The gene PSMD7 encodes the protein RPN8, also known as S12. RPN8, the human homologue of Mov-34, is a non-ATPase component of the 19S regulatory complex (Dubiel et al. 1995). Two 19S regulatory complexes bind to each end of the 20S proteasome to form the 26S proteasome.
The proteasome plays an important role in the cell, but its mechanism of action is not well understood. Although the proteasome was first isolated in 1979 (DeMartino and Goldberg 1979), the essential role the proteasome plays within a cell was not realized until 1990 (Fujiwara et al. 1990). The proteasome is the main component in the intracellular protein degradation pathway. This pathway was once thought to be a relatively unimportant part of the cell, but is now recognized to play an important role in regulating the lifetime of cellular proteins (Spataro et al. 1998).
Degradation is an important process within the cell (Glickman and Ciechanover 2002). The ultimate state of a cell is governed by the set of cellular proteins that exist within it. Cellular proteins are regulated both through creation (transcription/translation) and through degradation (ubiquitin–proteasome pathway). The degradation of proteins is just as important as the creation of them when studying regulatory processes within the cell. Since the understanding of the proteasome is immature, fundamental studies on assembly, activity, and control within the proteasome may well reveal important information about the complex and, in turn, the regulation of cellular proteins.
The proteasome plays an important role in regulating protein concentrations inside eukaryotic cells through the ubiquitin–proteasome pathway (Hochstrasser 1995). Ubiquitin is a small (76 amino acid) protein that may be specifically conjugated to lysine residues on a protein. This ubiquitin conjugation signals the proteasome to degrade the protein into short polypeptides. Proteins are ubiquitinated through a process that involves three classes of enzymes called E1, E2, and E3 (Weissman 2001). First an E1 enzyme activates the ubiquitin by modifying the C-terminal glycine residue. Simultaneously, an E3 enzyme binds to the target protein. Finally, an E2 enzyme transfers the activated ubiquitin to the E3/target complex, resulting in a ubiquitin tagged target protein that will be degraded by the 26S proteasome in eukaryotic cells. There are many (dozens) of each class of enzyme, and the combination of E2/E3 enzyme is thought to give specificity to the degradation (Glickman and Ciechanover 2002).
The eukaryotic proteasome consists of several components. The entire complex is known as the 26S proteasome. The 26S complex is composed of a single 20S proteasome (total mass 700 kDa, composed of 28 subunits) and two 19S multi-subunit complexes. The 20S proteasome has a cylindrical shape, and the 19S complexes form caps for the two ends of the 20S proteasome (Orlowski and Wilk 2000). The activity of the proteasome can be modified by the substitution of different subunits. In cultured cells, stimulation by gamma interferon changes the composition of the proteasome (Tanka 1994). A sub-complex known as the 11S regulator replaces the entire 19S sub-complex. Three of the 28 proteins in the 20S proteasome are replaced with completely different proteins. It is thought that these changes enhance the production of polypeptides suitable for MHC class I antigen presentation. This form of the proteasome is called the immunoproteasome (Van den Eynde and Morel 2001).
A heterogeneous population of proteasomes exists within cells. A structural study using transmission electron microscopy of individual 26S proteasomes isolated from both Drosophila melanogaster and Xenopus laevis revealed that the 19S cap has an extraordinary flexible attachment to the 20S proteasome core (Walz et al. 1998). This study also revealed that one portion of the 19S cap has at least four different conformations. The heterogeneous population implies that the proteasome is used as a general framework for protein degradation machinery.
The standard model of the proteasome’s protein degradation mechanism works like this: a protein binds to the 19S subunit at one end of the proteasome, the protein is unfolded and fed into the 20S proteasome, the protein is cleaved inside the 20S barrel, and the cleavage products pass out the other end of the proteasome. The model is based upon structural (Walz et al. 1998; Unno et al. 2002) and kinetic measurements (Stein et al. 1996; Akopian et al. 1997; Kisselev et al. 1998; Nussbaum et al. 1998; Peters et al. 2002) of the proteasome. However, there are other models consistent with this data. For instance, some (Nussbaum et al. 1998) suggest that degradation products may exit through the sides of the 20S proteasome. The structural data (Walz et al. 1998; Unno et al. 2002) makes it clear that the 20S proteasome is symmetric, with no preferred direction.
Polyubiquitinated proteins can be degraded by the proteasome. The 26S proteasome is a large (approximately 2000 kDa) complex composed of three different components. The core component, called the 20S proteasome is a barrel-shaped protease, with the active sites protected inside the barrel. The 20S core is regulated through several mechanisms. A complex termed the 19S binds to the top and bottom of the 20S core to form the 26S proteasome (Fukunaga et al. 2010; Isono et al. 2004). The 19S complex is thought to recognize, bind, and unfold poly ubiquitinated proteins. These proteins are then fed into the barrel of the 20S proteasome, where they are degraded into polypeptide chains. Other complexes, such as the PA28 complex, can also bind to the 20S core, and these are thought to regulate the proteasome in different ways (Bochtler et al. 1999; Wigley et al. 1999).
Several domains and motifs for RPN8 have been identified. These include the C-terminal KEKE motif, a putative site of protein–protein interaction (Realini et al. 1994), the Jun activation-domain binding protein (JAB) domain, originally described as a regulator of transcription, the MPR1p and PAD1p N-terminal (MPN) domain which, along with surrounding sequence, has been shown to be important for pairing with S13 (Rpn11/POH1) (Fu et al. 2001), and possibly even weak homology to a MAPKK activation loop motif (Seeger et al. 1998).
The RPN8 MPN domain has been crystallized and the crystal structure has been solved. This structure showed that the MPN domain contains a metalloprotease fold, as expected, but this metalloprotease fold is surprisingly unable to coordinate a metal ion (Sanches et al. 2007).
RPN8 is cytosolic and localized around the nucleus. This was determined using an antibody raised against amino acids 1–205 of recombinant RPN8 to localize the 26S proteasome in human JU77 mesothelioma cells (Seeger et al. 1998). Furthermore, phosphorylated RPN8 was found by using an anti-RPN8 antibody that reacted with both the phosphorylated and unphosphorylated forms from 26S proteasomes immunoprecipitated from human L-132 cells (Mason et al. 1998). These data identified phosphorylated RPN8 migrating above 50 kDa, whereas unphosphorylated RPN8 migrates at 40 kDa (Braun et al. 1999).
Roles in Human Disease
The proteasome has been implicated in multiple diseases. Defects in proteasome activity are associated with chronic human neurodegenerative diseases. Protein aggregates are found in some patients with Alzheimer’s (Ciechanover and Brundin 2003), Parkinson’s (Dauer and Przedborski 2003), and Huntington’s (Rubinsztein 2006) diseases. These aggregates are thought to form when the proteasome is unable to keep up with the amount of ubiquitinated protein being produced by the cell. The ubiquitin–proteasome pathway of protein degradation is also the target of cancer-related deregulation. Proteasome inhibitors form a new class of anticancer drugs and are under study for use in pancreatic, colon, lung, breast, prostate, and ovarian cancers (Voorhees and Orlowski 2006). One proteasome inhibitor, Velcade, has been approved by the FDA for treatment of multiple myeloma.
Certain mutations in the gene BRCA1 are known to lead to a predisposition to developing breast cancer. These cases account for less than 10% of all breast cancers. The protein product of BRCA1 exhibits E3 (ubiquitin protein ligase) activity. Furthermore, it was shown that cancer-predisposing mutations in BRCA1 also lead to a loss of this E3 activity (Ruffner et al. 2001). This clearly implicates the ubiquitin–proteasome pathway as a potential mechanism in the development of breast cancer. Other defects leading to the aberrant regulation of the ubiquitin–proteasome pathway (e.g., E1, E2, E3, or proteasome subunits) may be responsible for a greater percentage of breast carcinomas.
RPN8/PSMD7 is one of a cohort of 231 genes whose expression levels were significantly associated with clinical outcome in breast cancer patients (van’t Veer et al. 2002).
In cell line studies, RPN8 appears to be functionally related to transformed cells. RPN8 was posttranslationally modified in six normal breast epithelial cell lines, but not four transformed cell lines. Rpn8 was unique among proteasome subunits in this characteristic. Modified RPN8 has identical mass, but different isoelectric points then unmodified RPN8, see Fig. 3 in (Thompson et al. 2004). Modified RPN8 does not associate with the 26S proteasome, but does associate with a separate high molecular weight complex, see Fig. 5 in (Thompson et al. 2004). Finally, modified RPN8 is localized to the nuclei of normal cells, whereas unmodified RPN8 is only present in the cytoplasm.
The differential RPN8 protein expression and nuclear localization observed between the normal and cancer cell lines suggests that RPN8 has multiple functions associated with normal and cancer phenotypes. For instance, while mRNA levels are essentially the same between normal and cancer cell lines, the difference in levels of modified RPN8 is great. Thus, mechanisms that modulate posttranslational modifications of RPN8 in normal cells are disrupted in each of the cancer cell lines studied. Furthermore, no differences exist in the protein expression pattern of six other proteasome subunits between the ten cell lines, with the exception of S10a, lending significance to the differences observed with RPN8.
In summary, RPN8 exists in normal cell lines with at least two different posttranslational modifications, but in transformed cell lines with only one. The modified form of RPN8 does not associate with the 26S proteasome, but does associate with the immunoproteasome. These data suggest a differential nuclear function of modified and unmodified RPN8 in cancer cells.
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