Cell-cycle-dependent cortical localization of pEg3 protein kinase in Xenopus and human cells

Protein kinase pEg3 belongs to the evolutionarily conserved KIN1/PAR-1/MARK family, whose members are involved in a variety of functions, including cell polarity, microtubule stability, intracellular signalling and the cell cycle. Activity and phosphorylation of pEg3 are cell-cycle dependent and rise to maximum levels during mitosis. pEg3 was shown to interact with and phosphorylate phosphatase CDC25B, and to potentially control cell-cycle progression. Subcellular localization of pEg3 was investigated in Xenopus and human cultured cells.

By expression of GFP (green fluorescent protein)-tagged pEg3 and indirect immunofluorescence with specific antibodies, pEg3 was found to be localized in the cytoplasm and the nucleus in interphase cells. During mitosis pEg3 was also found in the cytoplasm. From anaphase to telophase, a proportion of the protein was detected at the cell cortex. The cortical distribution in mitotic cells was dependent on F-actin, because the actin-depolymerization-inducing drugs cytochalasin D or latrunculin A prevented pEg3 cortical localization. The protein lacking the conserved C-terminal domain was not detected at the cell cortex, whereas the C-terminal domain was targeted to the cell periphery. In contrast with full-length pEg3, the cortical localization of the C-terminal domain and construct lacking the N-terminal domain was cell-cycle independent, and these constructs were found at the cell periphery in interphase cells.

pEg3 is localized at the cell periphery specifically during mitosis. The C-terminal domain is the only pEg3 domain found to be necessary and sufficient for cortical targeting. Cortical distribution of pEg3 also requires the F-actin cytoskeleton. The cell-cycle-independent cortical localization of the pEg3 C-terminal domain and a construct lacking the N-terminal domain indicates that a negative control mechanism involving the pEg3 catalytic N-terminal domain probably acts to prevent pEg3 cortical distribution during interphase. These results suggest that pEg3 might play a role at the cell cortex during mitosis.