Complex with MG-132. The 150 µM protein solution was incubated with the MG-132 inhibitor in non-saturating conditions, namely 1:1 and 1:2 LMS following our standard protocol to obtain crystals in space group P212121 with the entire dimer/a.u.. The two binding sites of our resulting crystal structures, solved at 1.85 Å and 1.60 Å resolution respectively (PDBid: 8P55, and 8P54), showed clear dissimilarities: The difference electron density map of one subunit showed a continuous positive electron density that well fit the MG-132 moiety, while in the other subunit no residual electron density was present, suggesting an empty pocket (Fig. ). Even after refinement, no further density appeared in the second binding site (Fig. ). This establishes single occupancy of the ligand.