Vacuolar Turnover of rRNA Mediated by An Autophagy‐Dependent Mechanism Is Necessary To Maintain Cellular Homeostasis

Abstract

Ribosomes are major components of all cells. Much is understood about their biosynthesis and several pathways have been characterized for the elimination of misassembled ribosomal subunits. However, little is known about the decay of stable ribosomes and recycling of rRNA. RNase T2 ribonucleases represent one of the most widely distributed RNase families, and they are conserved in all eukaryotes, many prokaryotes, and some viruses. Our work showed that RNS2, a class II RNase T2 in Arabidopsis thaliana, is essential for normal ribosomal RNA decay. Null rns2‐2 plants show constitutive autophagy, RNA accumulation in the vacuole, and longer rRNA half‐life. Autophagosomes and autophagic bodies in rns2‐2 mutants contain RNA and ribosomes, suggesting that autophagy is activated as an attempt to compensate for loss of rRNA degradation. Total RNA levels increase in the rns2‐2, in the autophagy mutants atg9‐4 and atg5‐1 which are deficient in core autophagy machinery, and in rns2‐2atg9‐4 and rns2‐2atg5‐1 double mutants. Interestingly, rns2‐2atg5‐1 had significantly higher total RNA than the single mutants and the rns2‐2atg9‐4 double mutant. These results suggest that, together with RNS2, autophagy has a role in RNA turnover. In addition, rRNA accumulates in the vacuole in rns2‐2 mutants. Vacuolar accumulation of rRNA was blocked by disrupting autophagy via an rns2‐2atg5‐1 double mutant but not by an rns2‐2atg9‐4 double mutant. Furthermore, atg5‐1 vacuoles had significantly lower amounts of rRNA than WT or atg9‐4. Thus, it is clear that ATG5 and ATG9 have different roles in the turnover of rRNA. Our results suggest that an ATG5‐dependet selective autophagic process transport rRNA to the vacuole for degradation by RNS2. Disruption of this process in the rns2‐2 mutant triggers general macroautophagy which depends on ATG9 and ATG5, as a compensatory mechanism to continue the regular turnover of ribosomes and to re‐establish cellular homeostasis.Support or Funding InformationThis work was supported by grant no. MCB‐1051818 from the United States National Science Foundation to G.C.M. and D.C.B.