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.

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, alpha1-7beta1-7beta1-7alpha1-7 (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 alpha-ring formation and keeps the alpha-rings competent for the subsequent formation of half-proteasomes. Thus, our results identify a mechanism for the correct assembly of 20S 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 carboxylterminal 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.

The 20S proteasome is the catalytic core of the 26S proteasome. It comprises four stacked rings of seven subunits each, a 1-7 b 1-7 b 1-7 a 1-7 . Recent studies indicated that proteasome-specific chaperones and b-subunit appendages assist in the formation of a-rings and dimerization of halfproteasomes, but the process involved in the assembly of b-rings is poorly understood. Here, we clarify the mechanism of b-ring formation on a-rings by characterizing assembly intermediates accumulated in cells depleted of each b-subunit. Starting from b2, incorporation of b-subunits occurs in an orderly manner dependent on the propeptides of b2 and b5, and the C-terminal tail of b2. Unexpectedly, hUmp1, a chaperone functioning at the final assembly step, is incorporated as early as b2 and is required for the structural integrity of early assembly intermediates. We propose a model in which b-ring formation is assisted by both intramolecular and extrinsic chaperones, whose roles are partially different between yeast and mammals.

In response to proteasome dysfunction, mammalian cells upregulate proteasome gene expression by activating Nrf1. Nrf1 is an endoplasmic reticulum-resident transcription factor that is continually retrotranslocated and degraded by the proteasome. Upon proteasome inhibition, Nrf1 escapes degradation and is cleaved to become active. However, the processing enzyme for Nrf1 remains obscure. Here we show that the aspartyl protease DNA-damage inducible 1 homolog 2 (DDI2) is required to cleave and activate Nrf1. Deletion of DDI2 reduced the cleaved form of Nrf1 and increased the full-length cytosolic form of Nrf1, resulting in poor upregulation of proteasomes in response to proteasome inhibition. These defects were restored by adding back wild-type DDI2 but not protease-defective DDI2. Our results provide a clue for blocking compensatory proteasome synthesis to improve cancer therapies targeting proteasomes.DOI: http://dx.doi.org/10.7554/eLife.18357.001