p34^cdc2-mediated phosphorylation mobilizes microtubule-organizing centers from the apical intermediate filament scaffold in CACO-2 epithelial cells

We have shown previously that centrosomes and other microtubule-organizing centers (MTOCs) attach to the apical intermediate filament (IF) network in CACO-2 cells. In this cell line, intermediate filaments do not disorganize during mitosis. Therefore, we speculated that the trigger of the G₂-M boundary may also detach MTOCs from their IF anchor. If that was the case, at least one of the proteins involved in the attachment must be phosphorylated by p34^cdc2 (cdk1). Using confocal microscopy and standard biochemical analysis, we found that p34^cdc2-mediated phosphorylation indeed released MTOCs from IFs in permeabilized cells. In isolated, immunoprecipitated multiprotein complexes containing both γ-tubulin and cytokeratin 19, p34^cdc2 phosphorylated only one protein, and phosphorylation released cytokeratin 19 from the complexes. We conclude that this as yet unidentified protein is a part of the molecular mechanism that attaches MTOCs to IFs in interphase.