Nuclear import and export signals enable rapid nucleocytoplasmic shuttling of the atypical protein kinase C λ

The atypical protein kinase C (PKC) isoenzymes, lambda /L-and zeta PKC, play important roles in cellular signaling pathways regulating proliferation, differentiation, and cell survival. By using green fluorescent protein (G;FP) fusion proteins, we found that wild-type lambda PKC localized predominantly to the cytoplasm, whereas both a kinase-defective mutant and an activation loop mutant accumulated in the nucleus. We have mapped a functional nuclear localization signal (NLS) to the N-terminal part of the zinc finger domain of lambda PKC. Leptomycin B treatment induced rapid nuclear accumulation of GFP-lambda as well as endogenous lambda PKC suggesting the existence of a CRM1-dependent nuclear export signal (NES), Consequently, we identified a functional leucine-rich NES in the linker region between the zinc finger 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- and zeta PKC is regulated by intramolecular interactions between the N-terminal part, including the pseudosubstrate sequence, and the catalytic domain. Thus, either deletion of the N-terminal region, including the pseudosubstrate sequence, or a point mutation in this sequence leads to nuclear accumulation 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 is much less efficiently imported into the nucleus. This difference can be explained by the different relative strengths of the NLS and NES in zeta PKC compared with zeta PKC.