1. Introduction
1.3 NEDD8
1.3.1 Substrates and functions of NEDD8
A B
Figure 4. Crystal structure of NEDD8 (A) and overlay with ubiquitin and SUMO1 (B)
(A) The globular NEDD8 structure reveals a β‐grasp fold with four antiparallel β‐sheets and an α‐helix on top of them, being characteristic for ubiquitin and all UBLs (protein data base 1NDD; modeled using Pymol). The C‐terminal double glycine motif is marked in green.
(B) Overlay of ubiquitin (blue), SUMO‐1 (green) and NEDD8 (red). Ubiquitin and the UBLs SUMO‐1 and NEDD8 exhibit a highly similar structure characterized by the β‐grasp fold (Welchman et al., 2005).
Investigation of NEDD8 expression in different tissues using northern blot and immuno‐
cytochemical analysis revealed NEDD8 mRNA is enriched in brain and skeletal muscle, and that NEDD8 protein is predominantly found in the nucleus whereas ubiquitin is equally distributed in the cell (Kamitani et al., 1997).
In mice, knockout of the catalytic subunit of the NEDD8 E1 enzyme, UBA3, was shown to be lethal in utero at the periimplantation state. The reason for this phenomenon lies in the involvement of the NEDD8 pathway in cell cycle progression and morphogenesis, underlining its indispensability for early development (Tateishi et al., 2001). In S. cerevisiae, however, depletion of the NEDD8 homologue Rub1, as well as the respective E1 or the E2 enzymes does not affect normal cell growth (Liakopoulos et al., 1998).
1.3.1 Substrates and functions of NEDD8
1.3.1.1 Cullins
In 1998, Cullin4a was described as the first substrate of the NEDD8‐conjugation system (Osaka et al., 1998). In the meantime, it turned out that modification of all canonical cullins with NEDD8 is crucial to execute their function (Hori et al., 1999; Ohh et al., 2002). Cullins are components of multi‐protein complexes, the cullin‐RING ligases, which play important roles in cell growth, development, signal transduction, transcriptional control, genomic integrity and tumor suppression (see also 1.1.1). In mammals, there are six canonical cullins (Cullin1, Cullin2, Cullin3, Cullin4a, Cullin4b, Cullin5) and three atypical cullins (APC2, Cullin7 and PARC) known
N N
C C
which together build more than 500 distinct multi‐subunit complexes (Petroski and Deshaies, 2005a; Skaar et al., 2007; Zachariae et al., 1998).
Cullin‐RING ligases usually consist of cullins serving as scaffold proteins, substrate adaptors, which recognize and bind the respective substrate, and a RING domain protein with the catalytic activity to ubiquitinate the substrate. NEDDylation of the cullin subunit at a conserved lysine residue leads to a conformational change which in turn recruits the ubiquitin‐loaded E2 enzyme (Duda et al., 2008; Kawakami et al., 2001). In addition, NEDDylation seems to promote dimerization of cullin‐RING ligases through their substrate recognition subunit (Wimuttisuk and Singer, 2007). Recently, it was suggested that rotation of the RING domain of the E3 also plays a crucial role in the activation of the cullin‐RING complex (Calabrese et al., 2011). Both the NEDD8‐ and the ubiquitin E3 ligase activity of the complex are carried out by either RBX1 or RBX2 (Petroski and Deshaies, 2005a). Moreover, Dcn1 in yeast and DCNL proteins in humans serve as a scaffold‐type E3 ligases which interact with the respective cullin and the NEDD8 E2 at the same time thereby enhancing NEDDylation of cullins and thus, the ubiquitination activity of cullin‐RING complexes (Kurz et al., 2008; Monda et al., 2013; Yang et al., 2007).
As already mentioned, cullin complexes share a general composition. Dependent on the cullin, however, several different substrate adaptors can be used. So called SCF complexes consist of Cullin1, RBX1, the adaptor protein Skp1 and an F‐box protein which specifically interacts with the substrate to be degraded (Lyapina et al., 1998). One of the substrates for the SCF complex with Skp2 as F‐box protein is p21, an important regulator of cell proliferation and differentiation. Phosphorylation of p21 by Cdk2‐Cyclin E enhances its recognition and ubiquitination by SCFSkp2, leading to its rapid degradation (Bornstein et al., 2003). Additionally, not only cell cycle inhibitors but also cell cycle activators like Cyclin E are substrates for SCFSkp2 (Nakayama et al., 2000). In contrast to Cullin1, Cullin2 and Cullin5 assemble with elongin B/C as adaptor and a SOCS‐box containing protein as substrate receptor. A well‐studied substrate of the Cullin2‐RING ligase is HIFα, an important player in oxygen metabolism. A complex formed by HIFα and HIFβ under hypoxic conditions controls the expression of several genes like VEGF or erythropoietin (Maxwell, 2003). Oxygen‐dependent hydroxylation of HIFα leads to its recognition by the tumor suppressor protein pVHL, a substrate receptor of the Cullin2‐complex, and its subsequent ubiquitination and degradation (Ivan et al., 2001; Jaakkola et al., 2001;
Petroski and Deshaies, 2005a; Yu et al., 2001).
Deactivation of cullin‐RING complexes is achieved through removal of NEDD8 by the CSN5 subunit of the COP9 signalosome which belongs to the family of JAMM metalloproteases (Cope et al., 2002; Lyapina et al., 2001). Another mechanism to silence cullin‐RING complexes is the binding of Cand1 (Cullin‐Associated and NEDDylation‐Dissociated 1) which inhibits NEDDylation and the assembly of the whole complex by specifically binding to the region of the
that Cand1 also serves as an important exchange factor for cullin‐RING ligase adaptors (Pierce et al., 2013).
1.3.1.2 NEDD8 and transcriptional regulation
In the last decade, several new substrates of NEDD8 were described, giving insights into the role of NEDD8 in various cellular functions. Interestingly, a majority of these substrates is involved in transcriptional regulation.
The tumor suppressor protein p53, which plays a major role in the regulation of cell cycle arrest and apoptosis, is not only a substrate for ubiquitin but also for ubiquitin‐like proteins such as SUMO or NEDD8 (Kruse and Gu, 2009; Rodriguez et al., 1999; Scheffner et al., 1993; Xirodimas et al., 2004). NEDDylation of p53 by the RING ligase Hdm2 was shown to inhibit its transcriptional activity. In the case of p53, Hdm2 displays a dual specificity since it had also been described as an E3 ligase for the ubiquitination of p53 (Honda et al., 1997; Xirodimas et al., 2004).
Modification of p53 with NEDD8 is also promoted by FBXO11 and specifically inhibited by the histone acetyltransferase Tip60 (Abida et al., 2007; Dohmesen et al., 2008). Interestingly, C‐
terminal fusions of p53 with ubiquitin and NEDD8 to mimic its modification with these proteins showed that p53‐ubiquitin is rather found in the cytoplasm, whereas fusions of p53 with NEDD8 localize in the nucleus (Carter and Vousden, 2008).
Another member of the p53 family, TAp73, also serves as a substrate for Hdm2‐dependent NEDDylation. Modification of TAp73 with NEDD8 inhibits its transcriptional activity which can in part be explained by its localization to the cytoplasm (Watson et al., 2006).
In 2008, some ribosomal proteins were found to be modified with NEDD8 causing enhanced stability (Xirodimas et al., 2008). Further investigation of the ribosomal protein L11 revealed that its NEDDylation leads to a localization to the nucleolus (Sundqvist et al., 2009). Upon nucleolar stress, L11 is deNEDDylated and recruited to promoters of p53 regulated genes where it interacts with several co‐activators. In addition, binding of L11 to Hdm2 at these promoter sites inhibits interaction of p53 with Hdm2, thereby promoting transactivation of p53 target genes. Interestingly, ribosome biogenesis is not affected under deNEDDylation conditions in spite of reduced L11 levels (Mahata et al., 2011).
NEDDylation also seems to play an important role in the regulation of NFκB activity. On the one hand, suppression of the transcriptional activity of NFκB by BCA3 is dependent on its modification with NEDD8 (Gao et al., 2006). On the other hand, TRIM40‐catalyzed NEDDylation of IKKγ, an inhibitor of NFκB signaling, enhances the repression of NFκB (Noguchi et al., 2011).
Another protein which is regulated by NEDDylation is AICD, the intracellular domain of the amyloid precursor protein (APP). APP is found in plaques that accumulate in brains of Alzheimer patients. Cleaving of APP by secretases leads to the formation of AICD amongst others, whose role in Alzheimer development and progression is only poorly understood. Modification of AICD
with NEDD8 prevents the interaction with its co‐activator Fe65 and the histone acetyltransferase Tip60, resulting in an inhibition of the transactivator function for genes involved in e.g. cell growth and motility (Lee et al., 2008; Muller et al., 2008).
1.3.1.3 Further substrates and functions of NEDD8
Modification with NEDD8 does not only play roles in transcriptional regulation but also in the regulation of protein stability. The ribosomal protein L11 and the E3 ligase Hdm2 as well as PINK1, a protein involved in Parkinson´s disease, show an enhanced stability upon modification with NEDD8 (Choo et al., 2012; Xirodimas et al., 2004; Xirodimas et al., 2008).
In addition, NEDDylation regulates the stability of distinct receptors. Ubiquitination of EGFR is mediated by the RING‐ligase c‐Cbl, leading to its internalization and lysosomal degradation.
C‐Cbl is also capable of NEDDylating EGFR and therefore displays a dual specificity as Hdm2 does for modification of p53 (Oved et al., 2006; Xirodimas et al., 2004). Modification of EGFR with NEDD8 leads to an increased turnover rate caused by intensified ubiquitination (Oved et al., 2006). In the case of steroid hormone receptors, NEDD8 was even found to be required for their ubiquitination and degradation. Therefore, inactivation of the NEDD8 pathway might be involved in the development of steroid hormone dependent tumors (Fan et al., 2003; Fan et al., 2002).
Parkin, a RING‐type E3 ligase which is frequently mutated in patients suffering from Parkinson´s disease, reveals an enhanced activity upon NEDDylation. Substrates of parkin take part in diverse cellular functions like transcription, neurotransmission, synaptic function or cell cycle control (Choo et al., 2012; Walden and Martinez‐Torres, 2012).
As a component of the Cullin2/elongin B/C complex, pVHL is involved in the regulation of oxygen‐dependent ubiquitination of HIFα (see 1.3.1.1). Interestingly, pVHL is also required for fibronectin matrix assembly, independent of Cullin2 (Stickle et al., 2004). Toggling the binding of pVHL to the cullin complex is achieved by its modification with NEDD8: NEDDylation leads to its interaction with fibronectin whereas its association with Cullin2 is inhibited (Russell and Ohh, 2008).
IAPs (Inhibitor of Apoptosis) are often found to be overexpressed in cancer, thereby contributing to cell proliferation and survival. IAPs are RING ubiquitin E3 ligases that negatively regulate caspase activity and additionally have an influence on cellular survival functions. In Drosophila and humans, effector caspases were identified to act as substrates for NEDDylation by IAPs leading to their inactivation (Broemer et al., 2010). However, it was also supposed that IAPs themselves, rather than caspases, might be substrates for NEDD8 (Nagano et al., 2012).
Very recently, both reduced levels of Ubc12 and inhibition of APPBP1/UBA3 were discovered to impair T‐cell proliferation and cytokine production. Thereupon, NEDDylation of Shc, an adapter
protein between the antigen receptor of T‐cells and the Erk‐pathway, was identified as an important event in T‐cell receptor signaling (Jin et al., 2013).
In proteomic analyses, many further potential substrates for NEDD8 were identified which are mainly involved in mRNA splicing, DNA replication and repair, chromatin remodeling and proteasomal degradation (Jones et al., 2008; Xirodimas et al., 2008).
Finally, one well‐studied interaction partner of NEDD8 which targets NEDD8 and its conjugates for proteasomal degradation is NUB1 (NEDD8 Ultimate Buster1) (Kamitani et al., 2001). By interacting with the S5a subunit of the proteasome, NUB1 not only delivers NEDD8 but also the UBL FAT10 for degradation (Hipp et al., 2004; Tanji et al., 2005). Interaction of NEDD8 with NUB1 additionally results in inhibition of NEDDylation and enhances ubiquitination of p53, leading to its cytoplasmic localization (Liu and Xirodimas, 2010).