1. Introduction
1.1 Ubiquitin‐proteasome system
1. Introduction
Protein function, localization and stability are regulated by posttranslational modifications which involve for example small chemical groups such as acetyl‐, methyl‐ or phosphate moieties, or larger groups serving as membrane anchors such as palmitoyl‐ or myristoyl groups (Han and Martinage, 1992; Magee and Courtneidge, 1985). With the discovery of the protein ubiquitin and its ability to serve as posttranslational modification in the 1980s, a new chapter of the functions of these modifications has begun. Ubiquitin was found to target proteins for proteasomal degradation thereby offering the cell a second major pathway to regulate protein levels in addition to lysosomal degradation. In the meantime, several proteins with high structural similarity to ubiquitin were identified most of which act as posttranslational modifiers controlling various cellular functions (Hershko and Ciechanover, 1998; Kerscher et al., 2006).
1.1 Ubiquitin‐proteasome system
Ubiquitin is a small protein of 76 aa and 8.6 kDa which is highly conserved among eukaryotic organisms (Hershko and Ciechanover, 1998). Structurally, ubiquitin is characterized by a globular domain with a β‐grasp fold and a flexible C terminus (Figures 1 and 4B) (Vijay‐Kumar et al., 1987).
Figure 1. Crystal structure of ubiquitin revealing the characteristic βgrasp fold
Ribbon representation of human ubiquitin (protein data base 1UBQ; modeled with Pymol). The structure of ubiquitin is characterized by four antiparallel β‐sheets grasping an α‐helix. The C terminus of ubiquitin protrudes from the globular domain (Vijay‐Kumar et al., 1987).
C
N
1.1.1 Ubiquitin‐conjugation cascade
In humans, expression of ubiquitin from one of the four different genes first leads to the formation of an inactive precursor protein (Baker and Board, 1987; Lund et al., 1985; Wiborg et al., 1985). Being rapidly processed by ubiquitin‐specific proteases, the functionally important C‐
terminal double glycine motif of ubiquitin is exposed (Wilkinson, 1997).
In an enzymatic cascade, ubiquitin is covalently attached to other proteins via its C terminus (“ubiquitination”). Ubiquitination occurs through several consecutive steps catalyzed by three (or four) different classes of enzymes: ubiquitin‐activating enzymes (E1), ubiquitin‐conjugating enzymes (E2) and ubiquitin ligases (E3) (Figure 2). In some cases, an E4 enzyme may be required for the formation of ubiquitin chains (Hershko and Ciechanover, 1998; Hoppe, 2005).
In a first step, the carboxyl group of the C‐terminal glycine of ubiquitin forms a high energy thioester linkage with an active site cysteine residue of one of two E1 enzymes, UBA1 or UBA6.
This step is ATP‐dependent and involves the formation of a ubiquitin adenylate intermediate (Jin et al., 2007; Pelzer et al., 2007; Pickart, 2001). The E2 is then able to accept activated ubiquitin from its cognate E1, forming a thioester linkage. In the last step, which in most cases requires the presence of an E3, ubiquitin is transferred to the lysine residue of a substrate protein by the formation of an isopeptide bond (Pickart, 2001) (Figure 2). An asparagine residue in the conserved HPNI/V motif next to the catalytic cysteine of the E2 enzyme plays a crucial role for accomplishing this transfer, as it might be important for oxyanion intermediate stabilization during lysine attack (Wu et al., 2003b).
Structural and functional analyses indicate that E3s consist of at least two functional domains:
one domain (i.e. RING/RING‐like or HECT) is interacting with the cognate E2 while the other mediates the specific interaction with the substrate, thereby conferring substrate specificity on the whole conjugation cascade. Depending on their mode of action, E3 ligases can be divided into two major classes: HECT and RING/RING‐like ligases (Metzger et al., 2012). HECT ligases contain a HECT (Homologous to E6AP Carboxyl Terminus) domain which consists of an N‐terminal and a C‐terminal lobe. The N‐terminal lobe is necessary for the interaction with the E2 enzyme, whereas the C‐terminal lobe contains a catalytic cysteine which forms a thioester linkage with ubiquitin before transferring it to the substrate (Huang et al., 1999).
Figure 2. Ubiquitinconjugation cascade
Ubiquitin is first activated by the E1 and then transferred to one of a number of E2 enzymes. RING E3 ligases facilitate the conjugation of ubiquitin to the substrate by acting as adaptors between E2s and substrates. In contrast, HECT E3 ligases form thioester bonds with ubiquitin and subsequently transfer it to the substrate. By repeating these steps, the substrate can be modified with several ubiquitin moieties (modified from (Di Fiore et al., 2003)).
Being encoded by more than 600 genes in mammals, RING ligases represent the largest family of E3 ligases (Li et al., 2008). This type of ligase is characterized by a RING (Really Interesting New Gene) domain containing the consensus motif C‐X2‐C‐X(9‐39)‐C‐X(1‐3)‐H‐X(2‐3)‐C/H‐X2‐C‐X(4‐
48)‐C‐X2–C (X stands for any aa), which is stabilized by two zinc ions (Lovering et al., 1993). By bringing the E2 enzyme and the substrate into close proximity, RING ligases allow a direct transfer of ubiquitin from the E2 to the substrate.
The family of RING ligases comprises ligases that function as monomer, homo‐ or heterodimer or as large multi‐subunit complexes. Most of the known RING ligases possess intrinsic E3 ligase activity, but there are also RING domain containing proteins that do not show activity by themselves, e.g. Bard1, Bmi1 and HdmX. Heterodimerization of these E3 ligases with another RING ligase (Brca1, Ring1B and Hdm2, respectively), which involves the interaction of the RING domains, leads to the stimulation of ligase activity of the latter (Hashizume et al., 2001; Linares et al., 2003; Wang et al., 2004). Moreover, other RING E3 ligases like RNF4 or TRAF6 form
ATP AMP+PPi
homodimers to execute their function (Liew et al., 2010; Yin et al., 2009). Recent data about the mechanism of ubiquitin transfer by RNF4 sheds light on why dimerization is necessary for RING ubiquitin ligase activity. Dimerization of the RING domains facilitates the interaction with both components of the ubiquitin‐charged E2 at the same time: one monomer binds to ubiquitin and the other one to the E2. By altering the conformation of the active site of the E2, the E2‐ubiquitin thioester bond is then activated and ubiquitin can be transferred to the substrate (Plechanovova et al., 2011; Plechanovova et al., 2012).
Being composed of multiple subunits, the APC/C complex and cullin‐RING ligases such as SCF form an additional type of RING E3 ligases. APC/C, a complex consisting of at least twelve subunits including the RING domain containing protein APC11, plays a major role in cell cycle regulation, especially in mitotic progression. Dependent on the substrate adaptor bound, a specific set of substrates is recognized, ubiquitinated and degraded by the proteasome (reviewed in (Peters, 2006)). The SCF complex belongs to the largest family of ubiquitin E3 ligases known, the cullin‐RING ligases (see chapter 1.3.1.1). It consists of Cullin1 as scaffold protein, the RING ligase RBX1, Skp1 as adaptor protein and an exchangeable F‐box protein recognizing a specific substrate. Not only cell cycle inhibitors like p21 or p27, but also oncogenic proteins such as Cyclin E or c‐Myc turned out to be substrates for SCF complexes, underlining its important role in controlling the cell cycle (summarized in (Kitagawa et al., 2009)).
1.1.2 Modes of ubiquitination
In most cases, ubiquitination of substrates occurs via isopeptide bond formation between the C‐
terminal glycine residue of ubiquitin and the ‐amino group of a lysine residue in the substrate.
Furthermore, there are few publications showing that serine, threonine and cysteine residues as well as the N‐terminal amino group of some proteins are used for the attachment of ubiquitin (Cadwell and Coscoy, 2005; Ciechanover and Ben‐Saadon, 2004; Shimizu et al., 2010; Wang et al., 2007b).
In addition to mono‐ or multiubiquitination where single ubiquitin moieties are conjugated to one or several distinct residues in the substrate protein, respectively, ubiquitin is also capable of forming “chains” (polyubiquitination) (Figure 3). Monoubiquitination was found to be involved in DNA damage response and endocytosis (reviewed in (Hicke, 2001)). For instance, Rad6‐ and Rad18‐dependent ubiquitination of the sliding clamp PCNA leads to the recruitment of translesion synthesis polymerases that are crucial for inducing the DNA damage tolerance pathway (Hoege et al., 2002; Kannouche et al., 2004). Moreover, multiubiquitination plays a particular role in the internalization and lysosomal degradation of plasma membrane receptors, e.g. receptor tyrosine kinases (Haglund et al., 2003).
Each of the seven internal lysine residues of ubiquitin (K6, K11, K27, K29, K33, K48 and K63) can be used for isopeptide bond formation with the C‐terminal carboxyl group of another ubiquitin moiety and hence, for the formation of polyubiquitin chains (Figure 3). Best characterized and most abundant are K11‐, K48‐ and K63‐linked chains (Ye and Rape, 2009). As an example, the E3 ligase complex APC/C triggers degradation of its mitotic substrates via formation of K11‐linked ubiquitin chains (Jin et al., 2008). Furthermore, the ubiquitin‐
conjugating enzyme Ubc6 which is involved in ER‐associated degradation (ERAD), a protein quality control system mainly localized in the ER membrane, is ubiquitinated and degraded via K11‐linkage of ubiquitin in yeast (Xu et al., 2009). Nonetheless, K48‐linked chains are a more common signal to target proteins for proteasomal degradation (Chau et al., 1989). For a long time it was believed that a minimal chain length of four ubiquitin moieties linked via K48 is absolutely required for the recognition by the proteasome (Thrower et al., 2000). This hypothesis is contradicted by recent studies indicating that monoubiquitinated or multiubiquitinated proteins like PAX3 or p105, respectively, can also be recognized and degraded by the proteasome (Boutet et al., 2007; Kravtsova‐Ivantsiv et al., 2009; Shabek et al., 2012). In contrast to K11‐ and K48‐chains, K63‐linked chains are relevant for a variety of non‐
proteolytic cellular processes like endocytosis, DNA repair or activation of kinases (Deng et al., 2000; Duncan et al., 2006; Spence et al., 1995). Mixed and forked chains, as well as chains formed on internal lysine residues other than K11, K48 and K63 of ubiquitin are still under intense investigation (Figure 3).
Figure 3. Modes of ubiquitination
Ubiquitin is primarily attached to the ‐amino group of one or more lysine residues of a substrate (mono‐ and multiubiquitination, respectively) or, in rare cases, to the N terminus of a substrate (not shown). It contains seven internal lysine residues all of which can serve as an acceptor for another ubiquitin moiety, thereby forming ubiquitin chains (polyubiquitination). K11‐, K48‐, K63‐linked and linear chains, which are linked via C‐ and N terminus of two ubiquitin moieties, function in proteasomal degradation, DNA‐repair or intracellular signaling, whereas the function
1.1.3 Ubiquitin recycling and the proteasome
Processing of ubiquitin precursor proteins as well as recycling of ubiquitin is carried out by deubiquitinating enzymes (DUBs) which mainly exhibit cysteine protease activity. The two major classes of DUBs are UCHs (Ubiquitin COOH‐terminal Hydrolases) that preferentially cleave ubiquitin from substrates and USPs (Ubiquitin‐Specific Proteases) that additionally hydrolyze isopeptide bonds between two ubiquitin moieties. In addition, OUTs (otubain proteases), JAMM metalloproteases and MJDs were found to function as DUBs (summarized in (Sorokin et al., 2009)).
Some DUBs are associated with or even part of the 26S proteasome which degrades 80‐90 % of intracellular proteins (Rock et al., 1994). The 26S proteasome consists of the 20S core particle and two 19S regulatory particles. The core proteasome is composed of 14 α‐ and 14 β‐subunits forming four heptameric rings which build a channel in whose inner part the target protein is hydrolyzed. Three of the β‐subunits possess proteolytic activities ensuring efficient cleavage of the target: subunit β1 exhibits caspase‐like activity, β2 has trypsin‐like and β5 chymotrypsin‐
like activity. The regulatory particles contain subunits that fulfill three important functions:
interaction with the ubiquitinated substrate, cleaving ubiquitin from the substrate (by isopeptidases) and unfolding the target protein (by ATPases) (reviewed in (Murata et al., 2009;
Sorokin et al., 2009)).