Phosphorylation of proteasomes in mammalian cells

20S proteasomes are large (700 kDa) proteinase complexes which form the cen tral catalytic core of larger complexes (26S proteasomes or PA28-20S comple xes) formed by association with regulatory particles. These larger complexe s are involved in diverse regulatory processes in the cell including cyclin breakdown, proteolytic control of transcription factors and other short-li ved regulatory proteins, and antigen presentation. In order to carry out th ese diverse functions the proteasome complexes must be held under tight reg ulatory control. The early recognition of potential phosphorylation sites i n a number of core and regulatory subunits suggested that some control of t he complexes activities may be via phosphorylation. We have investigated th e role of phosphorylation in determining proteasome localization, activitie s and association with regulatory complexes.