1. Volume 19, Issue 5, Pages 581-593 (September 2005)
Regulating the Regulators: Control of Protein Ubiquitination and Ubiquitin-like Modifications by Extracellular Stimuli 
Min Gao, Michael Karin 
Molecular Cell 
Volume 19, Issue 5, Pages (September 2005)
DOI: /j.molcel
Copyright © 2005 Elsevier Inc. Terms and Conditions

2. Figure 1 Overview of the Ubiquitin Conjugation Pathway
Ubiquitin is first activated by a ubiquitin-activating enzyme E1; activated ubiquitin is then transferred to a ubiquitin-conjugating enzyme E2; a ubiquitin ligase E3 facilitates the transfer of ubiquitin from E2 to the protein substrate. There are two major classes of E3 ubiquitin ligases: proteins with a HECT catalytic domain and proteins with a RING finger adaptor domain. Additional E3s, such as those containing a U box, were recently described. Substrates marked with a Lys48-linked polyubiquitin chain are selectively targeted to 26S proteasome-mediated degradation, whereas certain substrates conjugated with mono- or multiubiquitins or Lys63-linked polyubiquitin chains are targeted for endocytosis or are enabled to engage in new protein-protein interactions. The polyubiquitin chain can be removed from the substrate by a deubiquitinating enzyme.
Molecular Cell  , DOI: ( /j.molcel )
Copyright © 2005 Elsevier Inc. Terms and Conditions

3. Figure 2 Role of Protein Ubiquitination in NF-κB Activation
The IKK complex can be activated in response to engagement of cell surface receptors such as the IL-1 receptor (IL-1R) or the type1 TNF receptor (TNFR1). (I) IKK phosphorylates IκBs, thereby allowing their recognition by the RING finger E3-SCFβTrCP. This results in polyubiquitination of the IκBs and subsequent degradation by 26S proteasome. The liberated NF-κB p50:Rel dimer with an exposed nuclear localization sequence (NLS) can now enter the nucleus to stimulate transcription of numerous target genes. (II) Engagement of IL-1R leads to oligomerization of receptor signaling complexes followed by self-ubiquitination of the RING finger E3 TRAF6 through conjugation of Lys63-linked polyubiquitin chains. TAB2 binds to the polyubiquitin chains of TRAF6, and the TAB2-associated TAK1 kinase is activated, presumably by autophosphorylation. TAK1 further phosphorylates and activates the IKK complex. (III) Engagement of TNFR1 leads to TRAF2 and RIP1 polyubiquitination through a Lys63 linkage, allowing binding of TAB2 and activation of TAK1, which phosphorylates and activates the IKK complex. TNFR1 stimulation also induces association of A20 with RIP1. The DUB domain of A20 removes the Lys63-linked polyubiquitin chains from RIP1 and replaces them with Lys48-linked polyubiquitin chains, which signal RIP1 proteasomal degradation. Both the deubiquitination and ubiquitination activities of A20 negatively regulate NF-κB signaling. (IV) TCR stimulation leads to phosphorylation of PDK1. Activated PDK1 phosphorylates PKCθ, which recruits IKK to lipid rafts. PDK1 also recruits the CARD11-Bcl10-MALT1 complex, probably through its phosphorylation, and the latter ubiquitinates IKKγ/NEMO. Ubiquitination of IKKγ/NEMO leads to trans-autophosphorylation of the IKK catalytic subunits, culminating in their activation.
Molecular Cell  , DOI: ( /j.molcel )
Copyright © 2005 Elsevier Inc. Terms and Conditions

4. Figure 3 Regulation of E3 Activities by Extracellular Stimuli
(I) Mdm2 possesses an E3 activity that mediates self-ubiquitination as well as polyubiquitination of p53. Stress stimuli induce the phosphorylation of Mdm2 by the protein kinase Akt. Phosphorylated Mdm2 promotes p53 ubiquitination and degradation, while its self-ubiquitination is inhibited. At the same time, DNA damage induces p53 phosphorylation by stress-activated protein kinases (SAPK), which reduces its binding to Mdm2, thereby inhibiting p53 polyubiquitination and degradation. (II) Nedd4 binds and ubiquitinates ENaC. Aldosterone increases transcription of the gene encoding the protein kinase SGK, which binds and phosphorylates Nedd4. This weakens the binding of Nedd4 to ENaC, leading to ENaC stabilization. (III) Itch is a HECT domain E3 ligase that targets JunB ubiquitination in T cells. Engagement of T cell receptor results in rapid JNK activation followed by Itch phosphorylation. This modification activates Itch’s E3 ligase activity, resulting in accelerated degradation of JunB and attenuation of IL-4 gene transcription.
Molecular Cell  , DOI: ( /j.molcel )
Copyright © 2005 Elsevier Inc. Terms and Conditions