Figure 4. Proteasome inhibition regulates protein turnover rates.

The human bronchial epithelial cells (HBE) were treated for 2 hours (2 h, partial) or overnight (O/N, broad) with low dose proteasome inhibitor (MG-132, 1 µM) and accumulation of newly synthesized ubiquitinated protein was monitored over time by metabolic labeling and immunoprecipitation. Lower levels of ubiquitinated proteins in overnight MG132 treated cells during protein synthesis indicate that decreased proteasomal activity inhibits protein synthesis as a feedback inhibition loop. Lower proteasomal activity inhibits protein turnover rates (proteostasis-imbalance) by inhibiting both protein- synthesis and degradation.