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Review
. 2012 Sep;160(1):56-64.
doi: 10.1104/pp.112.199869. Epub 2012 Jun 22.

Ubiquitin on the move: the ubiquitin modification system plays diverse roles in the regulation of endoplasmic reticulum- and plasma membrane-localized proteins

Damian D Guerra  1 , Judy Callis
Affiliations
Review

Ubiquitin on the move: the ubiquitin modification system plays diverse roles in the regulation of endoplasmic reticulum- and plasma membrane-localized proteins

Damian D Guerra et al. Plant Physiol. 2012 Sep.
No abstract available

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Figures

Figure 1.
Figure 1.
The ubiquitination enzyme cascade. A, The modification of eukaryotic intracellular proteins with the small protein ubiquitin (Ub; gray sphere) depends on a three-enzyme cascade. E1 (tan oval) activates ubiquitin at the expense of two ATP equivalents by forming a high-energy thioester bond between the E1 active-site Cys and the C terminus of ubiquitin. Ubiquitin is then passed to an active-site Cys on E2 (yellow octagon). Lastly, the Ub∼E3 thioester is brought into proximity with the desired protein target (red oval) by a scaffold protein called E3 (blue). Ubiquitin can be added to a variety of nucleophilic groups on the target protein, including the N terminus (Cα1-NH2), Lys residues (K-NH2), Ser and Thr residues (S/T-OH), and Cys residues (C-SH). B, Types of E3s. Multisubunit RING E3s, such as CULLIN RING Ligases (CRLs), recognize substrates with adaptor proteins (i.e. F-box proteins). Single-subunit RINGs like MDM2 consist of one polypeptide with binding sites for the E2 (the RING domain) and the target protein. The U-box is a modified RING domain lacking zinc-binding sites. HECT-type E3s form a Ub∼E3 thioester prior to ubiquitin being transferred to the target. APC, Anaphase-promoting complex.
Figure 2.
Figure 2.
Fates of ubiquitinated proteins. The fate of a ubiquitinated target protein, typically mediated by ubiquitin-binding proteins (UBP; dark green), depends on the number of ubiquitin moieties attached to the substrate, the type of ubiquitin-ubiquitin linkage, and whether the substrate protein is membrane associated. A, Monoubiquitinated integral membrane proteins are internalized, leading to activation or vacuolar degradation upon exit from the sorting endosome (SE). B, Polyubiquitinated integral membrane proteins can be degraded either by vacuoles or, via ERAD, the proteasome. C, Polyubiquitinated soluble proteins, in which ubiquitin moieties are linked via Lys-48, are proteasomally degraded; in contrast, Lys-63-linked substrates tend not to be proteolyzed but have been shown to play roles in DNA repair. D, Monoubiquitination and multiubiquitination of soluble proteins can bring about activation or inhibition of a protein’s activity.
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