Protein kinase C-theta is specifically localized on centrosomes and kinetochores in mitotic cells

In this study we provide evidence that the protein kinase C (PKC)-theta iso enzyme is recruited on to the mitotic spindle in dividing murine erythroleu kaemia (MEL) cells and associates specifically with centrosome and kinetoch ore structures. None of the other PKC isoenzymes (-alpha, -delta, -epsilon, -mu and -zeta) expressed by MEL cells shows this localization on the mitot ic spindle. An identical subcellular distribution of PKC-theta is also obse rved in dividing murine P3 myeloma cells and human LAN-5 neuroblastoma cell s, indicating that this PKC isoenzyme interacts with the mitotic apparatus in mammalian cells. In phorbol-ester-treated non-growing MEL cells, a rapid change in the intracellular distribution of PKC-theta occurs. Under these conditions, PKC-theta is translocated from the nuclear to the cytosolic cel l compartment, an event that is accompanied by phosphorylation of the PKC-t heta molecule and is followed by its down-regulation. The recovery of cell growth capacity results in the concomitant reappearance of PKC-theta. Furth ermore, when MEL cells acquire the differentiated non-growing phenotype, th e level of PKC-theta is reduced to less than 5%, suggesting that this PKC i soenzyme is no longer required. We propose that, unlike other members of th e PKC family, PKC-theta may play a role in cell proliferation.