Localization of Phosphorylated Forms of Bcl-2 in Mitosis : Co-Localization with Ki-67 and Nucleolin in Nuclear Structures and on Mitotic Chromosomes

Abstract

Bcl-2 phosphorylation is a normal physiological process occurring at mitosis or duringmitotic arrest induced by microtubule damaging agents. The consequences of Bcl-2phosphorylation on its function are still controversial. To better understand the role ofBcl-2 phosphorylation in mitosis, we studied the subcellular localization ofphosphorylated forms of Bcl-2. Immunofluorescence experiments performed insynchronized HeLa cells indicate for the first time that mitotic phosphorylated forms ofBcl-2 can be detected in nuclear structures in prophase cells together with nucleolin andKi-67. In later mitotic stages, as previously described, phosphorylated forms of Bcl-2are localized on mitotic chromosomes. In addition, we demonstrate that Bcl-2 in thesestructures is at least in part phosphorylated on the T56 residue. Then, coimmunoprecipitationexperiments reveal that, in cells synchronized at the onset ofmitosis, Bcl-2 is present in a complex with nucleolin, cdc2 kinase and PP1 phosphatase.Taken together, these data further support the idea that Bcl-2 could have a new functionat mitosis.