STRUCTURAL INVESTIGATION OF PROTEASOME INHIBITION

The novel proteolytic mechanism of the 20S proteasome from T. acidophi lum has been investigated by X-ray crystallography using small-molecul e inhibitors and substrate analogues. The 20S proteasome degrades unfo lded substrates into small peptides of a defined length. Calpain inhib itor II, chymostatin and lactacystin all bind in the previously identi fied active site pocket near Thr1 of all fourteen beta-subunits. The c hromogenic substrate analogue Suc-LLVY-AMC binds in the same pocket of the proteolytically inactive T1A mutant of the beta-subunit, but with a significantly altered geometry. The heavy-atom cluster Ta6Br122+ us ed in X-ray structure determination occupies seven sites in the inner compartment of the proteasome and exhibits inhibition of the chymotryp sin-like activity. Other effecters of proteasome activity showed no si gnificant difference in electron density.