C-terminal truncation of Dlk/ZIP kinase leads to abrogation of nuclear transport and high apoptotic activity

Dlk (also termed ZIP kinase) is a novel serine/threonine kinase with a uniq ue C-terminal domain that is rich in arginine and contains three putative N LS motifs and a functional lecuine zipper. Dlk is indeed localized in the n ucleus where it shows a speckled distribution, To elucidate the biological functions of Dlk, we wanted to identify the signals relevant for nuclear tr ansport and further the nuclear structures which Dlk binds to. Expression o f various deletion and point mutations of Dlk as GFP fusion proteins reveal ed that the leucine zipper is required for association with speckles and th e most C-terminal NLS is necessary and sufficient for nuclear transport. In terestingly, a C-terminal deletion mutant defective for nuclear transport e xhibited a pronounced colocalization with actin filaments and, even more st rikingly, was a very potent inducer of apoptosis, This apoptotic activity w as abrogated, however, when this mutant was retargeted to the nucleus via a heterologous NLS from large T, indicating that Dlk only exerts an apoptoti c activity in the cytoplasm, To identify the speckle like structures to whi ch Dlk binds we performed immunofluorescence analyses with antibodies direc ted against representative marker proteins of replication, transcription, o r splicing centers. None of these marker proteins revealed a colocalization with Dlk. Instead, we found a partial colocalization with PML bodies which seem to play a key role in regulation of apoptosis, Taken together, these data strongly suggest a functional role for Dlk in control of cell survival which is dependent on its subcellular localization.