An evolutionarily distinct chaperone promotes 20S proteasome α-ring assembly in plants

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

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.

Oxidative and salt stresses alter the 26S proteasome holoenzyme and associated protein profiles in Arabidopsis thaliana

BACKGROUND

The 26S proteasome, canonically composed of multi-subunit 19S regulatory (RP) and 20S core (CP) particles, is crucial for cellular proteostasis. Proteasomes are re-modeled, activated, or re-localized and this regulation is critical for plants in response to environmental stresses. The proteasome holoenzyme assembly and dissociation are therefore highly dynamic in vivo. However, the stoichiometric changes of the plant proteasomes and how proteasome associated chaperones vary under common abiotic stresses have not been systematically studied.

RESULTS

Here, we studied the impact of abiotic stresses on proteasome structure, activity, and interacting partners in Arabidopsis thaliana. We analyzed available RNA expression data and observed that expressions of proteasome coding genes varied substantially under stresses; however, the protein levels of a few key subunits did not change significantly within 24 h. Instead, a switch in the predominant proteasome complex, from 26S to 20S, occurs under oxidative or salt stress. Oxidative stress also reduced the cellular ATP content and the association of HSP70-family proteins to the 20S proteasome, but enhanced the activity of cellular free form CP. Salt stress, on the other hand, did not affect cellular ATP level, but caused subtle changes in proteasome subunit composition and impacted bindings of assembly chaperones. Analyses of an array of T-DNA insertional mutant lines highlighted important roles for several putative assembly chaperones in seedling establishment and stress sensitivity. We also observed that knockout of PBAC1, one of the α-ring assembly chaperones, resulted in reduced germination and tearing of the seed coat following sterilization.

CONCLUSIONS

Our study revealed an evolutionarily conserved mechanism of proteasome regulation during oxidative stress, involving dynamic regulation of the holoenzyme formation and associated regulatory proteins, and we also identified a novel role of the PBAC1 proteasome assembly chaperone in seed coat development.