Circadian regulation of macromolecular complex turnover and proteome renewal

Although energetically costly, maintaining protein homeostasis is indispensable for normal cellular function and long-term health. In mammalian cells and tissues, daily variation in global protein synthesis has been observed, but the changes in total protein levels are comparatively minimal. Here, we used several different pulse-labelling strategies to investigate the proteome-wide relationship between protein synthesis and abundance. We show that protein degradation varies in-phase with protein synthesis, facilitating rhythms in turnover rather than abundance. This results in rhythmic daily proteome renewal whilst minimising changes in proteome composition. Coupled rhythms in synthesis and turnover are especially salient to macromolecular protein complexes, in particular the ribosome, which we observed to nascently assemble with a 24-hour oscillation and no overall changes in ribosome abundance. Finally, circadian turnover and proteasomal degradation rhythms render cells and mice more sensitive to proteotoxic stress at specific times, potentially contributing to daily rhythms in the efficacy of proteasomal inhibitors against cancer. Our findings suggest that circadian rhythms function to minimise the bioenergetic cost of protein homeostasis through temporal consolidation of protein turnover.