A 'garbage can' for ribosomes: how eukaryotes degrade their ribosomes

Ribosome synthesis is a major metabolic activity that involves hundreds of individual reactions, each of which is error-prone. Ribosomal insults occur in cis (alteration in rRNA sequences) and in trans (failure to bind to, or loss of, an assembly factor or ribosomal protein). In addition, specific growth conditions, such as starvation, require that excess ribosomes are turned over efficiently. Recent work indicates that cells evolved multiple strategies to recognize specifically, and target for clearance, ribosomes that are structurally and/or functionally deficient, as well as in excess. This surveillance is active at every step of the ribosome synthesis pathway and on mature ribosomes, involves nearly entirely different mechanisms for the small and large subunits, and requires specialized subcellular organelles.