- We have constructed expression plasmids carrying protein kinase C (PKC) cDNAs with deletions in the coding region. Two truncated molecules, consisting only of the kinase domain of PKC-α, were generated by removing parts of the cDNA coding for the regulatory region. Another mutant molecule was created by deleting 95 amino acids from the C-terminal part of the molecule. The full-length cDNA coding for PKC-α and its deletion constructs was expressed in COS cells. Using cell fractionation experiments and immunofluorescence staining, we demonstrate here that in contrast to the cytosolic localization of full-length PKC-α, the truncated forms, coding only for the kinase domain, were found exclusively in the cell nucleus. Further subfractionation of nuclei isolated from these transfected cells indicated partial association with the nuclear envelopes. Expression of the cDNA lacking the C-terminal part of the molecule in COS cells encoded a truncated molecule that was found both in the cytosol and in the nucleus. We also show that translocation of full-length PKC-α molecules to the cell nucleus occurred in response to phorbol ester treatment. Thus, it appears that accumulation of PKC-α in the nucleus results either by phorbol ester activation or by deletions of specific regions of the molecule. A molecular mechanism for the nuclear translocation of phorbol ester-activated PKC-α or its truncated molecules is suggested.