In mammalian cells proteasomes can be activated by two different types of regulatory complexes which bind to the ends of the proteasome cylinder. Addition of two 19S (PA700; ATPase) complexes forms the 26S proteasome, which is responsible for ATP-dependent non-lysosomal degradation of intracellular proteins, whereas 11S complexes (PA28; REG) have been implicated in antigen processing. The PA28 complex is upregulated in response to γ-interferon (γ-IFN) as are three non-essential subunits of the 20S proteasome. In the present study we have investigated the effects of γ-IFN on the level of different proteasome complexes and on the phosphorylation of proteasome subunits. After treatment of cells with γ-IFN, the level of 26S proteasomes decreased and there was a concomitant increase in PA28Őproteasome complexes. However, no free 19S regulatory complexes were detected. The majority of the γ-IFN-inducible proteasome subunits LMP2 and LMP7 were present in PA28Őproteasome complexes, but these subunits were also found in 26S proteasomes. The level of phosphorylation of both 20S and 26S proteasome subunits was found to decrease after γ-IFN treatment of cells. The C8 alpha subunit showed more than a 50% decrease in phosphorylation, and the phosphorylation of C9 was only barely detectable after γ-IFN treatment. These results suggest that association of regulatory components to 20S proteasomes is regulated, and that phosphorylation of proteasome alpha subunits may be one mode of regulation.

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