1
|
Thomas T, Salcedo-Tacuma D, Smith DM. Structure, Function, and Allosteric Regulation of the 20S Proteasome by the 11S/PA28 Family of Proteasome Activators. Biomolecules 2023; 13:1326. [PMID: 37759726 PMCID: PMC10526260 DOI: 10.3390/biom13091326] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/17/2023] [Revised: 08/24/2023] [Accepted: 08/24/2023] [Indexed: 09/29/2023] Open
Abstract
The proteasome, a complex multi-catalytic protease machinery, orchestrates the protein degradation essential for maintaining cellular homeostasis, and its dysregulation also underlies many different types of diseases. Its function is regulated by many different mechanisms that encompass various factors such as proteasome activators (PAs), adaptor proteins, and post-translational modifications. This review highlights the unique characteristics of proteasomal regulation through the lens of a distinct family of regulators, the 11S, REGs, or PA26/PA28. This ATP-independent family, spanning from amoebas to mammals, exhibits a common architectural structure; yet, their cellular biology and criteria for protein degradation remain mostly elusive. We delve into their evolution and cellular biology, and contrast their structure and function comprehensively, emphasizing the unanswered questions regarding their regulatory mechanisms and broader roles in proteostasis. A deeper understanding of these processes will illuminate the roles of this regulatory family in biology and disease, thus contributing to the advancement of therapeutic strategies.
Collapse
Affiliation(s)
- Taylor Thomas
- Department of Biochemistry and Molecular Medicine, School of Medicine, West Virginia University, 64 Medical Center Drive, Morgantown, WV 26506, USA
| | - David Salcedo-Tacuma
- Department of Biochemistry and Molecular Medicine, School of Medicine, West Virginia University, 64 Medical Center Drive, Morgantown, WV 26506, USA
| | - David M. Smith
- Department of Biochemistry and Molecular Medicine, School of Medicine, West Virginia University, 64 Medical Center Drive, Morgantown, WV 26506, USA
- Department of Neuroscience, Rockefeller Neuroscience Institute, West Virginia University, Morgantown, WV 26506, USA
| |
Collapse
|
2
|
Thomas TA, Smith DM. Proteasome activator 28γ (PA28γ) allosterically activates trypsin-like proteolysis by binding to the α-ring of the 20S proteasome. J Biol Chem 2022; 298:102140. [PMID: 35714770 PMCID: PMC9287138 DOI: 10.1016/j.jbc.2022.102140] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/07/2022] [Revised: 06/06/2022] [Accepted: 06/08/2022] [Indexed: 11/21/2022] Open
Abstract
Proteasome activator 28γ (PA28γ/REGγ) is a member of the 11S family of proteasomal regulators that is constitutively expressed in the nucleus and implicated in various diseases, including certain cancers and systemic lupus erythematosus. Despite years of investigation, how PA28γ functions to stimulate proteasomal protein degradation remains unclear. Alternative hypotheses have been proposed for the molecular mechanism of PA28γ, including the following: (1) substrate selection, (2) allosteric upregulation of the trypsin-like (T-L) site, (3) allosteric inhibition of the chymotrypsin-like (CT-L) and caspase-like (C-L) sites, (4) conversion of the CT-L or C-L sites to new T-L sites, and (5) gate opening alone or in combination with a previous hypothesis. Here, by mechanistically decoupling gating effects from active site effects, we unambiguously demonstrate that WT PA28γ allosterically activates the T-L site. We show PA28γ binding increases the Kcat/Km by 13-fold for T-L peptide substrates while having little-to-no effect on hydrolysis kinetics for CT-L or C-L substrates. Furthermore, mutagenesis and domain swaps of PA28γ reveal that it does not select for T-L peptide substrates through either the substrate entry pore or the distal intrinsically disordered region. We also show that a previously reported point mutation can functionally switch PA28γ from a T-L activating to a gate-opening activator in a mutually exclusive fashion. Finally, using cryogenic electron microscopy, we visualized the PA28γ-proteasome complex at 4.3 Å and confirmed its expected quaternary structure. The results of this study provide unambiguous evidence that PA28γ can function by binding the 20S proteasome to allosterically activate the T-L proteolytic site.
Collapse
Affiliation(s)
- Taylor A Thomas
- Department of Biochemistry, Robert C. Byrd Health Sciences Center, West Virginia University, Morgantown, West Virginia, USA
| | - David M Smith
- Department of Biochemistry, Robert C. Byrd Health Sciences Center, West Virginia University, Morgantown, West Virginia, USA; Department of Neuroscience, Robert C. Byrd Health Sciences Center, West Virginia University, Morgantown, West Virginia, USA; WVU Rockefeller Neuroscience Institute, Morgantown, West Virginia, USA; WVU Cancer Institute, Morgantown, West Virginia, USA.
| |
Collapse
|
3
|
Cascio P. PA28γ: New Insights on an Ancient Proteasome Activator. Biomolecules 2021; 11:228. [PMID: 33562807 PMCID: PMC7915322 DOI: 10.3390/biom11020228] [Citation(s) in RCA: 31] [Impact Index Per Article: 10.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/02/2021] [Revised: 02/02/2021] [Accepted: 02/03/2021] [Indexed: 02/06/2023] Open
Abstract
PA28 (also known as 11S, REG or PSME) is a family of proteasome regulators whose members are widely present in many of the eukaryotic supergroups. In jawed vertebrates they are represented by three paralogs, PA28α, PA28β, and PA28γ, which assemble as heptameric hetero (PA28αβ) or homo (PA28γ) rings on one or both extremities of the 20S proteasome cylindrical structure. While they share high sequence and structural similarities, the three isoforms significantly differ in terms of their biochemical and biological properties. In fact, PA28α and PA28β seem to have appeared more recently and to have evolved very rapidly to perform new functions that are specifically aimed at optimizing the process of MHC class I antigen presentation. In line with this, PA28αβ favors release of peptide products by proteasomes and is particularly suited to support adaptive immune responses without, however, affecting hydrolysis rates of protein substrates. On the contrary, PA28γ seems to be a slow-evolving gene that is most similar to the common ancestor of the PA28 activators family, and very likely retains its original functions. Notably, PA28γ has a prevalent nuclear localization and is involved in the regulation of several essential cellular processes including cell growth and proliferation, apoptosis, chromatin structure and organization, and response to DNA damage. In striking contrast with the activity of PA28αβ, most of these diverse biological functions of PA28γ seem to depend on its ability to markedly enhance degradation rates of regulatory protein by 20S proteasome. The present review will focus on the molecular mechanisms and biochemical properties of PA28γ, which are likely to account for its various and complex biological functions and highlight the common features with the PA28αβ paralog.
Collapse
Affiliation(s)
- Paolo Cascio
- Department of Veterinary Sciences, University of Turin, Largo P. Braccini 2, 10095 Grugliasco, Italy
| |
Collapse
|
4
|
Xie SC, Metcalfe RD, Hanssen E, Yang T, Gillett DL, Leis AP, Morton CJ, Kuiper MJ, Parker MW, Spillman NJ, Wong W, Tsu C, Dick LR, Griffin MDW, Tilley L. The structure of the PA28-20S proteasome complex from Plasmodium falciparum and implications for proteostasis. Nat Microbiol 2019; 4:1990-2000. [PMID: 31384003 DOI: 10.1038/s41564-019-0524-4] [Citation(s) in RCA: 27] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/24/2018] [Accepted: 06/25/2019] [Indexed: 11/09/2022]
Abstract
The activity of the proteasome 20S catalytic core is regulated by protein complexes that bind to one or both ends. The PA28 regulator stimulates 20S proteasome peptidase activity in vitro, but its role in vivo remains unclear. Here, we show that genetic deletion of the PA28 regulator from Plasmodium falciparum (Pf) renders malaria parasites more sensitive to the antimalarial drug dihydroartemisinin, indicating that PA28 may play a role in protection against proteotoxic stress. The crystal structure of PfPA28 reveals a bell-shaped molecule with an inner pore that has a strong segregation of charges. Small-angle X-ray scattering shows that disordered loops, which are not resolved in the crystal structure, extend from the PfPA28 heptamer and surround the pore. Using single particle cryo-electron microscopy, we solved the structure of Pf20S in complex with one and two regulatory PfPA28 caps at resolutions of 3.9 and 3.8 Å, respectively. PfPA28 binds Pf20S asymmetrically, strongly engaging subunits on only one side of the core. PfPA28 undergoes rigid body motions relative to Pf20S. Molecular dynamics simulations support conformational flexibility and a leaky interface. We propose lateral transfer of short peptides through the dynamic interface as a mechanism facilitating the release of proteasome degradation products.
Collapse
Affiliation(s)
- Stanley C Xie
- Department of Biochemistry and Molecular Biology, Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Melbourne, Victoria, Australia
| | - Riley D Metcalfe
- Department of Biochemistry and Molecular Biology, Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Melbourne, Victoria, Australia
| | - Eric Hanssen
- Department of Biochemistry and Molecular Biology, Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Melbourne, Victoria, Australia.,Advanced Microscopy Facility, Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Melbourne, Victoria, Australia
| | - Tuo Yang
- Department of Biochemistry and Molecular Biology, Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Melbourne, Victoria, Australia
| | - David L Gillett
- Department of Biochemistry and Molecular Biology, Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Melbourne, Victoria, Australia
| | - Andrew P Leis
- Advanced Microscopy Facility, Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Melbourne, Victoria, Australia
| | - Craig J Morton
- Department of Biochemistry and Molecular Biology, Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Melbourne, Victoria, Australia
| | | | - Michael W Parker
- Department of Biochemistry and Molecular Biology, Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Melbourne, Victoria, Australia.,Australian Cancer Research Foundation Rational Drug Discovery Centre, St Vincent's Institute of Medical Research, Fitzroy, Victoria, Australia
| | - Natalie J Spillman
- Department of Biochemistry and Molecular Biology, Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Melbourne, Victoria, Australia
| | - Wilson Wong
- Infection and Immunity Division, The Walter and Eliza Hall Institute, Parkville, Victoria, Australia
| | - Christopher Tsu
- Oncology Clinical R&D, Takeda Pharmaceuticals International Co., Cambridge, MA, USA
| | - Lawrence R Dick
- Oncology Clinical R&D, Takeda Pharmaceuticals International Co., Cambridge, MA, USA
| | - Michael D W Griffin
- Department of Biochemistry and Molecular Biology, Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Melbourne, Victoria, Australia.
| | - Leann Tilley
- Department of Biochemistry and Molecular Biology, Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Melbourne, Victoria, Australia.
| |
Collapse
|
5
|
Hwang H, Jin Z, Krishnamurthy VV, Saha A, Klein PS, Garcia B, Mei W, King ML, Zhang K, Yang J. Novel functions of the ubiquitin-independent proteasome system in regulating Xenopus germline development. Development 2019; 146:dev172700. [PMID: 30910828 PMCID: PMC6503979 DOI: 10.1242/dev.172700] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/11/2018] [Accepted: 03/20/2019] [Indexed: 01/22/2023]
Abstract
In most species, early germline development occurs in the absence of transcription with germline determinants subject to complex translational and post-translational regulations. Here, we report for the first time that early germline development is influenced by dynamic regulation of the proteasome system, previously thought to be ubiquitously expressed and to serve 'housekeeping' roles in controlling protein homeostasis. We show that proteasomes are present in a gradient with the highest levels in the animal hemisphere and extending into the vegetal hemisphere of Xenopus oocytes. This distribution changes dramatically during the oocyte-to-embryo transition, with proteasomes becoming enriched in and restricted to the animal hemisphere and therefore separated from vegetally localized germline determinants. We identify Dead-end1 (Dnd1), a master regulator of vertebrate germline development, as a novel substrate of the ubiquitin-independent proteasomes. In the oocyte, ubiquitin-independent proteasomal degradation acts together with translational repression to prevent premature accumulation of Dnd1 protein. In the embryo, artificially increasing ubiquitin-independent proteasomal degradation in the vegetal pole interferes with germline development. Our work thus reveals novel inhibitory functions and spatial regulation of the ubiquitin-independent proteasome during vertebrate germline development.
Collapse
Affiliation(s)
- Hyojeong Hwang
- Department of Comparative Biosciences, University of Illinois at Urbana-Champaign, 2001 South Lincoln Avenue, 3411 Veterinary Medicine Basic Sciences Building, Urbana, IL 61802, USA
| | - Zhigang Jin
- Department of Comparative Biosciences, University of Illinois at Urbana-Champaign, 2001 South Lincoln Avenue, 3411 Veterinary Medicine Basic Sciences Building, Urbana, IL 61802, USA
- College of Chemistry and Life Sciences, Zhejiang Normal University, 688 Yingbin Road, Jinhua, Zhejiang 321004, China
| | - Vishnu Vardhan Krishnamurthy
- Department of Biochemistry, University of Illinois at Urbana-Champaign, 600 S Mathews, 314B Roger Adams Laboratory, Urbana, IL 61801, USA
| | - Anumita Saha
- Epigenetics Institute, Department of Biochemistry and Biophysics, Perelman School of Medicine at the University of Pennsylvania, 3400 Civic Center Blvd, Philadelphia, PA 19104, USA
| | - Peter S Klein
- Department of Medicine (Hematology-Oncology), Perelman School of Medicine at the University of Pennsylvania, Philadelphia, PA 19104, USA
| | - Benjamin Garcia
- Epigenetics Institute, Department of Biochemistry and Biophysics, Perelman School of Medicine at the University of Pennsylvania, 3400 Civic Center Blvd, Philadelphia, PA 19104, USA
| | - Wenyan Mei
- Department of Comparative Biosciences, University of Illinois at Urbana-Champaign, 2001 South Lincoln Avenue, 3411 Veterinary Medicine Basic Sciences Building, Urbana, IL 61802, USA
| | - Mary Lou King
- Department of Cell Biology, University of Miami Miller School of Medicine, 1011 NW 15th St, Miami, FL 33136, USA
| | - Kai Zhang
- Department of Biochemistry, University of Illinois at Urbana-Champaign, 600 S Mathews, 314B Roger Adams Laboratory, Urbana, IL 61801, USA
| | - Jing Yang
- Department of Comparative Biosciences, University of Illinois at Urbana-Champaign, 2001 South Lincoln Avenue, 3411 Veterinary Medicine Basic Sciences Building, Urbana, IL 61802, USA
| |
Collapse
|
6
|
Raule M, Cerruti F, Benaroudj N, Migotti R, Kikuchi J, Bachi A, Navon A, Dittmar G, Cascio P. PA28αβ reduces size and increases hydrophilicity of 20S immunoproteasome peptide products. ACTA ACUST UNITED AC 2014; 21:470-480. [PMID: 24631123 DOI: 10.1016/j.chembiol.2014.02.006] [Citation(s) in RCA: 47] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/25/2013] [Revised: 12/20/2013] [Accepted: 02/03/2014] [Indexed: 11/25/2022]
Abstract
The specific roles that immunoproteasome variants play in MHC class I antigen presentation are unknown at present. To investigate the biochemical properties of different immunoproteasome forms and unveil the molecular mechanisms of PA28 activity, we performed in vitro degradation of full-length proteins by 20S, 26S, and PA28αβ-20S immunoproteasomes and analyzed the spectrum of peptides released. Notably, PA28αβ-20S immunoproteasomes hydrolyze proteins at the same low rates as 20S alone, which is in line with PA28, neither stimulating nor preventing entry of unfolded polypeptides into the core particle. Most importantly, binding of PA28αβ to 20S greatly reduces the size of proteasomal products and favors the release of specific, more hydrophilic, longer peptides. Hence, PA28αβ may either allosterically modify proteasome active sites or act as a selective "smart" sieve that controls the efflux of products from the 20S proteolytic chamber.
Collapse
Affiliation(s)
- Mary Raule
- Department of Veterinary Sciences, University of Turin, 10095 Grugliasco, Italy
| | - Fulvia Cerruti
- Department of Veterinary Sciences, University of Turin, 10095 Grugliasco, Italy
| | - Nadia Benaroudj
- Unité Biologie des Spirochètes, Institut Pasteur, 75015 Paris, France
| | - Rebekka Migotti
- Mass Spectrometry Core Unit, Max-Delbrück Center for Molecular Medicine, 13125 Berlin, Germany
| | - Julia Kikuchi
- Mass Spectrometry Core Unit, Max-Delbrück Center for Molecular Medicine, 13125 Berlin, Germany
| | - Angela Bachi
- IFOM, FIRC Institute of Molecular Oncology, 20139 Milan, Italy
| | - Ami Navon
- Department of Biological Regulation, The Weizmann Institute of Science, 76100 Rehovot, Israel
| | - Gunnar Dittmar
- Mass Spectrometry Core Unit, Max-Delbrück Center for Molecular Medicine, 13125 Berlin, Germany
| | - Paolo Cascio
- Department of Veterinary Sciences, University of Turin, 10095 Grugliasco, Italy.
| |
Collapse
|
7
|
Sugiyama M, Sahashi H, Kurimoto E, Takata SI, Yagi H, Kanai K, Sakata E, Minami Y, Tanaka K, Kato K. Spatial arrangement and functional role of α subunits of proteasome activator PA28 in hetero-oligomeric form. Biochem Biophys Res Commun 2013; 432:141-5. [PMID: 23376067 DOI: 10.1016/j.bbrc.2013.01.071] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/16/2013] [Accepted: 01/20/2013] [Indexed: 11/30/2022]
Abstract
A major form of proteasome activator PA28 is a heteroheptamer composed of interferon-γ-inducible α and β subunits, which share approximately 50% amino acid identity and possess distinct insert loops. This activator forms a complex with the 20S proteasome and thereby stimulates proteasomal degradation of peptides in an ATP-independent manner, giving rise to smaller antigenic peptides presented by major histocompatibility complex class I molecules. In this study, we performed biophysical and biochemical characterization of the structure and function of the PA28 hetero-oligomer. Deuteration-assisted small-angle neutron scattering demonstrated three α and four β subunits are alternately arranged in the heptameric ring. In this arrangement, PA28 loops surround the central pore of the heptameric ring (site for peptide entry). Activating the 20S proteasome with a PA28 mutant that lacked the α subunit loops cleaved model substrates longer than a nonapeptide with better efficiency when compared to wild-type PA28. Based on these data, we hypothesize that the flexible PA28 loops act as gatekeepers, which function to select the length of peptide substrates to be transported between the proteolytic chamber and the extra-proteasomal medium.
Collapse
Affiliation(s)
- Masaaki Sugiyama
- Research Reactor Institute, Kyoto University, Osaka 590-0494, Japan.
| | | | | | | | | | | | | | | | | | | |
Collapse
|
8
|
Liu G, Zheng W, Chen X. Molecular cloning of proteasome activator PA28-beta subunit of large yellow croaker (Pseudosciana crocea) and its coordinated up-regulation with MHC class I alpha-chain and beta 2-microglobulin in poly I:C-treated fish. Mol Immunol 2006; 44:1190-7. [PMID: 16901544 DOI: 10.1016/j.molimm.2006.06.024] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/17/2006] [Accepted: 06/20/2006] [Indexed: 10/24/2022]
Abstract
Antigenic peptides presented on MHC class I molecules to cytotoxic T-cells are generated in the cytosol by the 20S proteasome. Two activators PA28-alpha and PA28-beta, which are inducible by interferon-gamma (IFN-gamma), activate the latent 20S proteasome, thus playing an important role in the processing of MHC class I antigen. Molecular properties and function in the MHC class I antigen processing of PA28 have been well studied and documented in mammals while little is known in fish. In the present study, we reported the cloning of a PA28-beta gene homologue from the spleen of large yellow croaker (Pseudosciana crocea), an economically important marine fish (LycPA28-beta). The full-length cDNA of LycPA28-beta is 1133 nucleotides (nt) encoding a protein of 245 amino acids (aa), with a putative molecular weight of 27.7 kDa. The deduced protein shares 76, 69, 61, 60, 59, 57 and 57% sequence identity to sequences found in zebrafish, flounder, pig, rat, mouse, cattle and human, respectively. The deduced LycPA28-beta contains a PA28-beta subunit-specific insert in the region corresponding to the KEKE motif of the known PA28-alpha (Region B), a conserved activation loop (Region C) and a highly homologous C-terminal region among all three PA28 subunits (Region E), and a characteristic proline-rich motif (Region A) and a potential protein kinase C recognition site (Region D). Western blot analysis of various tissues indicated that LycPA28-beta was constitutively expressed in kidney, liver, spleen and intestine, and weakly expressed in muscle tissue, but not detected in gills, heart and brain. The LycPA28-beta expression was significantly up-regulated in kidney, liver, spleen, intestine and muscle tissues, and also induced in gills after 72 h of treatment with a viral micmic, polyinosinic polycytidynic acid (poly I:C). The transcriptional analysis of LycPA28-beta and MHC class I alpha-chain (alpha-chain) and beta(2)-microglobulin (beta(2)m) in spleens of poly I:C-induced large yellow croaker was further performed by RT-PCR. The results showed that the expression of LycPA28-beta and class I alpha-chain and beta(2)m genes was coordinately up-regulated by poly I:C, suggesting that induction of the MHC class I antigen processing and presentation pathway may be required for the antiviral immune response triggered poly I:C in large yellow croaker.
Collapse
Affiliation(s)
- Guozhu Liu
- Key Laboratory of Marine Biogenetic Resources, Third Institute of Oceanography, State Oceanic Administration, 178 Daxue Road, Xiamen 361005, PR China
| | | | | |
Collapse
|
9
|
Abstract
The 20S proteasome is a multicatalytic protein complex that plays an important role in intracellular protein degradation from archaebacteria to eukaryotes. This complex is made up of two copies each of seven different alpha (alpha) and seven different beta (beta) subunits arranged into four stacked rings (alpha7beta7beta7alpha7). Although the proteasome's cylindrical structure is conserved, the subunit composition of the 20S protein complex varies during the evolution, and the number of subunits increases from archaebacteria to mammals. To fully characterize the 20S proteasome subunit composition and understand the subunit functions, we, the authors of this chapter, have developed and employed various mass spectrometry (MS)-based approaches to generate a comprehensive profile of the 20S proteasomes from rat liver and Tropanosoma brucei. We have identified 7 alpha and 10 beta subunits, including 7 essential and 3 nonessential beta subunits from rat 20S proteasome complex using two-dimensional (2-D) gel electrophoresis and tandem MS (MS/MS). In addition, multiple isoforms of most of the subunits were determined; indicating the composition of rat 20S proteasome complex was much more complicated than expected. Further analysis of the intact protein molecular weight of each subunit using LC-MS confirmed the heterogeneous population of the 20S proteasome and revealed that many of the experimental measured molecular weights do not correspond well with the theoretical values deduced from the sequences in protein databases. This finding is mostly due to the sequence errors in the protein databases and possible posttranslational modifications. Although the protein sequences of rat 20S proteasome are present in the databases, the sequences of the 20S proteasome from T. brucei were not available at the time when the analysis was carried out. To determine the subunit composition of the 20S proteasome from T. brucei, we developed a homology-based database searching tool to identify unknown proteins based on the novel sequences determined by de novo sequencing using MS/MS. As a result, 14 subunits (7 alpha and 7 beta) were identified on the 2-D gel, which was later confirmed by the full-length sequences. Using the same approach, we also identified and characterized an activator protein, PA26, from T. brucei. The purified recombinant PA26 self-assembles into a heptamer ring, which can bind and activate the 20S proteasome from T. brucei as well as rat. Compared to the human PA28 complex, PA26 may be the prototype activator protein involved in proteasomal protein degradation. Therefore, the MS-based strategy developed here for identification of the known and unknown protein complexes can be generalized for the study of other protein complexes.
Collapse
Affiliation(s)
- Lan Huang
- Science I, University of California, Irvine, USA
| | | |
Collapse
|
10
|
Masters EI, Pratt G, Förster A, Hill CP. Purification and Analysis of Recombinant 11S Activators of the 20S Proteasome: Trypanosoma brucei PA26 and Human PA28α, PA28β, and PA28γ. Methods Enzymol 2005; 398:306-21. [PMID: 16275338 DOI: 10.1016/s0076-6879(05)98025-7] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/23/2023]
Abstract
Proteasomes perform the bulk of nonlysosomal degradation of aberrant, damaged, misfolded, and naturally short-lived regulatory proteins in eukaryotic cells. They are approximately 700-kDa assemblies whose hollow architecture sequesters the proteolytic sites inside a central chamber, thereby ensuring that the activity of isolated proteasomes is repressed. In vivo, proteasomes are activated by protein complexes, including the 11S activators (PA28 and PA26), which bind to one or both ends of the barrel-shaped structure. This chapter describes protocols for the purification of recombinant 11S regulators, characterization of their ability to stimulate proteasome activity, and crystallization of proteasome complexes.
Collapse
Affiliation(s)
- Eugene I Masters
- Department of Biology, University of Utah, Salt Lake City, UT 84103, USA
| | | | | | | |
Collapse
|
11
|
Gao X, Li J, Pratt G, Wilk S, Rechsteiner M. Purification procedures determine the proteasome activation properties of REGγ (PA28γ). Arch Biochem Biophys 2004; 425:158-64. [PMID: 15111123 DOI: 10.1016/j.abb.2004.03.021] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/24/2003] [Revised: 03/11/2004] [Indexed: 10/26/2022]
Abstract
The proteasome activation properties of recombinant REG gamma molecules depend on purification procedures. Prior to ammonium sulfate precipitation recombinant REG gamma activates the trypsin-like catalytic subunit of the proteasome; afterwards it activates all three catalytic subunits. The expanded activation specificity is accompanied by reduced stability of the REG gamma heptamer providing support for the idea that a "tight" REG gamma heptamer suppresses the proteasome's chymotrypsin-like and postglutamyl-preferring active sites. In an attempt to determine whether REG gamma synthesized in mammalian cells also exhibits restricted activation properties, extracts were prepared from several mammalian organs and cell lines. Surprisingly, endogenous REG gamma was found to be largely monomeric. In an alternate approach, COS7 cells were cotransfected with plasmids expressing FLAG-REG gamma and REG gamma. The expressed FLAG-REG gamma molecules were shown to form oligomers with untagged REG gamma subunits, and the mixed oligomers preferentially activated the proteasome's trypsin-like subunit. Thus, REG gamma molecules synthesized in mammalian cells also exhibit restricted activation properties.
Collapse
Affiliation(s)
- Xiaolin Gao
- Department of Biochemistry, University of Utah School of Medicine, 50 N Medical Drive, Salt Lake City, UT 84132, USA
| | | | | | | | | |
Collapse
|
12
|
Stohwasser R, Holzhütter HG, Lehmann U, Henklein P, Kloetzel PM. Hepatitis B virus HBx peptide 116-138 and proteasome activator PA28 compete for binding to the proteasome alpha4/MC6 subunit. Biol Chem 2003; 384:39-49. [PMID: 12674498 DOI: 10.1515/bc.2003.005] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
Abstract
PA28 is a modulator of the 20S proteasome. The PA28 binding sites on the 20S proteasome are still not well defined. Using yeast two-hybrid interaction assays and proteasome inactivation kinetics we provide evidence that the proteasome alpha4 subunit is one of the PA28 binding sites. This finding is supported by the observation that a hepatitis B virus X protein-derived polypeptide habouring the alpha4 proteasome subunit binding motif impairs the activation of 20S proteasomes by PA28.
Collapse
Affiliation(s)
- Ralf Stohwasser
- Department of Food Chemistry and Preventive Nutrition, German Institute of Human Nutrition Potsdam-Rehbrücke (DIfE), Arthur-Scheunert-Allee 114-116, D-14558 Bergholz-Rehbrücke, Germany
| | | | | | | | | |
Collapse
|
13
|
Zwickl P, Seemüller E, Kapelari B, Baumeister W. The proteasome: a supramolecular assembly designed for controlled proteolysis. ADVANCES IN PROTEIN CHEMISTRY 2002; 59:187-222. [PMID: 11868272 DOI: 10.1016/s0065-3233(01)59006-3] [Citation(s) in RCA: 42] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Affiliation(s)
- P Zwickl
- Department of Molecular Structural Biology, Max-Planck Institute for Biochemistry, Am Klopferspitz 18a, 82152 Martinsried, Germany
| | | | | | | |
Collapse
|
14
|
Abstract
Although substantial progress has been made in understanding the biochemical properties of 11S regulators since their discovery in 1992, we still only have a rudimentary understanding of their biological role. As discussed above, we have proposed a model in which the alpha/beta complex promotes the production of antigenic peptides by opening the exit port of the 20S proteasome (Whitby et al. 2000). There are other possibilities, however, that are not exclusive of the exit port hypothesis. For example the alpha/beta complex may promote assembly of immunoproteasome as suggested by Preckel et al. 1999, or it may function as a docking module and conduit for the delivery of peptides to the ER lumen (Realini et al. 1994b). There are also unanswered structural and mechanistic questions. Higher resolution data are needed to discern important structural details of the PA26/20S proteasome complex. The models for binding and activation that are suggested from the structural data have to be tested by mutagenesis and biochemical analysis. What is the role of homolog-specific inserts? Will cognate regulator/proteasome complexes show conformational changes that are not apparent in the currently available crystal structures, including perhaps signs of allosteric communication between the regulator and the proteasome active sites?
Collapse
Affiliation(s)
- C P Hill
- Biochemistry Department, University of Utah Medical School, 50 N Medical Drive, Salt Lake City, UT 84132, USA
| | | | | |
Collapse
|
15
|
Li J, Gao X, Ortega J, Nazif T, Joss L, Bogyo M, Steven AC, Rechsteiner M. Lysine 188 substitutions convert the pattern of proteasome activation by REGgamma to that of REGs alpha and beta. EMBO J 2001; 20:3359-69. [PMID: 11432824 PMCID: PMC125523 DOI: 10.1093/emboj/20.13.3359] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/14/2023] Open
Abstract
11S REGs (PA28s) are multimeric rings that bind proteasomes and stimulate peptide hydrolysis. Whereas REGalpha activates proteasomal hydrolysis of peptides with hydrophobic, acidic or basic residues in the P1 position, REGgamma only activates cleavage after basic residues. We have isolated REGgamma mutants capable of activating the hydrolysis of fluorogenic peptides diagnostic for all three active proteasome beta subunits. The most robust REGgamma specificity mutants involve substitution of Glu or Asp for Lys188. REGgamma(K188E/D) variants are virtually identical to REGalpha in proteasome activation but assemble into less stable heptamers/hexamers. Based on the REGalpha crystal structure, Lys188 of REGgamma faces the aqueous channel through the heptamer, raising the possibility that REG channels function as substrate-selective gates. However, covalent modification of proteasome chymotrypsin-like subunits by 125I-YL3-VS demonstrates that REGgamma(K188E)'s activation of all three proteasome active sites is not due to relaxed gating. We propose that decreased stability of REGgamma(K188E) heptamers allows them to change conformation upon proteasome binding, thus relieving inhibition of the CT and PGPH sites normally imposed by the wild-type REGgamma molecule.
Collapse
Affiliation(s)
- J Li
- Department of Biochemistry, University of Utah School of Medicine, Salt Lake City, UT 84132-0001, USA
| | | | | | | | | | | | | | | |
Collapse
|
16
|
Abstract
The proteasome activators known as 11S REG or PA28 were discovered about 10 years ago. They are homo- or heteroheptameric rings that bind to the ends of 20S proteasomes and activate cleavage of peptides but not folded proteins. In this article, we focus on structural features of three homologous REG subunits (termed alpha, beta, gamma) that contribute to their oligomerization, proteasome binding and proteasome activation. We review a number of published studies on the biochemical properties of REGs and present new results in which N-terminal sequences and sequences flanking REG activation loops have been exchanged between homologs. Characterization of these chimeras and previously constructed C-terminal chimeras reveal that N-terminal and loop flanking sequences affect oligomerization, whereas C-terminal sequences are essential for proteasome binding. None of these regions is responsible for the broad activation specificity of REGs alpha/beta versus the narrow specificity of REGgamma. Rather, mutation in a single residue lining the channel through the REGgamma heptamer changes the activation property of the gamma homolog to match that of REGs alpha and beta.
Collapse
Affiliation(s)
- J Li
- Department of Biochemistry, University of Utah School of Medicine, Salt Lake City 84132-0001, USA
| | | |
Collapse
|
17
|
Masson P, Andersson O, Petersen UM, Young P. Identification and characterization of a Drosophila nuclear proteasome regulator. A homolog of human 11 S REGgamma (PA28gamma ). J Biol Chem 2001; 276:1383-90. [PMID: 11027688 DOI: 10.1074/jbc.m007379200] [Citation(s) in RCA: 42] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
We report the cloning and characterization of a Drosophila proteasome 11 S REGgamma (PA28) homolog. The 28-kDa protein shows 47% identity to the human REGgamma and strongly enhances the trypsin-like activities of both Drosophila and mammalian 20 S proteasomes. Surprisingly, the Drosophila REG was found to inhibit the proteasome's chymotrypsin-like activity against the fluorogenic peptide succinyl-LLVY-7-amino-4-methylcoumarin. Immunocytological analysis reveals that the Drosophila REG is localized to the nucleus but is distributed throughout the cell when nuclear envelope breakdown occurs during mitosis. Through site-directed mutagenesis studies, we have identified a functional nuclear localization signal present in the homolog-specific insert region. The Drosophila PA28 NLS is similar to the oncogene c-Myc nuclear localization motif. Comparison between uninduced and innate immune induced Drosophila cells suggests that the REGgamma proteasome activator has a role independent of the invertebrate immune system. Our results support the idea that gamma class proteasome activators have an ancient conserved function within metazoans and were present prior to the emergence of the alpha and beta REG classes.
Collapse
Affiliation(s)
- P Masson
- Department of Molecular Biology, Stockholm University, S-10691 Stockholm, Sweden
| | | | | | | |
Collapse
|
18
|
Voges D, Zwickl P, Baumeister W. The 26S proteasome: a molecular machine designed for controlled proteolysis. Annu Rev Biochem 2000; 68:1015-68. [PMID: 10872471 DOI: 10.1146/annurev.biochem.68.1.1015] [Citation(s) in RCA: 1383] [Impact Index Per Article: 57.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
In eukaryotic cells, most proteins in the cytosol and nucleus are degraded via the ubiquitin-proteasome pathway. The 26S proteasome is a 2.5-MDa molecular machine built from approximately 31 different subunits, which catalyzes protein degradation. It contains a barrel-shaped proteolytic core complex (the 20S proteasome), capped at one or both ends by 19S regulatory complexes, which recognize ubiquitinated proteins. The regulatory complexes are also implicated in unfolding and translocation of ubiquitinated targets into the interior of the 20S complex, where they are degraded to oligopeptides. Structure, assembly and enzymatic mechanism of the 20S complex have been elucidated, but the functional organization of the 19S complex is less well understood. Most subunits of the 19S complex have been identified, however, specific functions have been assigned to only a few. A low-resolution structure of the 26S proteasome has been obtained by electron microscopy, but the precise arrangement of subunits in the 19S complex is unclear.
Collapse
Affiliation(s)
- D Voges
- Max-Planck-Institut für Biochemie, Martinsried, Germany
| | | | | |
Collapse
|
19
|
Li J, Gao X, Joss L, Rechsteiner M. The proteasome activator 11 S REG or PA28: chimeras implicate carboxyl-terminal sequences in oligomerization and proteasome binding but not in the activation of specific proteasome catalytic subunits. J Mol Biol 2000; 299:641-54. [PMID: 10835274 DOI: 10.1006/jmbi.2000.3800] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The REG homologs, alpha, beta and gamma, activate mammalian proteasomes in distinct ways. REGalpha and REGbeta activate the trypsin-like, chymotrypsin-like and peptidylglutamyl-preferring active sites, whereas REGgamma only activates the proteasome's trypsin-like subunit. The three REG homologs differ in carboxyl-terminal sequences that are located next to activation loops on their proteasome binding surface. To assess the importance of these carboxyl-terminal sequences in the activation of specific proteasome beta catalytic subunits, we characterized chimeras in which 8 or 12 residues were exchanged among the three proteins. Like the wild-type molecule, REGalpha chimeras activated all three proteasome catalytic subunits regardless of the carboxyl-terminal sequence. However, REGalpha-beta chimeras activated the proteasome at lower concentrations than wild-type REGalpha and higher levels of REGalpha-gamma chimeras were needed for maximal activation because exchanged carboxyl-terminal sequences can stabilize (REGalpha-beta) or destabilize (REGalpha-gamma) the REGalpha heptamer. REGgamma chimeras were equivalent to REGgamma in their activation properties, but they bound the proteasome less tightly than the wild-type molecule. REGbeta chimeras also bound the proteasome more weakly than wild-type REGbeta and were virtually unable to activate it. Our findings demonstrate that the carboxyl-terminal sequences of REG subunits can affect heptamer stability and proteasome affinity, but they do not determine which proteasome beta subunits become activated.
Collapse
Affiliation(s)
- J Li
- Department of Biochemistry, University of Utah School of Medicine, Salt Lake City, UT, 84132, USA
| | | | | | | |
Collapse
|
20
|
Abstract
There are two immune responses in vertebrates: humoral immunity is mediated by circulating antibodies, whereas cytotoxic T lymphocytes (CTL) confer cellular immunity. CTL lyse infected cells upon recognition of cell-surface MHC Class I molecules complexed with foreign peptides. The displayed peptides are produced in the cytosol by degradation of host proteins or proteins from intracellular pathogens that might be present. Proteasomes are cylindrical multisubunit proteases that generate many of the peptides eventually transferred to the cell surface for immune surveillance. In mammalian proteasomes, six active sites face a central chamber. As this chamber is sealed off from the enzyme's surface, there must be mechanisms to promote entry of substrates. Two protein complexes have been found to bind the ends of the proteasome and activate it. One of the activators is the 19 S regulatory complex of the 26 S proteasome; the other activator is '11 S REG' [Dubiel, Pratt, Ferrell and Rechsteiner (1992) J. Biol. Chem. 267, 22369-22377] or 'PA28' [Ma, Slaughter and DeMartino (1992) J. Biol. Chem. 267, 10515-10523]. During the past 7 years, our understanding of the structure of REG molecules has increased significantly, but much less is known about their biological functions. There are three REG subunits, namely alpha, beta and gamma. Recombinant REGalpha forms a ring-shaped heptamer of known crystal structure. 11 S REG is a heteroheptamer of alpha and beta subunits. REGgamma is also presumably a heptameric ring, and it is found in the nuclei of the nematode work Caenorhabditis elegans and higher organisms, where it may couple proteasomes to other nuclear components. REGalpha and REGbeta, which are abundant in vertebrate immune tissues, are located mostly in the cytoplasm. Synthesis of REG alpha and beta subunits is induced by interferon-gamma, and this has led to the prevalent hypothesis that REG alpha/beta hetero-oligomers play an important role in Class I antigen presentation. In the present review we focus on the structural properties of REG molecules and on the evidence that REGalpha/beta functions in the Class I immune response.
Collapse
|
21
|
West RW, Kruger B, Thomas S, Ma J, Milgrom E. RLR1 (THO2), required for expressing lacZ fusions in yeast, is conserved from yeast to humans and is a suppressor of SIN4. Gene 2000; 243:195-205. [PMID: 10675628 DOI: 10.1016/s0378-1119(99)00510-7] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
We isolated a mutation (rlr1-1; required for lacZ RNA) in the Saccharomyces cerevisiae (Sc) RLR1 gene as a suppressor of sin4, a component of the Mediator subcomplex of the RNA polymerase II holoenzyme and a determinant of chromatin structure. RLR1 encodes a deduced protein found also in fission yeast, nematode worms, and humans. The presence of these orthologs suggests that Rlr1 family members comprise a class of putative KEKE motif-containing proteins, characteristic of certain chaperones as well as regulators and subunits of the mammalian 20S proteasome. A role for RLR1 (THO2) in transcription appears to occur at a step subsequent to transcription initiation (see also Piruat, J.I. and Aguilera, A., 1998. EMBO J. 17, 4859-4872); Sc genes fused to the reporter gene lacZ were expressed at a very low level, while the corresponding native chromosomal genes were expressed at approximately normal levels in rlr1 mutants. Our studies show that rlr1 mutations cause a wide range of growth defects in addition to their novel affect on lacZ.
Collapse
Affiliation(s)
- R W West
- Department of Biochemistry, State University of New York, Health Science Center at Syracuse, Syracuse, NY, USA
| | | | | | | | | |
Collapse
|
22
|
Yao Y, Huang L, Krutchinsky A, Wong ML, Standing KG, Burlingame AL, Wang CC. Structural and functional characterizations of the proteasome-activating protein PA26 from Trypanosoma brucei. J Biol Chem 1999; 274:33921-30. [PMID: 10567354 DOI: 10.1074/jbc.274.48.33921] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The activated 20 S proteasome, which has been found only in mammalian cells, is composed of two heptamer rings of an activator protein on each end of the 20 S proteasome and is inducible by interferon-gamma. A 20 S proteasome has been recently identified in a protozoan pathogen Trypanosoma brucei, but there has been no experimental evidence yet for the presence of a 26 S proteasome. Instead, an activated form of 20 S proteasome was isolated from this organism, which has significantly enhanced peptidase activities. It consists of an additional activator protein with an estimated molecular mass of 26 kDa (PA26) (To, W. Y., and Wang, C. C. (1997) FEBS Lett. 404, 253-262). The profile and sequences of tryptic peptides from PA26 were determined by mass spectrometry; no matches were found in the data base. The peptide sequences were used in reverse transcriptase-polymerase chain reaction to isolate a full-length cDNA clone encoding PA26. The protein sequence thus derived from it indicates little sequence identity with those of mammalian activator proteins PA28 alpha, beta, or gamma. There is only a single copy of PA26 gene in T. brucei. Purified recombinant PA26 polymerizes spontaneously to form heptamer ring with an outer diameter of 8.5 nm. The ring binds and activates 20 S proteasomes from T. brucei as well as rat, whereas human PA28alpha can neither bind nor activate T. brucei 20 S proteasome. The former is thus apparently more ubiquitous than PA28 in its capability of binding to and activating 20 S proteasomes. Its presence in T. brucei may also suggest a more ancient origin of proteasome activator proteins and a much wider involvement in protein degradation among other eukaryotic organisms than was originally envisaged.
Collapse
MESH Headings
- Amino Acid Sequence
- Animals
- Blotting, Southern
- Cloning, Molecular
- Cysteine Endopeptidases/chemistry
- Cysteine Endopeptidases/genetics
- Cysteine Endopeptidases/metabolism
- DNA, Complementary/chemistry
- DNA, Complementary/genetics
- DNA, Protozoan/analysis
- DNA, Protozoan/genetics
- Electrophoresis, Gel, Two-Dimensional
- Electrophoresis, Polyacrylamide Gel
- Enzyme Activation
- Gene Dosage
- Histidine/genetics
- Immunoblotting
- Insect Proteins/chemistry
- Insect Proteins/genetics
- Insect Proteins/metabolism
- Mass Spectrometry/methods
- Molecular Sequence Data
- Multienzyme Complexes/chemistry
- Multienzyme Complexes/genetics
- Multienzyme Complexes/metabolism
- Proteasome Endopeptidase Complex
- Protein Conformation
- Protein Isoforms/genetics
- Protozoan Proteins
- Rats
- Recombinant Fusion Proteins/chemistry
- Recombinant Fusion Proteins/genetics
- Recombinant Fusion Proteins/ultrastructure
- Sequence Alignment
- Sequence Analysis, DNA
- Sequence Analysis, Protein
- Sequence Homology, Amino Acid
- Trypanosoma brucei brucei/chemistry
- Trypanosoma brucei brucei/enzymology
- Trypanosoma brucei brucei/genetics
Collapse
Affiliation(s)
- Y Yao
- Department of Pharmaceutical Chemistry, Howard Hughes Medical Institute, University of California, San Francisco, California 94143, USA
| | | | | | | | | | | | | |
Collapse
|
23
|
Zhang Z, Clawson A, Rechsteiner M. The proteasome activator 11 S regulator or PA28. Contribution By both alpha and beta subunits to proteasome activation. J Biol Chem 1998; 273:30660-8. [PMID: 9804839 DOI: 10.1074/jbc.273.46.30660] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The proteasome 11 S regulator (REG) consists of two homologous subunits, REGalpha and REGbeta. Each subunit is capable of activating the proteasome, and when combined, they form superactive REGalpha/REGbeta complexes. We have previously shown that a highly conserved loop in the REGalpha crystal structure is critical for proteasome activation. We now show that hetero-oligomers formed from REGalpha activation loop mutants and wild-type REGbeta or vice versa are partially active. By contrast, hetero-oligomers bearing mutations in the activation loops of REGalpha and REGbeta subunits are inactive, demonstrating that both alpha and beta subunits contribute to proteasome activation. We have also characterized REG proteins with mutations near or at their C termini. Partially active REGalpha(Y249C) and REGalpha(M247V) and an inactive REGalpha subunit bearing five additional C-terminal amino acids formed active hetero-oligomers with REGbeta. REGbeta subunits lacking 1, 2, or 9 C-terminal amino acids did not bind or activate the proteasome, but each of these mutants formed partially active hetero-oligomers with the monomer REGalpha(N50Y). However, hetero-oligomers formed from REG subunits lacking the last 14 amino acids were unable to bind the proteasome. Thus, C-terminal regions of both alpha and beta subunits are required for hetero-oligomers to bind the proteasome.
Collapse
Affiliation(s)
- Z Zhang
- Department of Biochemistry, University of Utah, Salt Lake City, Utah 84132, USA
| | | | | |
Collapse
|