Nuclear import of protein kinase C occurs by a mechanism distinct from the mechanism used by proteins with a classical nuclear localization signal

Protein kinase C does not have any known nuclear localization signal but, nevertheless, is redistributed from the cytoplasm to the nucleus upon various stimuli. In NIH 3T3 fibroblasts stimulation with phorbol ester leads to a translocation of protein kinase C alpha to the plasma membrane and into the cell nucleus. We compared the mechanism of protein kinase C alpha's transport into the nucleus with the transport mechanism of a protein with a classical nuclear localization signal at several steps. To this end, we co-microinjected fluorescently labeled bovine serum albumin to which a nuclear localization signal peptide was coupled, together with substances interfering with conventional nuclear protein import. Thereafter, the distribution of both the nuclear localization signal-bearing reporter protein and protein kinase C alpha was analyzed in the same cells. We can show that, in contrast to the nuclear localization signal-dependent transport, the phorbol ester-induced transport of protein kinase C alpha is not affected by microinjection of antibodies against the nuclear import factor p97/importin/karyopherin beta or microinjection of non-hydrolyzable GTP-analogs. This suggests that nuclear import of protein kinase C alpha is independent of p97/importin/karyopherin beta and independent of GTP. At the nuclear pore there are differences between the mechanisms too, since nuclear transport of protein kinase C alpha cannot be inhibited by wheat germ agglutinin or an antibody against nuclear pore complex proteins. Together these findings demonstrate that the nuclear import of protein kinase C alpha occurs by a mechanism distinct from the one used by classical nuclear localization signal-bearing proteins at several stages.