Hela cells proteasome interacts with leucine-rich polypeptides and contains a phosphorylated subunit

The proteasome is a multicatalytic proteinase complex composed of several non-identical protein subunits with molecular weights ranging from 20 to 35 kDa. To approach the mechanisms modulating the activity of this protease, we have investigated the possible interaction of this particle with specific polypeptides as well as the phosphorylation status of its subunits. A specific antiserum was used to immunoprecipitate this particle under native conditions. Three major polypeptides, characterized by molecular masses of 53, 59 and 77 kDa co-immunoprecipitated specifically with the proteasome. Labelling experiments indicated that these proteins are leucine-rich and contain very few methionine residues. None of them were phosphorylated in vivo in normal cell growth conditions, in contrast to one of the proteasome subunit (30 kDa). These results indicate that, in vivo, the proteasome is probably associated with leucine-rich polypeptides and that this protease is a kinase substrate.