A cell-permeable, potent and selective proteasome inhibitor. A Streptomyces metabolite that is thought to bind irreversibly to the active site N-terminal threonine residue of the catalytic β-subunit of the 20S proteasome, thereby inhibiting its chymotrypsin and trypsin-like activities. Induces neurite outgrowth in Neuro 2a neuroblastoma cells and has been reported to induce apoptosis in human monoblast U937 cells. Also inhibits NF-κB activation (IC50 = 10 μM). MW (observed): 376.42.