Rapid Cytoplasmic Turnover of Yeast Ribosomes in Response to Rapamycin Inhibition of TOR

Abstract

The target of rapamycin (TOR) pathway is the central regulator of cell growth in eukaryotes. Inhibition of TOR by rapamycin elicits changes in translation attributed mainly to altered translation initiation and repression of the synthesis of new ribosomes. Using quantitative analysis of rRNA, we found that the number of existing ribosomes present in a Saccharomyces cerevisiae culture during growth in rich medium rapidly decreases by 40 to 60% when the cells are treated with rapamycin. This process is not appreciably affected by a suppression of autophagy, previously implicated in degradation of ribosomes in eukaryotes upon starvation. Yeast cells deficient in the exosome function or lacking its cytoplasmic Ski cofactors show an abnormal pattern of rRNA degradation, particularly in the large ribosomal subunit, and accumulate rRNA fragments after rapamycin treatment and during diauxic shift. The exosome and Ski proteins are thus important for processing of rRNA decay intermediates, although they are probably not responsible for initiating rRNA decay. The role of cytoplasmic nucleases in rapamycin-induced rRNA degradation suggests mechanistic parallels of this process to nutrient-controlled ribosome turnover in prokaryotes. We propose that ribosome content is regulated dynamically in eukaryotes by TOR through both ribosome synthesis and the cytoplasmic turnover of mature ribosomes.

The Saccharomyces cerevisiae protein Stm1p facilitates ribosome preservation during quiescence

“…It is well established that yeast entering quiescence lose a substantial percentage of their ribosomes, either through ribophagy or by alternative mechanisms [6,21]. However, while protein synthesis may drop $300-fold upon entry into quiescence [22], not all ribosomes are lost.…”

Purine auxotrophy: Possible applications beyond genetic marker

“…If environmental conditions change, old mRNA will be degraded and will provide source of nucleotides before relatively costly biosynthesis of purines or pyrimidines starts. Also, degradation of rRNA has been described as response during starvation to provide cells with additional energy and nitrogen sources (Pestov & Shcherbik, ; Xu et al, ). Therefore, we hypothesize that if the supply of new purines is halted (stopping de novo synthesis or purine supply from the media), the RNA nucleotide pool could form a source for purines in the cytoplasm.…”

MRF Family Genes Are Involved in Translation Control, Especially under Energy-Deficient Conditions, and Their Expression and Functions Are Modulated by the TOR Signaling Pathway

“…The authors proposed that ribosomes not engaged in translation, and consequently present as subunits, are sensitive to endoribonuclease cleavage and subsequent degradation (Zundel et al, 2009). Furthermore, TOR inactivation by rapamycin rapidly decreased the cellular ribosome numbers by 40 to 60% in yeast, correlated with rRNA degradation by cytoplasmic nucleases (Pestov and Shcherbik, 2012). These results suggest that TOR controls both new ribosome biosynthesis and degradation of mature ribosomes, in order to adjust the size of the translation machinery to changing environmental conditions.…”

“…Recently, it was reported that TOR inhibition by rapamycin triggers a rapid decrease (40-60%) in ribosome content in yeast, through rapid cytoplasmic turnover of the existing ribosomes (Pestov and Shcherbik, 2012). We performed virus-induced gene silencing (VIGS) using the tobacco rattle virus (TRV) system against TOR in MRF1 OE (#1) Arabidopsis plants, to analyze phosphorylation status and ribosome association of Flag:MRF1 in TOR-silenced leaf cells ( Figured 10B and 10C).…”