PLAKOPHILINS 1A AND 1B - WIDESPREAD NUCLEAR PROTEINS RECRUITED IN SPECIFIC EPITHELIAL-CELLS AS DESMOSOMAL PLAQUE COMPONENTS

The cytokeratin-binding, basic 80.5 kDa polypeptide plakophilin 1 (''b and 6 protein'' of bovine muzzle desmosome fractions) has originally b een described as a single molecular species, localized to desmosomal p laques of certain cell types, mostly stratified squamous epithelia and complex epithelia. We now report that this protein exists in at least two different isoforms: 726 amino acids (aa), plakophilin 1a; and 747 aa, plakophilin Ib. This reflects the splicing of the 21 aa-encoding exon 7 of the human plakophilin-1 gene and that each mRNA splice form can occur in two polyadenylation forms of 2.7 kb and 5.3 kb. Antibodie s recognizing either isoform and/or others that are specific for the e xon-encoded sequence of form 1b have allowed, in combination with immu nolocalization protocols minimizing losses of diffusible proteins, the detection of both isoforms in the nucleoplasm of diverse kinds of cul tured cells and tissues, including desmosome-forming cells as well as cells that never form desmosomes. The protein has also been identified in manually isolated nuclei (germinal vesicles) of Xenopus laevis ooc ytes. Plakophilin 1a accumulates in nuclei as shown by suitable immuno localization protocols and upon overexpression following transfection with cDNAs, but is also located in desmosomes of stratified and comple x epithelia. By contrast, isoform 1b has been found exclusively in nuc lei, even in cells connected by desmosomes immunostained with plakophi lin 1a-reactive antibodies. We conclude that plakophilins 1a and 1b ar e constitutive nuclear proteins encoded by the same gene, which is not expressed in relation to epithelial differentiation path ways, wherea s the additional appearance of plakophilin 1a in desmosomal plaques of stratified and complex epithelia is regulated by an as yet unknown me chanism of differentiation-dependent topogenic recruitment. Possible f unctions of plakophilins are discussed in relation to recent reports o f the involvement of other members of the armadillo/plakoglobin multig ene family of proteins in cell surface-gene regulation signalling path ways.