The 26S proteasome is an abundant multi-subunit complex, which, in addition to lysosomes, represents a major cellular "protein degradation factory". The proteasome complex possesses protease, ATPase/helicase, and RNAse enzymatic activities, which are used by the latter to regulate various physiological processes. Recent findings have revealed an important role of proteasomes in transcriptional regulation. Although proteasomes are well documented to undergo various post-translational modifications, little is known about their functional significance, in particular in the process of gene regulation in response to various forms of stress. Here, we review the data on the role of proteasomes in gene regulation and their post-translational modifications as well as discuss potential mechanisms by which proteasomal activity may be regulated by genotoxic stress.

26S proteasome is a large multi-subunit protein complex involved in proteolytic degradation of proteins. In addition to its canonical proteolytic activity, the proteasome is also associated with recently characterized endoribonuclease (endo-RNAse) activity. However, neither functional significance, nor the mechanisms of its regulation are currently known. In this report, we show that 26S proteasome is able to hydrolyze various cellular RNAs, including AU-rich mRNA of c-myc and c-fos. The endonucleolytic degradation of these mRNAs is exerted by one of the 26S proteasome subunits, PSMA5 (α5). The RNAse activity of 26S proteasome is differentially affected by various extra-cellular signals. Moreover, this activity contributes to the process of degradation of c-myc mRNA during induced differentiation of K562 cells, and may be controlled by phosphorylation of the adjacent subunits, PSMA1 (α6) and PSMA3 (α7). Collectively, the data presented in this report suggest a causal link between cell signalling pathways,...

A large group of prion‐associated proteins was identified in yeast cells using a new approach, comparative analysis of pellet proteins of crude cell lysates in isogenic strains of Saccharomyces cerevisiae differing by their prion composition. Two‐dimensional (2D) electrophoresis followed by MALDI analysis of the pellet proteins of [PSI⁺] and [psi⁻] strains after prion elimination by GuHCl and prion transmission by cytoduction permitted identification of ca. 40 proteins whose aggregation state correlated with the change of prion(s) content. Approximately half of these proteins belonged to chaperones and to enzymes of glucose metabolism. Chaperones are known to be involved in prion metabolism and are expected to be present in prion‐containing aggregates, but glucose metabolism enzymes are not predicted to be present. Nevertheless, several recent data suggest that their presence is not incidental. We detected six proteins involved in oxidative stress response and eight in translation. Also notable is a protease. Most of the identified proteins seem to be prion‐associated, but we cannot exclude the possibility that several proteins may propagate as prions. Copyright © 2009 John Wiley & Sons, Ltd.

The intensity of amyloid‐bound thioflavine T fluorescence was studied in crude lysates of yeast strains carrying mutations in the ADE1 or ADE2 genes and accumulating the red pigment (a result of polymerization of aminoimidazoleribotide), and in white isogenic strains–either adenine prototrophs or carrying mutations at the first stages of purine biosynthesis. We found that the red pigment leads to a drop of amyloid content. This result, along with the data on separation of protein polymers of white and red strains in PAGE, suggests that the red pigment inhibits amyloid fibril formation. The differences in transmission of the thioflavine T fluorescence pattern by cytoduction and in blot‐hybridization of pellet proteins of red and white [PSI⁺] strains with Sup35p antibodies confirmed this conclusion. Purified red pigment treatment also led to a decrease of fluorescence intensity of thioflavine T bound to insulin fibrils and to yeast pellet protein aggregates from [PSI⁺] strains. This suggests red pigment interaction with amyloid fibrils. Comparison of pellet proteins from red and white isogenic strains separated by 2D‐electrophoresis followed by MALDI analysis has allowed us to identify 48 pigment‐dependent proteins. These proteins mostly belong to functional classes of chaperones and proteins involved in glucose metabolism, closely corresponding to prion‐dependent proteins that we characterized previously. Also present were some proteins involved in stress response and proteolysis. We suppose that the red pigment acts by blocking certain sites on amyloid fibrils that, in some cases, can lead in vivo to interfere with their contacts with chaperones and the generation of prion seeds. Copyright © 2011 John Wiley & Sons, Ltd.