Modulation of the PA28 alpha-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 ad ded to an incubation mixture of recombinant PA28 alpha plus 20S proteasome the peptidyl alcohol antagonizes the stimulation of the chymotrypsin-like a ctivity by PA28 alpha in a dose-dependent manner (IC50 = 30 mu M). This eff ect is selective for the chymotrypsin-like activity. Stimulation of the pep tidyl-glutamyl peptide bond hydrolyzing activity of the proteasome by PA28 alpha is not affected by the peptidyl alcohol. The ovalbumin immunodominant epitope SIINFEKL is hydrolyzed by the PA28 alpha-activated 20S proteasome to SIINF and SIINFE in approximately equimolar amounts. Addition of the pep tidyl alcohol to an incubation mixture of PA28 alpha, 20S proteasome and SI INFEKL, shifts the ratio of products in favor of SIINFE. A similar shift in favor of postglutamyl cleavages occurs with the extended peptide LEQLESIIN FEKLTE. By altering the ratio of products produced by the PA28 alpha-activa ted proteasome, the peptidyl alcohol acts as a proteasome modulator. Protea some modulators represent a novel class of molecules with a potential for a ltering the processing of antigens by the PA28-proteasome complex for prese ntation by the MHC class I system. (C) 1999 Academic Press.