Regulation of proteasome complexes by gamma-interferon and phosphorylation

Proteasomes play a major role in non-lysosomal proteolysis and also in the processing of proteins for presentation by the MHC class I pathway. In anim al cells they exist in several distinct molecular forms which contribute to the different functions. 26S proteasomes contain the core 20S proteasome t ogether with two 19S regulatory complexes. Alternatively, PA28 complexes ca n bind to the ends of the 20S proteasome to form PA28-proteasome complexes and PA28-proteasome-19S hybrid complexes have also been described. Immunopr oteasome subunits occur in 26S proteasomes as well as in PA28-proteasome co mplexes. We have found differences in the subcellular distribution of the d ifferent forms of proteasomes. The gamma -interferon inducible PA28 alpha a nd beta subunits are predominantly located in the cytoplasm, while 19S regu latory complexes (present at significant levels only in 26S complexes) are present in the nucleus as well as in the cytoplasm. Immunoproteasomes are g reatly enriched at the endoplasmic reticulum (ER) where they may facilitate the generation of peptides for transport into the lumen of the ER. We have also investigated the effects of gamma -interferon on the levels and subce llular distribution of inducible subunits and regulator subunits. In each c ase gamma -interferon was found to increase the level but not to alter the distribution. Several subunits of proteasomes are phosphorylated including alpha subunits C8 (alpha7) and C9 (alpha3), and ATPase subunit S4 (rpt2). O ur studies have shown that gamma -interferon treatment decreases the level of phosphorylation of proteasomes. We have investigated the role of phospho rylation of C8 by casein kinase Pi by site directed mutagenesis. The result s demonstrate that phosphorylation at either one of the two sites is essent ial for the association of 19S regulatory complexes and that the ability to undergo phosphorylation at both sites gives the most efficient incorporati on of C8 into the 26S proteasome. (C) 2001 Societe francaise de biochimie e t biologie moleculaire / Editions scientifiques et medicales Elsevier SAS.