The Molecular Mechanisms Governing the Assembly of the Immuno- and Thymoproteasomes in the Presence of Constitutive Proteasomes

Abstract

The proteasome is a large protein complex responsible for proteolysis in cells. Though the proteasome is widely conserved in all eukaryotes, vertebrates additionally possess tissue-specific proteasomes, termed immunoproteasomes and thymoproteasomes. These specialized proteasomes diverge from constitutive proteasomes in the makeup of their catalytic 20S core particle (CP), whereby the constitutive β1, β2, and β5 catalytic subunits are replaced by β1i, β2i, and β5i in immunoproteasomes, or β1i, β2i, and β5t in thymoproteasomes. However, as constitutive β1, β2, and β5 are also present in tissues and cells expressing immuno- and thymoproteasomes, the specialized proteasomes must be able to selectively incorporate their specific subunits. Here, we review the mechanisms governing the assembly of constitutive and specialized proteasomes elucidated thus far. Studies have revealed that β1i and β2i are added onto the α-ring of the CP prior to the other β subunits. Furthermore, β5i and β5t can be incorporated independent of β4, whereas constitutive β5 incorporation is dependent on β4. These mechanisms allow the immuno- and thymoproteasomes to integrate tissue-specific β-subunits without contamination from constitutive β1, β2, and β5. We end the review with a brief discussion on the diseases caused by mutations to the immunoproteasome and the proteins involved with its assembly.

Keywords: PAC1–PAC2; PAC3–PAC4; UMP1; chaperone; immunoproteasome; intermediate proteasome; propeptide; proteasome; thymoproteasome.

Figure 1

Schematic diagram of the proteasome. (a) The 26S proteasome consists of the catalytic 20S core particle and the 19S regulatory particle. (b) The subunits composing the 20S core particle. The catalytic β1, β2, and β5 subunits are responsible for caspase-like, trypsin-like, and chymotrypsin-like activities, respectively. (c) Tissue specific proteasomes are generated by switching their catalytic subunits.

Figure 2

Assembly pathways of the constitutive, the immuno-, and the thymoproteasomes. There are two differences between the assembly pathway of the constitutive proteasome and that of the immuno- and the thymoproteasomes. The first difference is that β1i is incorporated earlier than constitutive β1. The second is that constitutive β5 is incorporated after β4, whereas β5i and β5t can be incorporated on the α-ring independent of β4, and either β4 or β5i and β5t can be incorporated into the β-ring immediately after the incorporation of β3.

Figure 3

Structure of the pre-15S intermediate complex. (a) Cryo-EM structure of the pre-15S CP intermediate complex. The N-terminus of Pba1 extends to the CP interior and interacts with Ump1 and the propeptide of β5. α7 is omitted from the original structure (PDB 7LS6) for clarity. (b) Interaction between β2, β5, and Ump1. Ump1 makes contacts with both the propeptides and main bodies of β2 and β5. PyMOL was used for visualization (https://pymol.org/2/support.html), accessed on 27 April 2022.

Figure 4

Incorporation of the three immunosubunits into the immunoproteasome. (a) β1i and β2i are mutually required for their incorporation during immunoproteasome assembly. Please note that the intermediate containing sole β1i or β2i with the α-ring has not been observed. (b) The precursor complex containing β1i and β2i promotes the incorporation of β5i. (c) The incorporation of β5i is necessary for removal of the β1i and β2i propeptides.