Proteasomal Degradation of Human Peptidyl Prolyl Isomerase Pin1-pointing Phospho Bcl2 toward Dephosphorylation

Abstract

Microtubule inhibitor-induced Bcl2 phosphorylation is detrimental to its antiapoptotic function. Phosphorylation of Bcl2 predominantly occurs on two serine residues (70 and 87) in cells arrested at G2-M phase by microtubule disarraying agents. Phospho Bcl2 can associate with a cis-trans peptidyl prolyl isomerase, Pini. Pini and its homologues are known to target the proline residue carboxyl terminal to the phosphorylated threonine or serine residue of mitotic phosphoproteins, such as Bcl2. However, it was not clear how an extranuclear protein could associate with nuclear Pini. The confocal images of the immunofluorescence studies employing phospho Bcl2-specific antibody developed in the laboratory demonstrated the translocation of phospho Bcl2 inside the nucleus. Interestingly, proteasomal degradation of Pini facilitates dephosphorylation of phospho Bcl2 due to longer exposure of Taxol. Here we show for the first time that proteasomal degradation of Pini is the key factor to determine the fate of phosphoforms of Bcl2. When Pini is degraded by proteasomes, phospho Bcl2 is converted to its native form. Thus, transient conformational change of Bcl2 due to association with peptidyl prolyl isomerase can contribute to irreversible apoptotic signaling.