Hideki Yashiroda

Molecular mechanisms of proteasome assembly

The 26S proteasome is a highly conserved protein degradation machine that consists of the 20S proteasome and 19S regulatory particles, which include 14 and 19 different polypeptides, respectively. How the proteasome components are assembled is a fundamental question towards understanding the process of protein degradation and its functions in diverse biological processes. Several proteasome-dedicated chaperones are involved in the efficient and correct assembly of the 20S proteasome. These chaperones help the initiation and progression of the assembly process by transiently associating with proteasome precursors. By contrast, little is known about the assembly of the 19S regulatory particles, but several hints have emerged.

A heterodimeric complex that promotes the assembly of mammalian 20S proteasomes

The 26S proteasome is a multisubunit protease responsible for regulated proteolysis in eukaryotic cells. It comprises one catalytic 20S proteasome and two axially positioned 19S regulatory complexes. The 20S proteasome is composed of 28 subunits arranged in a cylindrical particle as four heteroheptameric rings, α1–7β1–7β1–7α1–7 (refs 4, 5), but the mechanism responsible for the assembly of such a complex structure remains elusive. Here we report two chaperones, designated proteasome assembling chaperone-1 (PAC1) and PAC2, that are involved in the maturation of mammalian 20S proteasomes. PAC1 and PAC2 associate as heterodimers with proteasome precursors and are degraded after formation of the 20S proteasome is completed. Overexpression of PAC1 or PAC2 accelerates the formation of precursor proteasomes, whereas knockdown by short interfering RNA impairs it, resulting in poor maturation of 20S proteasomes. Furthermore, the PAC complex provides a scaffold for α-ring formation and keeps the α-rings competent for the subsequent formation of half-proteasomes. Thus, our results identify a mechanism for the correct assembly of 20S proteasomes.

The molecular chaperone Hsp90 plays a role in the assembly and maintenance of the 26S proteasome

Hsp90 has a diverse array of cellular roles including protein folding, stress response and signal transduction. Herein we report a novel function for Hsp90 in the ATP‐dependent assembly of the 26S proteasome. Functional loss of Hsp90 using a temperature‐sensitive mutant in yeast caused dissociation of the 26S proteasome. Conversely, these dissociated constituents reassembled in Hsp90‐dependent fashion both in vivo and in vitro; the process required ATP‐hydrolysis and was suppressed by the Hsp90 inhibitor geldanamycin. We also found genetic interactions between Hsp90 and several proteasomal Rpn (Regulatory particle non‐ATPase subunit) genes, emphasizing the importance of Hsp90 to the integrity of the 26S proteasome. Our results indicate that Hsp90 interacts with the 26S proteasome and plays a principal role in the assembly and maintenance of the 26S proteasome.

A novel proteasome interacting protein recruits the deubiquitinating enzyme UCH37 to 26S proteasomes

The 26S proteasome is a multisubunit protease responsible for regulated proteolysis in eukaryotic cells. It is composed of one catalytic 20S proteasome and two 19S regulatory particles attached on both ends of 20S proteasomes. Here, we describe the identification of Adrm1 as a novel proteasome interacting protein in mammalian cells. Although the overall sequence of Adrm1 has weak homology with the yeast Rpn13, the amino‐ and carboxyl‐terminal regions exhibit significant homology. Therefore, we designated it as hRpn13. hRpn13 interacts with a base subunit Rpn2 via its amino‐terminus. The majority of 26S proteasomes contain hRpn13, but a portion of them does not, indicating that hRpn13 is not an integral subunit. Intriguingly, we found that hRpn13 recruits UCH37, a deubiquitinating enzyme known to associate with 26 proteasomes. The carboxyl‐terminal regions containing KEKE motifs of both hRpn13 and UCH37 are involved in their physical interaction. Knockdown of hRpn13 caused no obvious proteolytic defect but loss of UCH37 proteins and decrease in deubiquitinating activity of 26S proteasomes. Our results indicate that hRpn13 is essential for the activity of UCH37.

Bul1, a new protein that binds to the Rsp5 ubiquitin ligase in Saccharomyces cerevisiae.

We characterized a temperature-sensitive mutant of Saccharomyces cerevisiae in which a mini-chromosome was unstable at a high temperature and cloned a new gene which encodes a basic and hydrophilic protein (110 kDa). The disruption of this gene caused the same temperature-sensitive growth as the original mutation. By using the two-hybrid system, we further isolated RSP5 (reverses Spt- phenotype), which encodes a hect (homologous to E6-AP C terminus) domain, as a gene encoding a ubiquitin ligase. Thus, we named our gene BUL1 (for a protein that binds to the ubiquitin ligase). BUL1 seems to be involved in the ubiquitination pathway, since a high dose of UBI1, encoding a ubiquitin, partially suppressed the temperature sensitivity of the bul1 disruptant as well as that of a rsp5 mutant. Coexpression of RSP5 and BUL1 on a multicopy plasmid was toxic for mitotic growth of the wild-type cells. Pulse-chase experiments revealed that Bul1 in the wild-type cells remained stable, while the bands of Bul1 in the rsp5 cells were hardly detected. Since the steady-state levels of the protein were the same in the two strains as determined by immunoblotting analysis, Bul1 might be easily degraded during immunoprecipitation in the absence of intact Rsp5. Furthermore, both Bul1 and Rsp5 appeared to be associated with large complexes which were separated through a sucrose gradient centrifugation, and Rsp5 was coimmunoprecipitated with Bul1. We discuss the possibility that Bul1 functions together with Rsp5 in protein ubiquitination.

Dissecting β-ring assembly pathway of the mammalian 20S proteasome

The 20S proteasome is the catalytic core of the 26S proteasome. It comprises four stacked rings of seven subunits each, α1–7β1–7β1–7α1–7. Recent studies indicated that proteasome‐specific chaperones and β‐subunit appendages assist in the formation of α‐rings and dimerization of half‐proteasomes, but the process involved in the assembly of β‐rings is poorly understood. Here, we clarify the mechanism of β‐ring formation on α‐rings by characterizing assembly intermediates accumulated in cells depleted of each β‐subunit. Starting from β2, incorporation of β‐subunits occurs in an orderly manner dependent on the propeptides of β2 and β5, and the C‐terminal tail of β2. Unexpectedly, hUmp1, a chaperone functioning at the final assembly step, is incorporated as early as β2 and is required for the structural integrity of early assembly intermediates. We propose a model in which β‐ring formation is assisted by both intramolecular and extrinsic chaperones, whose roles are partially different between yeast and mammals.

Control of IκBα proteolysis by the ubiquitin-proteasome pathway

It has recently been determined that the proteolytic destruction of IkappaB (inhibitor of NF-kappaB) by the ubiquitin-proteasome system plays a key role in the immediate elimination of IkappaB from the IkappaB-(NF-kappaB) complex which allows nuclear translocation of free NF-kappaB, thus leading to activation of a multitude of target genes. The SCF(Fbw1) (composed of Skp1, Cul-1, Roc1, and Fbw1) complex, identified as an IkappaBalpha-E3 ligase, binds and ubiquitylates IkappaBalpha phosphorylated by IkappaB kinase that has been activated in response to extracellular signals. The generating poly-ubiquitin chain is finally recognized by the 26S proteasome for ultimate degradation. In this NF-kappaB signalling pathway, it becomes clear that the SCF(Fbw1) activity is enhanced by a ubiquitin-like protein NEDD8 (equivalent to Rub1) that modifies Cul-1 in a manner analogous to ubiquitylation, and consequently, IkappaBalpha proteolysis is induced. NEDD8 is a new regulator of the SCF ubiquitin-ligase, functioning as a covalent modifier for proteolytic targeting at a physiological level.

p57Kip2 Is Degraded through the Proteasome in Osteoblasts Stimulated to Proliferation by Transforming Growth Factor β1

Cyclin-dependent kinase inhibitory proteins are negative regulators of the cell cycle. Although all the cyclin-dependent kinase inhibitory proteins may be involved in cell cycle control during a differentiation process, only p57Kip2 is shown to be essential for embryonic development. However, the role of p57 in the control of the cell cycle is poorly understood. Using osteoblasts derived from the calvaria of rat fetus, we show that p57 is accumulated in cells starved by low serum. Cyclin-dependent kinase 2 activity was suppressed in these cells with a significant amount bound to p57. Treatment of the cells with transforming growth factor β1 dramatically reduced the amount of p57, resulting in an activation of cyclin-dependent kinase 2 activity and the stimulation of cell proliferation. The decrease in p57 was inhibited by treating the cells with proteasome inhibitors, Z-Leu-Leu-Leu-aldehyde or lactacystin, but not with Z-Leu-Leu-aldehyde, which is an inhibitor of calpain, indicating that p57 is degraded through the proteasome pathway. p57 was also shown to be ubiquitinated in vitro. Because transforming growth factor β1 not only stimulates the growth but also inhibits the differentiation of the cells in this system, our results may suggest a possible involvement of p57 in the control of osteoblastic cell proliferation and differentiation.

Crystal structure of a chaperone complex that contributes to the assembly of yeast 20S proteasomes

Eukaryotic 20S proteasomes are composed of two α-rings and two β-rings, which form an αββα stacked structure. Here we describe a proteasome-specific chaperone complex, designated Dmp1–Dmp2, in budding yeast. Dmp1–Dmp2 directly bound to the α5 subunit to facilitate α-ring formation. In Δdmp1 cells, α-rings lacking α4 and decreased formation of 20S proteasomes were observed. Dmp1–Dmp2 interacted with proteasome precursors early during proteasome assembly and dissociated from the precursors before the formation of half-proteasomes. Notably, the crystallographic structures of Dmp1 and Dmp2 closely resemble that of PAC3—a mammalian proteasome-assembling chaperone; nonetheless, neither Dmp1 nor Dmp2 showed obvious sequence similarity to PAC3. The structure of the Dmp1–Dmp2–α5 complex reveals how this chaperone functions in proteasome assembly and why it dissociates from proteasome precursors before the β-rings are assembled.

Proteasomes and Molecular Chaperones : Cellular Machinery Responsible for Folding and Destruction of Unfolded Proteins

Molecular chaperones recognize proteins of non-native structure, prevent them from irreversible intracellular aggregation, and then act with regulatory co-chaperones in the conversion of proteins to be properly folded and in a functional state. However, not every non-native protein is folded successfully. Those proteins that are not accurately folded/ refolded are then directed to the ubiquitin-proteasome system (UPS) for destruction. Both chaperones and proteasomes act jointly together for selective removal of proteins with aberrant structure so as to keep protein homeostasis in cells. Though the precise nature of the cooperative linkage between chaperone and UPS pathways remains largely elusive so far, accumulating evidence from in vivo and in vitro studies shed some light on the molecular mechanisms that link proteasomes and molecular chaperones. This review focuses on how unfolded proteins are handled by these two machineries.