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 r egulatory complexes which bind to the ends of the proteasome cylinder. Addi tion of two 19 S (PA700; ATPase) complexes forms the 26 S proteasome, which is responsible for ATP-dependent non-lysosomal degradation of intracellula r proteins, whereas 11 S complexes (PA28; REG) have been implicated in anti gen processing. The PA28 complex is upregulated in response to gamma -inter feron (gamma -IFN) as are three non-essential subunits of the 270 S proteas ome. 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-pr oteasome complexes. However. no free 19 S regulatory complexes were detecte d. The majority of the gamma -IFN-inducible proteasome subunits LMP2 and LM P7 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 phosphoryla tion, and the phosphorylation of C9 was only barely detectable after gamma -IFN treatment. These results suggest that association of regulatory compon ents to 20 S proteasomes is regulated, and that phosphorylation of proteaso me alpha subunits may be one mode of regulation.