Plakophilin, armadillo repeats, and nuclear localization

Plakophilins are armadillo-repeat containing proteins, identified through their localization to desmosomes. Expressed in a wide range of tissues, plakophilins are largely nuclear in most cell types [Schmidt et al. (1997) Cell Tissue Res 290:481; Mertens et al. (1996) J. Cell Biol 135:1009]. Using Xenopus embryos and cultured A6 cells, together with myc- and green fluorescent protein (GFP)-tags, we found that both the N-terminal, non-armadillo repeat "head" and the C-terminal armadillo repeat-containing regions can enter nuclei. The "arm" repeat domain is predominantly cytoplasmic and concentrated at the cell cortex, whereas the head and full-length polypeptides are concentrated in the nucleus. The head domain can also be seen to decorate and disrupt keratin filament network organization in some cells. In the course of these studies, we found that the distribution of the myc-epitope and green fluorescence differed in fixed cells, e.g., while the green fluorescence of a myc- and GFP-tagged head domain polypeptide was usually exclusively nuclear, a substantial fraction of the myc-immunoreactivity was cytoplasmic. Treating cells with the translation inhibitor cycloheximide reduces the cytoplasmic myc-signal, suggesting that it represented nascent polypeptides awaiting folding and nuclear import. Based on these types of experiments, GFP can be seen as a marker of the distribution of the mature form of the tagged polypeptide.