topology of the polyubiquitin chain. Two primary classes of E3s have been described. The first is distinguished by the presence of a HECT (homologous to E6-AP carboxyl terminus) domain; the second by a RING (really interesting new gene)-finger domain. Both bind to E2s (3). The HECT E3s participate in the catalytic reaction by forming a thioester bond with Ub, via a conserved cysteine residue within the HECT dom...

Autoregulation of MARCH1 Expression by Dimerization and Autoubiquitination

“…U biquitination is a multistep process involving three enzymes called E1, E2, and E3 (1). In eukaryotes, only two E1s are responsible for the ATP-dependent activation of ubiquitin (2).…”

“…In a typical reaction, one of the at least 38 E2 enzymes interacts with activated ubiquitin and one of a near thousand E3 enzymes. The E3 then recognizes the target molecule and catalyzes the transfer of ubiquitin on a lysine or, less frequently, serine, cysteine, or threonine residues (1,(3)(4)(5). The E2/E3 combination and the localization of the complex confer the specificity to the reaction toward a given substrate (1,6).…”

“…The E3 then recognizes the target molecule and catalyzes the transfer of ubiquitin on a lysine or, less frequently, serine, cysteine, or threonine residues (1,(3)(4)(5). The E2/E3 combination and the localization of the complex confer the specificity to the reaction toward a given substrate (1,6). Addition of ubiquitin moieties can lead to lysosomal or proteasomal degradation of the target, to a change in subcellular localization, or to modulation of its interactions with different partners.…”

“…Addition of ubiquitin moieties can lead to lysosomal or proteasomal degradation of the target, to a change in subcellular localization, or to modulation of its interactions with different partners. The fate of ubiquitinated molecules will depend on the type of modification, which can be mono-, multi-, or polyubiquitination (1). There are seven lysines in ubiquitin and each can be modified by the addition of another ubiquitin moiety.…”

EGFR and ADAMs Cooperate to Regulate Shedding and Endocytic Trafficking of the Desmosomal Cadherin Desmoglein 2

“…E-cadherin trafficking can occur through clathrin-dependent, clathrin-independent, and caveolae-dependent pathways and can be regulated by RTKs (Lu et al, 2003;Bryant and Stow, 2004). In addition to regulating interactions with the cytoskeleton, RTK-dependent phosphorylation of cadherins and their associated proteins can contribute to the internalization and/or diversion of cadherins to a lysosomal degradation pathway (Fujita et al, 2002;d'Azzo et al, 2005;McLachlan and Yap, 2007). However, the contribution of desmosomal cadherin regulation to epithelial remodeling via endocytic trafficking has not been addressed, nor is the involvement of RTKs or MMPs understood.…”