The suppressor of cytokine signaling (SOCS) box consists of the BC box and the cullin 5 (Cul5) box, which interact with Elongin BC and Cul5, respectively. SOCS box-containing proteins have ubiquitin ligase activity mediated by the formation of a complex with the scaffold protein Cul5 and the RING domain protein Rbx2, and are thereby members of the cullin RING ligase superfamily. Cul5-type ubiquitin ligases have a variety of substrates that are targeted for polyubiquitination and proteasomal degradation. Here, we review the current knowledge on the identification of Cul5 and the regulation of its expression, as well as the signaling pathways regulated by Cul5 and how viruses highjack the Cul5 system to overcome antiviral responses. Identification and regulation of cullin 5Cullin 5 (Cul5) was originally identified as a vasopressin-activated calcium-mobilizing (VACM-1) protein, an arginine vasopressin (AVP) receptor [1]. AVP is a nonapeptide that regulates body fluid and blood pressure homeostasis. VACM-1 is recognized as Cul5 because of its homology to the Caenorhabditis elegans gene Cul5 [2,3]. Cul5 is expressed in many cells and organs, including endothelial cells, brain, kidney collecting tubule cells, and vascular endothelial cells [2,[4][5][6]7]. Cul5 inhibits cyclic AMP production, and this effect is reversed by staurosporin, a protein kinase A (PKA) inhibitor, or by mutating S730A, the PKA-dependent phosphorylation site in the Cul5 sequence in COS-1 cells [8]. The inhibitory effect of Cul5 on AVP-stimulated cAMP production is enhanced by a protein kinase C inhibitor [8]. CUL-5 expression is downregulated in 82 % (41/50) of breast tumors compared with matched normal tissues [9]. Overexpression of Cul5 in T47D breast cancer cells decreases cell growth and mitogen activated protein kinase (MAPK) phosphorylation [10], and Cul5 overexpression downregulates early growth response 1 (EGR-1) protein expression and upregulates Fas-L mRNA expression [10]. The regulation of both MAPK and EGR-1 pathways by 17β-estradiol led to the examination of estrogen-dependent T47D cell growth, which showed that Cul5 inhibits basal and 17β-estradiol-dependent cell growth and MAPK phosphorylation [11].Resveratrol (trans-3,5,4′-trihydroxystilbene), which inhibits tumor initiation and promotion, is a natural component of the human diet, and its wide range of biological activities has been demonstrated in vivo and in vitro [12][13][14][15]. The antiproliferative effect of resveratrol is significantly enhanced by Cul5 overexpression in T47D cells [16].The expression of Cul5 is regulated by several stimuli and pathways (Fig. 1). Resveratrol upregulates Cul5 expression and decreases T47D cell growth, suggesting that the antiproliferative effect of resveratrol is mediated by Cul5 [16]. Cul5 is a flexible scaffold protein with a preferred distribution of conformational states [17], and NEDD8 modification (neddylation) alters the conformation of Cul5 and activates it [18]. Cul5(S730A) accelerates cellular proliferation and induces an...

U biquitin-mediated proteolysis by the 26S proteasome plays an important role in the elimination of short-lived proteins (1), including those involved in cell cycle progression, cellular signaling in response to environmental stress or extracellular ligands, morphogenesis, secretion, DNA repair, and organelle biogenesis (2, 3). The pathway consists of two key steps, namely, the covalent attachment of multiple ubiquitin molecules to a target protein and the degradation of the ubiquitinated protein by the 26S proteasome complex. Several components act in concert to attach ubiquitin to a target protein, including a ubiquitin-activating enzyme (E1), a ubiquitin-conjugating enzyme (E2), and a ubiquitin-protein isopeptide ligase (E3). E3 is directly responsible for substrate recognition. On the basis of structural similarity, E3 enzymes are classified into three families: the HECT (homologous to E6-AP COOH terminus) family, the U-box family, and the RING finger-containing protein family.The elongin B and C-Cul2 or Cul5-SOCS box protein (ECS) family belongs to the cullin RING ligase (CRL) superfamily (4). pVHL, the protein product of the von Hippel-Lindau (VHL) tumor suppressor gene, is a member of the ECS family. pVHL forms a complex with elongins B and C, Cul2, and the RING finger protein Rbx1 (5, 6). The CRL2 pVHL complex has ubiquitin ligase activity and targets the hypoxia-inducible factor ␣ (HIF-␣) family of transcription factors (HIF-1 to -3␣) for proteasomal degradation (7). At normal oxygen levels, proline residues in the LXXLAP sequence motif of HIF-␣ proteins are hydroxylated by three prolyl hydroxylases (PHD1 to -3), and an in-depth study revealed that PHD2 is a critical enzyme for the hydroxylation of HIF-1␣ (8, 9). Hydroxylated HIF-␣ is targeted by pVHL for polyubiquitination and proteasomal degradation (10-12). Under conditions of hypoxia (low oxygen level), HIF-␣ is not hydroxylated by PHDs and is therefore not recognized or targeted for degradation by pVHL. The unhydroxylated HIF-␣ dimerizes with constitutively expressed HIF-1␤, also known as an aryl hydrocarbon receptor nuclear translocator (ARNT), and translocates to the nucleus, where it induces the transcription of downstream target genes, including the genes coding for vascular endothelial growth factor A (VEGFA), solute carrier family 2 member 1 (SLC2A1; also known as GLUT1), and platelet-derived growth factor (PDGF) (13). Loss of functional pVHL protein prevents the O 2 -dependent degradation of HIF-␣, resulting in constitutive expression of HIF-dependent genes and VHL disease, which is characterized by a variety of lesions, including hemangioblastomas, renal clear cell carcinomas, pheochromocytomas, pancreatic islet cell tumors, endolymphatic sac tumors, and papillary cystadenomas of the broad ligament (females) and epididymis (males) (13).Studies showing that heterozygous pVHL ϩ/Ϫ mice are phenotypically normal and VHL Ϫ/Ϫ mice die at embryonic day 10.5 (E10.5) to E12.5 (14), together with the existence of many pVHLinteracting proteins (13), a...

Eph receptor tyrosine kinases are important for cancer development and progression as well as in cellular repulsive responses. We determined that SOCS box-containing protein SPSB4 destabilizes EphB2 cytoplasmic fragments. SPSB4 is a novel ubiquitin ligase regulating EphB2-dependent cell repulsive responses.

Uematsu et al. show that ASB7 ubiquitinates DDA3, which facilitates Kif2a-mediated depolymerization of microtubules (MTs) for proteasomal degradation. The presence of MTs prevents the ASB7–DDA3 interaction, suggesting a feedback loop to appropriately regulate MT polymerization and spindle dynamics.

Ubiquitin ligase von Hippel–Lindau tumor suppressor (pVHL) negatively regulates protein levels of hypoxia-inducible factor-α (HIF-α). Loss of pVHL causes HIF-α accumulation, which contributes to the pathogenesis of von Hippel-Lindau (VHL) disease. In contrast, v-Myb avian myeloblastosis viral oncogene homolog–like 2 (MYBL2; B-Myb), a transcription factor, prevents VHL pathogenesis by regulating gene expression of HIF-independent pathways. Both HIF-α and B-Myb are targets of pVHL-mediated polyubiquitination and proteasomal degradation. Here, we show that knockdown of HIF-2α induces downregulation of B-Myb in 786-O cells, which are deficient in pVHL, and this downregulation is prevented by proteasome inhibition. In the presence of pVHL and under hypoxia-like conditions, B-Myb and HIF-2α are both upregulated, and the upregulation of B-Myb requires expression of HIF-2α. We also show that HIF-2α and B-Myb interact in the nucleus, and this interaction is mediated by the central region of HIF-2α and the C-terminal region of B-Myb. These data indicate that oncogenic HIF-2α stabilizes B-Myb to suppress VHL pathogenesis.