Identification of a protein kinase which phosphorylates a subunit of the 26S proteasome and changes in its activity during meiotic cell cycle in goldfish oocytes

The proteasome is involved in the progression of the meiotic cell cycle in fish oocytes. We reported that the α4 subunit of the 26S proteasome, which is a component of the outer rings of the 20S proteasome, is phosphorylated in immature oocytes and dephosphorylated in mature oocytes. To investigate the role of the phosphorylation, we purified the protein kinase from immature oocytes using a recombinant α4 subunit as substrate. A protein band which well corresponded to the kinase activity was identified as casein kinase Iα (CKIα). Two-dimensional (2D) PAGE analysis showed that part of the α4 subunit was phosphorylated by CKIα in vitro. This spot was detected in purified immature 26S proteasome but not in mature 26S proteasome, demonstrate that the α4 subunit is phosphorylated by CKIα meiotic cell cycle dependently.