Yeast Ribosomal Protein Rpl32 Modulates Ribosome Biogenesis, Ribosome Stability and the Cell Cycle*

How does the cell coordinate its two major activities of cell growth and cell division? To explore this, we have genetically depleted yeast ribosomal protein Rpl32 of the 60S ribosomal subunit, which is an essential protein for cell proliferation. 3-4 hours after Rpl32 depletion, the cell cycle arrests at G1. We have undertaken a kinetic analysis of the early cellular events to deduce the pathway from Rpl32 depletion to G1 arrest. Rpl32 depletion blocks pre-rRNA processing of the initial 35S pre-rRNA, thus preventing ribosomal biogenesis and nuclear export of 60S ribosomal subunits. Interestingly, the L25-GFP reporter transiently accumulates in a focal spot that resembles the nucleolar body/Cajal body. Amazingly, the inhibition of ribosome biogenesis in the nucleus is signalled to the cytoplasm where mature 18S and 25S rRNAs are degraded in a ribophagy-independent manner; Rpl32 protease-degradation uses de-ubiquitination. Nonetheless, the ribosomes that remain after degradation are sufficient for translation whose efficiency is unchanged through 6 hours after Rpl32 depletion, and the cell size and vacuole increase in size. The level of cyclin 1 mRNA is rapidly diminished after Rpl32 depletion and is a likely factor for the arrest of the cell cycle at G1.