You are studying P120, a protein of 120 KDa that is continuously synthesized in eukaryotic cells. Its cellular concentration, however, is maintained very low because P120 has a very short half-life of 1 minute. You observe that treating cells with Bortezomib, a drug that blocks the 26S proteasome activity, results in a rapid increase of the cytoplasmic concentration of P120. Based on this result, which hypothesis would you propose to explain the control of the half-life of P120