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 complex). Although the role(s) of PA28-20S proteasome complexes in cellular proteolytic processes remain to be defined, these particles have been implicated in antigen processing of major histocompatibility complex (MHC) class I molecules. Our results demonstrate that PA28 is phosphorylated as evidenced by 32P incorporation into a single PA28 species in rabbit reticulocytes. In reticulocytes as well as human erythrocytes, PA28 is normally found in a phosphorylated state as detected by phosphoserine antibody. In human erythrocytes, this antibody recognizes three polypeptides which are also detected by antibody to PA28 on Western blot analysis. Dephosphorylation with alkaline phosphatase treatment completely abolishes the ability of PA28 to activate hydrolysis of Suc-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 suggest that phosphorylation may function in transduction of cytokine and growth factor signals that, in turn, modulate antigen presentation and other processes which involve PA28-20S proteasome complexes.
Li
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N
Department of Cell Biology and Anatomy, New York Medical College, Valhalla 10595, USA. Li0@nymc.edu
Lerea
K M
KM
Etlinger
J D
JD
D000595:Amino Acid Sequence; D000818:Animals; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D004789:Enzyme Activation; D004912:Erythrocytes / Q000378:metabolism; D006801:Humans; D066298:In Vitro Techniques; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000378:metabolism*; D009124:Muscle Proteins; D009842:Oligopeptides / Q000737:chemistry; D010766:Phosphorylation; D046988:Proteasome Endopeptidase Complex; D011506:Proteins / Q000737:chemistry / Q000378:metabolism*; D011817:Rabbits; D012156:Reticulocytes / Q000378:metabolism; D013379:Substrate Specificity
D009097:Multienzyme Complexes; D009124:Muscle Proteins; D009842:Oligopeptides; C085047:PSME1 protein, human; D011506:Proteins; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex