Phosphorylation by COP9 Signalosome-Associated CK2 Promotes Degradation of p27 during the G1 Cell Cycle Phase

摘要

The cell cycle regulator p27~(Kip1)(p27)is controlled by 26S proteasome-mediated proteolysis by two different pathways.From the S till the G2 phase of the cell cycle,degradation of p27 takes place in the nucleus and is initiated by CDK2-dependent phosphorylation of threonine 187 with subsequent ubiquitination by the SCF~(Skp2)ubiquitin ligase.During the G1 cell cycle phase(G1),p27 breakdown is cytosolic and is initiated by nuclear export with subsequent ubiquitination by a RING finger ligase called kip1 ubiquitination complex.Here we show that the COP9 signalosome(CSN)is a regulator of p27 proteolysis during G1.The CSN interacts with p27 and the CSN-associated kinase CK2 phosphorylates p27 at two regions.One is central to the protein(amino acids 101-113),and the other was mapped near to the C-termirius(amino acids 170-189).Elimination of the putative C-terminal phosphorylation sites stabilizes ectopic p27 towards proteasomal degradation and abolishes CSN-p27 binding.Inhibition of CSN-associated kinase activity by curcumin attenuates loss of p27 upon cell cycle re-entry.Similar but not additive effects of the phosphoinositol-3-kinase blocker LY 290042 may point to a common pathway of CSN-associated CK2 and protein kinase B/Akt(Akt)in regulating p27 abundance.Akt is found in Flag pulldowns of lysates obtained from cells permanently expressing Flag-tagged CSN2,indicating that Akt is a novel kinase associated with the CSN.Thus,the CSN seems to regulate p27 proteolysis at G1 downstream of Ras-mediated signal pathways.