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

AbstractPI31 is a previously described inhibitor of 20S proteasomes. Using recombinant PI31 we have analyzed its effect on proteasomal hydrolyzing activity of short fluorogenic substrates and of a synthetic 40-mer polypeptide. In addition, we investigated its influence on the activation of 20S proteasome by the proteasome activator PA28. PI31 inhibits polypeptide degradation already at concentrations which only partially inhibit fluorogenic substrate turnover and immunosubunits do not influence the PI31 binding affinity. Furthermore our data demonstrate that PI31 is a potent competitor of PA28-mediated activation

A third interferon-gamma-induced subunit exchange in the 20S proteasome

The 20S proteasome is a protease complex of functional importance for antigen processing. Two of the 14 proteasome subunits, delta and MB1, can be replaced by the major histocompatibility complex (MHC)-encoded and interferon-gamma (IFN-gamma)-inducible subunits LMP2 and LMP7, respectively. LMP2 and LMP7 alter the cleavage site specificity of the 20S proteasome and are required for the efficient generation of T cell epitopes from a number of viral proteins and for optimal MHC class I cell surface expression. We compared the 20S proteasome subunit pattern from IFN-gamma-induced and non-induced mouse fibroblasts on two-dimensional gels and identified a third subunit exchange by microsequencing: the non-MHC-encoded subunit MECL-1 is induced by IFN-gamma and replaces a sofar barely characterized beta subunit designated 'MC14'. In analogy to LMP2 and LMP7, MECL-1 may be functional in MHC class I-restricted antigen presentation