PHOSPHORYLATION OF THE PROTEASOME ACTIVATOR PA28 IS REQUIRED FOR PROTEASOME ACTIVATION

PA28, also referred to as 11S regulator, is a potent activator of the peptidase activities of the proteasome (multicatalytic proteinase comp lex). Although the role(s) of PA28-20S proteasome complexes in cellula r proteolytic processes remain to be defined, these particles have bee n implicated in antigen processing of major histocompatibility complex (MHC) class I molecules. Our results demonstrate that PA28 is phospho rylated as evidenced by P-32 incorporation into a single PA28 species in rabbit reticulocytes. In reticulocytes as well as human erythrocyte s, PA28 is normally found in a phosphorylated state as detected by pho sphoserine antibody. In human erythrocytes, this antibody recognizes t hree polypeptides which are also detected by antibody to PA28 on Weste rn blot analysis. Dephosphorylation with alkaline phosphatase treatmen t completely abolishes the ability of PA28 to activate hydrolysis of S uc-Leu-Leu-Val-Tyr by proteasomes. After exposure to phosphatase, the three polypeptides are no longer recognized by phosphoserine antibody, although binding to PA28 antibody is unaffected. These results sugges t that phosphorylation may function in transduction of cytokine and gr owth factor signals that, in turn, modulate antigen presentation and o ther processes which involve PA28-20S proteasome complexes. (C) 1996 A cademic Press, Inc.