Covalent modification of the active site threonine of proteasomal β subunits and theEscherichia colihomolog HslV by a new class of inhibitors

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Abstract
The proteasome is a multicatalytic protease complex that plays a key role in diverse cellular functions. The peptide vinyl sulfone, carboxybenzyl-leucyl-leucyl-leucine vinyl sulfone (Z-L3VS) covalently inhibits the trypsin-like, chymotrypsin-like and, unlike lactacystin, also the peptidylglutamyl peptidase activity in isolated proteasomes, and blocks their function in living cells. Although described as a class of mechanism-based inhibitors for cysteine proteases, the peptide vinyl sulfone Z-L3VS and a125I-labeled nitrophenol derivative (125I-NIP-L3VS) covalently modify the active site threonine of the catalytic β subunits of the proteasome. Modification of Thermoplasma proteasomes demonstrates the requirement for a hydroxyl amino acid (threonine, serine) as nucleophile at the β subunit’s NH2terminus.125I-NIP-L3VS covalently modifies the HslV subunit of theEscherichia coliprotease complex HslV/HslU, a reaction that requires ATP, and supports a catalytic mechanism shared with that of the eukaryotic proteasome.