The proteasome inhibitor PI31 competes with PA28 for binding to 20S proteasomes

PI31 is a previously described inhibitor of 20S proteasomes. Using recombin ant PI31 we have analyzed its effect on proteasomal hydrolyzing activity of short fluorogenic substrates and of a synthetic 40-mer polypeptide. In add ition, we investigated its influence on the activation of 20S proteasome by the proteasome activator PA28. PI31 inhibits polypeptide degradation alrea dy at concentrations which only partially inhibit fluorogenic substrate tur nover and immunosubunits do not influence the PI31 binding affinity. Furthe rmore our data demonstrate that PI31 is a potent competitor of PA28-mediate d activation. (C) 1999 Federation of European Biochemical Societies.