NUCLEAR IMPORT OF PROTEIN-KINASE-C OCCURS BY A MECHANISM DISTINCT FROM THE MECHANISM USED BY PROTEINS WITH A CLASSICAL NUCLEAR-LOCALIZATIONSIGNAL

Protein kinase C does not have any known nuclear localization signal b ut, nevertheless, is redistributed from the cytoplasm to the nucleus u pon various stimuli. In NIH 3T3 fibroblasts stimulation with phorbol e ster leads to a translocation of protein kinase C alpha to the plasma membrane and into the cell nucleus. We compared the mechanism of prote in kinase C alpha's transport into the nucleus with the transport mech anism of a protein with a classical nuclear localization signal at sev eral steps. To this end, we co-microinjected fluorescently labeled bov ine serum albumin to which a nuclear localization signal peptide was c oupled, together with substances interfering with conventional nuclear protein import. Thereafter, the distribution of both the nuclear loca lization signal-bearing reporter protein and protein kinase C alpha wa s analyzed in the same cells. We can show that, in contrast to the nuc lear localization signal-dependent transport, the phorbol ester-induce d transport of protein kinase C alpha is not affected by microinjectio n of antibodies against the nuclear import factor p97/importin/karyoph erin beta or microinjection of nonhydrolyzable GTP-analogs. This sugge sts that nuclear import of protein kinase C alpha is independent of p9 7/importin/karyopherin beta and independent of GTP. At the nuclear por e there are differences between the mechanisms too, since nuclear tran sport of protein kinase C alpha cannot be inhibited by wheat germ aggl utinin 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 n uclear localization signal-bearing proteins at several stages.