Nuclear import and export signals enable rapid nucleocytoplasmic shuttlingof the atypical protein kinase C lambda

The atypical protein kinase C (PKC) isoenzymes, lambda /L-and zeta PKC, pla y important roles in cellular signaling pathways regulating proliferation, differentiation, and cell survival. By using green fluorescent protein (G;F P) fusion proteins, we found that wild-type lambda PKC localized predominan tly to the cytoplasm, whereas both a kinase-defective mutant and an activat ion loop mutant accumulated in the nucleus. We have mapped a functional nuc lear localization signal (NLS) to the N-terminal part of the zinc finger do main of lambda PKC. Leptomycin B treatment induced rapid nuclear accumulati on of GFP-lambda as well as endogenous lambda PKC suggesting the existence of a CRM1-dependent nuclear export signal (NES), Consequently, we identifie d a functional leucine-rich NES in the linker region between the zinc finge r and the catalytic domain of lambda PKC. The presence of both the MLS and NES enables a continuous shuttling of lambda PKC between the cytoplasm and nucleus. Our results suggest that the exposure of the NLS in both lambda- a nd zeta PKC is regulated by intramolecular interactions between the N-termi nal part, including the pseudosubstrate sequence, and the catalytic domain. Thus, either deletion of the N-terminal region, including the pseudosubstr ate sequence, or a point mutation in this sequence leads to nuclear accumul ation of lambda PKC. The ability of the two atypical PKC isoforms to enter the nucleus in HeLa cells upon leptomycin B treatment differs substantially . Although lambda PKC is able to enter the nucleus very rapidly, zeta PKC i s much less efficiently imported into the nucleus. This difference can be e xplained by the different relative strengths of the NLS and NES in zeta PKC compared with zeta PKC.