Abstract
BackgroundThe 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.ResultsHere, 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.ConclusionsOur 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.
Highlights
The 26S proteasome, canonically composed of multi-subunit 19S regulatory (RP) and 20S core (CP) particles, is crucial for cellular proteostasis
Many of the proteasome subunits are encoded by two paralogous genes and they can be very differently expressed under the same type of abiotic stress
RPT2a is induced while RPT2b is repressed under osmotic stress, and RPT5b is strongly induced while RPT5a is only slightly upregulated under various abiotic stresses
Summary
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. Plants are sessile and must adapt to and develop in fluctuating environmental conditions. Such a feat is possible partly due to the complicated and stringently regulated UPS that is involved in most cellular processes including embryogenesis, growth, senescence, hormone signalling and abiotic stress response [60, 61]. Recent studies in yeast and animals showed that the 26S proteasome macromolecule structure is highly dynamic, and its controlled and modular assembly may play vital roles in regulating protein degradation in rapid response to stresses [11, 24]
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