Phepropeptin A, a microbial secondary metabolite, is a proteasome inhibitor with an IC50 of 21 μg/mL[1].
References
[1] Sekizawa R, et al. Isolation and structural determination of phepropeptins A, B, C, and D, new proteasome inhibitors, produced by Streptomyces sp. J Antibiot (Tokyo). 2001 Nov;54(11):874-81. DOI:10.7164/antibiotics.54.874
