Recent studies on the subcellular distribution of cytoplasmic plaque proteins of intercellular junctions have revealed that a number of such proteins can also occur in the cyto-and the nucleoplasm. This occurrence in different, and distant locations suggest that some plaque proteins play roles in cytoplasmic and nuclear processes in addition to their involvement in cell-cell adhesive interactions. Plakophilin (PKP) 3, a member of the arm-repeat family of proteins, occurs, in a diversity of cell types, both as an architectural component in plaques of desmosomes and dispersed in cytoplasmic particles. In immuno-selection experiments using PKP3-specific antibodies, we have identified by mass spectrometric analysis the following RNA-binding proteins: Poly (A) binding protein (PABPC1), fragile-X-related protein (FXR1), and ras-GAP-SH3-binding protein (G3BP). Moreover, the RNA-binding proteins codistributed after sucrose gradient centrifugation in PKP3-containing fractions corresponding to 25-35 S and 45-55 S. When cells are exposed to environmental stress (e.g., heat shock or oxidative stress) proteins FXR1, G3BP, and PABPC1 are found, together with PKP3 or PKP1, in "stress granules" known to accumulate stalled translation initiation complexes. Moreover, the protein eIF-4E and the ribosomal protein S6 are also detected in PKP3 particles. Our results show that cytoplasmic PKP3 is constitutively associated with RNA-binding proteins and indicate an involvement in processes of translation and RNA metabolism. INTRODUCTIONOver the past decade, several studies on the subcellular distribution of the plaque proteins of adhering junctions have revealed that a number of such proteins are not only constituents of cell-cell contact structures but also found dispersed in the cytoplasm and nucleus. This dual location suggests that in addition to establishing and maintaining cell adhesive functions these proteins may also play roles in nuclear and ribonucleoprotein processing mechanisms. Such proteins include members of the arm-repeat family, which are characterized by variable numbers of an ϳ42-amino acid motif (Peifer et al., 1994). Among these nonjunctional functions the signaling roles of ␤-catenin and protein p120 have been rather well characterized (for reviews, see Anastasiadis and Reynolds, 2000;Huelsken and Birchmeier, 2001;Nelson and Nusse, 2004). For example, ␤-catenin is a key player in the Wnt pathway, directly mediating downstream events through transactivation of transcription factors of the Lef1/ TCF family to coordinate the activation of gene targets (Clevers and van de Wetering, 1997). Additional more recent evidence indicates that protein p120 also regulates cadherin turnover at the cell surface, thereby controlling the amount of cadherin available for cell adhesion (Kowalczyk and Reynolds, 2004;Reynolds and Roczniak-Ferguson, 2004) Less is known on the functions of the plakophilins (PKPs), which are characteristic plaque proteins of desmosomes and also occur nearly ubiquitously and constitutively in the cytoplasm...

Birt-Hogg-Dube (BHD) is a tumor suppressor gene syndrome associated with fibrofolliculomas, cystic lung disease, and chromophobe renal cell carcinoma. In seeking to elucidate the pathogenesis of BHD, we discovered a physical interaction between folliculin (FLCN), the protein product of the BHD gene, and p0071, an armadillo repeat containing protein that localizes to the cytoplasm and to adherens junctions. Adherens junctions are one of the three cell-cell junctions that are essential to the establishment and maintenance of the cellular architecture of all epithelial tissues. Surprisingly, we found that downregulation of FLCN leads to increased cell-cell adhesion in functional cell-based assays and disruption of cell polarity in a three-dimensional lumen-forming assay, both of which are phenocopied by downregulation of p0071. These data indicate that the FLCN-p0071 protein complex is a negative regulator of cell-cell adhesion. We also found that FLCN positively regulates RhoA activity and Rho-associated kinase activity, consistent with the only known function of p0071. Finally, to examine the role of Flcn loss on cell-cell adhesion in vivo, we utilized keratin-14 cre-recombinase (K14-cre) to inactivate Flcn in the mouse epidermis. The K14-Cre-Bhdflox/flox mice have striking delays in eyelid opening, wavy fur, hair loss, and epidermal hyperplasia with increased levels of mammalian target of rapamycin complex 1 (mTORC1) activity. These data support a model in which dysregulation of the FLCN-p0071 interaction leads to alterations in cell adhesion, cell polarity, and RhoA signaling, with broad implications for the role of cell-cell adhesion molecules in the pathogenesis of human disease, including emphysema and renal cell carcinoma.