Abstract
Abrogation of ribosome synthesis (ribosomal stress) leads to cell cycle arrest. However, the immediate cell response to cessation of ribosome formation and the transition from normal cell proliferation to cell cycle arrest have not been characterized. Furthermore, there are conflicting conclusions about whether cells are arrested in G2/M or G1, and whether the cause is dismantling ribosomal assembly per se, or the ensuing decreased number of translating ribosomes. To address these questions, we have compared the time kinetics of key cell cycle parameters after inhibiting ribosome formation or function in Saccharomyces cerevisiae. Within one-to-two hours of repressing genes for individual ribosomal proteins or Translation Elongation factor 3, configurations of spindles, spindle pole bodies began changing. Actin began depolarizing within 4 hours. Thus the loss of ribosome formation and function is sensed immediately. After several hours no spindles or mitotic actin rings were visible, but membrane ingression was completed in most cells and Ace2 was localized to daughter cell nuclei demonstrating that the G1 stage was reached. Thus cell division was completed without the help of a contractile actin ring. Moreover, cell wall material held mother and daughter cells together resulting in delayed cell separation, suggesting that expression or function of daughter gluconases and chitinases is inhibited. Moreover, cell development changes in very similar ways in response to inhibition of ribosome formation and function, compatible with the notion that decreased translation capacity contributes to arresting the cell cycle after abrogation of ribosome biogenesis. Potential implications for the mechanisms of diseases caused by mutations in ribosomal genes (ribosomopathies) are discussed.
Highlights
Ribosome biogenesis and cell cycle progression are both controlled by complicated networks
Translation capacity and the cell cycle promoter is completely repressed in glucose medium, while transcription of other r-proteins transcribed from the native promoters continues [41]
We have shown that decreased ribosomal translation rate by itself leads to G1 arrest
Summary
Ribosome biogenesis and cell cycle progression are both controlled by complicated networks Both processes feature hierarchical waves of proteins that have been studied extensively in Saccharomyces cerevisiae. As was originally shown for the assembly of bacterial ribosomes [4,5,6], the binding of r-proteins to precursor particles is hierarchical such that only a subset of proteins binds directly to the rRNA [7]. The binding of these primary proteins generates binding sites for a secondary wave etc. The time to build a ribosome in yeast is relatively short (about 10 minutes) [12] compared to the doubling time (about 90 minutes in rich glucose medium), but since the cell needs a large number of ribosomes in order to make sufficient protein for a new cell within a doubling time [13], a cell is building thousands of ribosome in parallel
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