The enzymes of the ubiquitylation pathway play a pivotal role in a number of cellular processes including the regulated and targeted proteasome-dependent degradation of substrate proteins. Three classes of enzymes are involved in the process of ubiquitylation; activating enzymes (E1s), conjugating enzymes (E2s) and protein ligases (E3s). Cullin-RING-Ligases (CRLs) are one of the largest classes of ubiquitin E3 ligases and the enzymes of the NEDDylation pathway play a pivotal role in the activation of these CRLs. Akin to ubiquitylation, the E1 activating enzyme (APP-BP1/UBA3 heterodimer) and the E2 conjugating enzymes (UBE2M or UBE2F) are involved in mammalian NEDDylation of the Cullin Ring Ligases (CRLs) (Meyer-Schaller et al., 2009; Huang et al., 2011; Morimoto et al., 2003). The human Cullin1-5 genes were first described by Kipreos et al. (1996). Cullin RING ligases (CRL) comprise the largest subfamily of ubiquitin ligases which are activated by Neddylation. CRLs are involved in cell cycle regulation, DNA replication and the DNA damage response (DDR). CRLs consist of several subunits including, a scaffold protein (cullin) and a Ring finger protein; either Rbx1 (Cul1-4) or Rbx2 (Cul5) that binds a ubiquitin E2 Ube2M or Ube2F respectively (Sarikas et al., 2011; Skowyra et al., 1997). The first CRL to be identified was named Skp1/Cullin or Cdc53/F-box (SCF) from Saccharomyces cerevisiae. Many CRL E3 ligases have additional linker proteins such as Elongin B/C associated with Cul2 and DDB1 associated with Cul4. The Elongin B/C-Cul2 or Cul5-SOCS box (ECS) family also belongs to the CRL superfamily (Kile et al, 2002). SCF and ECS ubiquitin ligases have structural similarities in that both contain Rbx1 or Rbx2 as a RING finger protein and Cul1, Cul2 or Cul5 as a scaffold protein (Kile et al., 2002; Kamura et al., 2004). The von Hippel-Lindau (VHL) complex is a ubiquitin ligase which targets the Hypoxia Inducible Factor alpha (HIF) family of transcription factors for proteasomal degradation. The complex comprises pVHL, the Cul2/Rbx1 subunit and the BC box protein Elongin B/C. Loss of functional pVHL protein prevents the oxygen dependent degradation of HIF1 resulting in constitutive expression of HIF1 dependent genes and consequently VHL disease (Okumura et al., 2012). Huang G, Kaufman A J, Ramanathan Y, Singh B. (2011) SCCRO (DCUN1D1) promotes nuclear translocation and assembly of the neddylation E3 complex, J Biol Chem 286, 10297-10304.
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