Enzymes degraded under high light maintain proteostasis by transcriptional regulation in Arabidopsis.

Photoinhibitory high light stress in Arabidopsis leads to increases in markers of protein degradation and transcriptional up-regulation of proteases and proteolytic machinery, but proteostasis is largely maintained. We find significant increases in the in vivo degradation rate for specific molecular chaperones, nitrate reductase, glyceraldehyde-3 phosphate dehydrogenase, and phosphoglycerate kinase and other plastid, mitochondrial, peroxisomal, and cytosolic enzymes involved in redox shuttles. Coupled analysis of protein degradation rates, mRNA levels, and protein abundance reveal that 57% of the nuclear-encoded enzymes with higher degradation rates also had high light­induced transcriptional responses to maintain proteostasis. In contrast, plastid-encoded proteins with enhanced degradation rates showed decreased transcript abundances and must maintain protein abundance by other processes. This analysis reveals a light-induced transcriptional program for nuclear-encoded genes, beyond the regulation of the photosystem II (PSII) D1 subunit and the function of PSII, to replace key protein degradation targets in plants and ensure proteostasis under high light stress.

Particle radiation-induced dysregulation of protein homeostasis in primary human and mouse neuronal cells.

Space particle radiations may cause significant damage to proteins and oxidative stress in the cells within the central nervous system and pose a potential health hazard to humans in long-term manned space explorations. Dysregulation of the ubiquitin-proteasome system as evidenced by abnormal accumulation of polyubiquitin (pUb) chain linkages has been implicated in several age-related neurodegenerative disorders by mechanisms that may involve the inter-neuronal spread of toxic misfolded proteins, the induction of chronic neuroinflammation, or the inappropriate inhibition or activation of key enzymes, which could lead to dysfunction in, for example, proteolysis, or the accumulation of post-translationally-modified substrates.In this study, we employed a quantitative proteomics method to evaluate the impact of particle-radiation induced alterations in three major pUb-linked chains at lysine residues Lys-48 (K-48), Lys-63 (K-63), and Lys-11 (K-11), and probed for global proteomic changes in mouse and human neural cells that were irradiated with low doses of 250 MeV proton, 260 MeV/u silicon or 1 GeV/u iron ions. We found significant accumulation in K-48 linkage after 1 Gy protons and K-63 linkage after 0.5 Gy iron ions in human neural cells. Cells derived from different regions of the mouse brain (cortex, striatum and mesencephalon) showed differential sensitivity to particle radiation exposure. Although none of the linkages were altered after proton exposure, both K-48 and K-63 linkages in mouse striatal neuronal cells were elevated after 0.5 Gy of silicon or iron ions. Changes were also seen in proteins commonly used as markers of neural progenitor and stem cells, in DNA binding/damage repair and cellular redox pathways. In contrast, no significant changes were observed at the same time point after proton irradiation. These results suggest that the quality of the particle radiation plays a key role in the level, linkage and cell type specificity of protein homeostasis in key populations of neuronal cells.

CSNAP, the smallest CSN subunit, modulates proteostasis through cullin-RING ubiquitin ligases.

The cullin-RING ubiquitin E3 ligase (CRL) family consists of ~250 complexes that catalyze ubiquitylation of proteins to achieve cellular regulation. All CRLs are inhibited by the COP9 signalosome complex (CSN) through both enzymatic (deneddylation) and nonenzymatic (steric) mechanisms. The relative contribution of these two mechanisms is unclear. Here, we decouple the mechanisms using CSNAP, the recently discovered ninth subunit of the CSN. We find that CSNAP reduces the affinity of CSN toward CRL complexes. Removing CSNAP does not affect deneddylation, but leads to global effects on the CRL, causing altered reproductive capacity, suppressed DNA damage response, and delayed cell cycle progression. Thus, although CSNAP is only 2% of the CSN mass, it plays a critical role in the steric regulation of CRLs by the CSN.

Mechanisms of Photodamage and Protein Turnover in Photoinhibition.

Rapid protein degradation and replacement is an important response to photodamage and a means of photoprotection by recovering proteostasis. Protein turnover and translation efficiency studies have discovered fast turnover subunits in cytochrome b6f and the NAD(P)H dehydrogenase (NDH) complex, in addition to PSII subunit D1. Mutations of these complexes have been linked to enhanced photodamage at least partially via cyclic electron flow. Photodamage and photoprotection involving cytochrome b6f, NDH complex, cyclic electron flow, PSI, and nonphotochemical quenching proteins have been reported. Here, we propose that the rapid turnover of specific proteins in cytochrome b6f and the NDH complex need to be characterised and compared with the inhibition of PSII by excess excitation energy and PSI by excess electron flux to expand our understanding of photoinhibition mechanisms.