Phosphorylation of proteasomes in mammalian cells.

20S proteasomes are large (700 kDa) proteinase complexes which form the central catalytic core of larger complexes (26S proteasomes or PA28-20S complexes) formed by association with regulatory particles. These larger complexes are involved in diverse regulatory processes in the cell including cyclin breakdown, proteolytic control of transcription factors and other short-lived regulatory proteins, and antigen presentation. In order to carry out these diverse functions the proteasome complexes must be held under tight regulatory control. The early recognition of potential phosphorylation sites in a number of core and regulatory subunits suggested that some control of the complexes activities may be via phosphorylation. We have investigated the role of phosphorylation in determining proteasome localization, activities and association with regulatory complexes.