Withaferin A (WA) is a steroidal lactone purified from medicinal plant "Indian Winter Cherry" that is widely researched for its variety of properties, including antitumor effects. However, the primary molecular target of WA is unknown. By chemical structure analysis, we hypothesized that Withaferin A might be a natural proteasome inhibitor. Computational modeling studies consistently predict that C1 and C24 of WA are highly susceptible toward a nucleophilic attack by the hydroxyl group of N-terminal threonine of the proteasomal chymotrypsin subunit beta5. Furthermore, WA potently inhibits the chymotrypsin-like activity of a purified rabbit 20S proteasome (IC50=4.5 microM) and 26S proteasome in human prostate cancer cultures (at 5-10 microM) and xenografts (4-8 mg/kg/day). Inhibition of prostate tumor cellular proteasome activity in cultures and in vivo by WA results in accumulation of ubiquitinated proteins and three proteasome target proteins (Bax, p27, and IkappaB-alpha) accompanied by androgen receptor protein suppression (in androgen-dependent LNCaP cells) and apoptosis induction. Treatment of WA under conditions of the aromatic ketone reduction, or reduced form of Celastrol, had significantly decreased the proteasome-inhibitory and apoptosis-inducing activities. Treatment of human prostate PC-3 xenografts with WA for 24 days resulted in 70% inhibition of tumor growth in nude mice, associated with 56% inhibition of the tumor tissue proteasomal chymotrypsinlike activity. Our results demonstrate that the tumor proteasome beta5 subunit is the primary target of WA, and inhibition of the proteasomal chymotrypsin-like activity by WA in vivo is responsible for, or contributes to, the antitumor effect of this ancient medicinal compound.