E3 Ligase

Sachdev S, Bruhn L, Sieber H, et al (2001) PIASy, a nuclear matrix-associated SUMO E3 ligase, represses LEF1 activity by sequestration into nuclear bodies. Genes Dev 15 3088-3103... 

Sharrocks AD (2006) PIAS proteins and transcriptional regulation - more than just SUMO E3 ligases Genes Dev 20 754-758... 

The SCF, a ubiquitin ligase complex, consists of the piimaiy subunits Skpl, Cullin and Rbx/Rocl. While the Rbx/Cul components form the E3 ligase catalytic core... 

The third type of E3 ligases is represented by the polycomb protein 2 (Pc2), which was reported to enhance sumoylation of the substrate CtBP. N- and C-terminal domains in Pc2 that have been implicated in CtBP sumoylation do not resemble known E3 ligases. Like RanBP2, Pc2 expression is restricted to higher eukaryotes. 

In concordance with the central role of ubiquitin modification in multiple cellular functions perturbations of this system are associated with a variety of diseases. Defects in the control of cell cycle regulators by the ubiquitin proteasome system are connected to cancer progression and many E3 ligases were originally identified as oncogenes. 

As the ubiquitin proteasome pathway is a main route for protein clearance it is not surprising that in protein-opathies (disease caused by aggregate prone proteins) like sporadic Parkinson- or Huntington disease proteasome activity is reduced. Autosomal recessive loss of function of the E3 ligase parkin is the molecular base for one of the most common forms of familial Parkinson disease. 

We utilized this technique to analyze Scrapper gene-dehcient (SCR-KO) mice.21 SCRAPPER, a protein that we have recently reported, is localized at synapses in neurons. It is a ubiquitin E3 ligase that is involved in the decomposition of RIM (Rab3-interacting molecule) 1, an important regulator of synaptic plasticity, and thus regulates synaptic transmissions.22... 

The first clue to a relationship between Cbl and ubiquitylation came from Stanley and co-workers who demonstrated that c-Cbl was mono-ubiquitylated in response to CSE-1 (colony stimulating fiictor-1) [193]. Yarden and colleagues demonstrated an association between Cbl recruitment to receptors and their ubiquitylation and down-regulation. C-Cbl E3 ligase activity was subsequently demonstrated by several groups [31, 184, 186). 

Y., Osaka, F., Omata, M., and Tanaka, K. NEDD8 recruits E2-ubiquitin to SCF E3 ligase. EMBO J. 

Murata, S., Minami, Y., Minami, M., Chiba, T., and Tanaka, K. CHIP is a chaperone-dependent E3 ligase that ubiquitylates unfolded protein. EMBO Rep. 2001, 2, 1133-8. 

The essential components of the SCF ubiquitin E3 ligase include Skpl, Cul-1/ Cdc53, one of many F-box proteins, and the RINC-H2-finger protein Rod (Rbxl or Hrtl) (Figure 6.2). Although initial studies did not reveal the presence of a fourth component of the SCF complex [14, 15], later work showed that a RINC-H2-finger protein, Rod, is an essential subunit of the SCF complex [3]. The SCF complex thus contains three invariable components (Rod, Cull, and Skpl) which provide a core structure to which one of the many substrate-specific subunits (F-box proteins) binds. The Rocl-Cull-Skpl core also independently interacts with the ubiquitin E2-conjugating enzyme to couple ubiquitin transfer to the substrates [3]. One of the E-box proteins binds directly to a specific substrate and such interaction facilitates the polyubiquitination of the substrate by ubiquitin... 

Fig. 6.2. The structures of SCF, Cul2-Elongin B-C, and Cul3-BTB/POZ ubiquitin E3 ligase complexes with the bound substrates and E2 enzymes.

Skpl serves as an adaptor protein that provides a molecular link between Cull/ Rod and the F-box proteins [4, 5]. The Skpl protein contains two separate protein-interaction domains that are conserved among its family members between species [21]. The N-terminal region of Skpl (- l-70 a.a.) interacts with Cull while the C-terminal half (100-163 a.a.) binds the F-box proteins [21]. The use of Skpl as an adaptor to link the core ubiquitin E3 ligase components of Cull/Rocl with numerous and diverse substrate-targeting subunits, the F-box proteins, represents a strategy to specifically target many proteins for ubiquitination... 

F-box proteins serve as the substrate-targeting subunit of the SCF ubiquitin E3 ligase [5]. They are structurally diverse but they all contain a relatively conserved signature motif of about 45-50 amino acids [5]. This motif, the F-box, was initially... 

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