Translational Activity Controls Ribophagic Flux and Turnover of Distinct Ribosome Pools

Abstract

Ribosomes are among the most abundant and complex machineries in the cell, however, the turnover 15 of their subunits remains poorly understood. Here, we apply proteomic flux and cryo-electron 16 microscopy analyses to interrogate the ribosome life cycle in human cells. We show that subpopulations 17 of ribosomal subunits coexist, which vary in turnover kinetics and structure. Specifically, 80S ribosomes 18 have a much longer half-life than free 40S and 60S ribosomal subunits, indicating that they represent 19 distinct subunit pools that poorly intermix. Translation inhibition starkly increases the pool-size of 80S 20 ribosomes in a translationally idle state and induces ribophagy of old ribosomes, ultimately rejuvenating 21 the ribosome fleet. Our findings provide a comprehensive model for ribosome turnover and its regulation 22 via translational activity.