gamma-Interferon decreases the level of 26 S proteasomes and changes the pattern of phosphorylation

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