A potent, reversible, and cell-permeable proteasome inhibitor. Initiates neurite outgrowth in PC12 cells with an optimal concentration of 10 µM. Reduces the degradation of ubiquitin-conjugated proteins in mammalian cells and permeable strains of yeast by the 26S complex without affecting its ATPase or isopeptidase activities. Blocks cleavage of poly(ADP-ribose) polymerase and apoptosis in thymocytes. It activates JNK-1, responsible for apoptosis initiation in response to cell stress.