The crystal structures of the yeast 20S proteasome core particle (CP) in complex with Salinosporamides A (NPI-0052; 1) and B (4) were solved at < 3 A resolution. Each ligand is covalently bound to Thr10(y) via an ester linkage to the carbonyl derived from the beta-lactone ring of the inhibitor. In the case of 1, nucleophilic addition to the beta-lactone ring is followed by addition of C-30 to the chloroethyl group, giving rise to a cyclic ether. The crystal structures were compared to that of the omuralide/CP structure solved previously, and the collective data provide new insights into the mechanism of inhibition and irreversible binding of 1. Upon opening of the beta-lactone ring, C-30 assumes the position occupied by a water molecule in the unligated enzyme and hinders deacylation of the enzyme-ligand complex. Furthermore, the resulting protonation state of Thr1NH(2) deactivates the catalytic N-terminus.