Modulation of the PA28α-20S proteasome interaction by a peptidyl alcohol

The peptidyl alcohol N-benzyloxycarbonyl-Ile-Glu(O-t-Bu)-Ala-leucinol is a mild activator of the chymotrypsin-like activity of the proteasome. When added to an incubation mixture of recombinant PA28α plus 20S proteasome the peptidyl alcohol antagonizes the stimulation of the chymotrypsin-like activity by PA28α in a dose-dependent manner (IC₅₀ = 30 μM). This effect is selective for the chymotrypsin-like activity. Stimulation of the peptidyl- glutamyl peptide bond hydrolyzing activity of the proteasome by PA28α is not affected by the peptidyl alcohol. The ovalbumin immunodominant epitope SIINFEKL is hydrolyzed by the PA28α-activated 20S proteasome to SIINF and SIINFE in approximately equimolar amounts. Addition of the peptidyl alcohol to an incubation mixture of PA28α, 20S proteasome and SIINFEKL shifts the ratio of products in favor of SIINFE. A similar shift in favor of postglutamyl cleavages occurs with the extended peptide LEQLESIINFEKLTE. By altering the ratio of products produced by the PA28α-activated proteasome, the peptidyl alcohol acts as a proteasome modulator. Proteasome modulators represent a novel class of molecules with a potential for altering the processing of antigens by the PA28-proteasome complex for presentation by the MHC class I system.