Plakophilin 2: a critical scaffold for PKCα that regulates intercellular junction assembly

Plakophilins (PKPs) are armadillo family members related to the classical cadherin-associated protein p120ctn. PKPs localize to the cytoplasmic plaque of intercellular junctions and participate in linking the intermediate filament (IF)-binding protein desmoplakin (DP) to desmosomal cadherins. In response to cell–cell contact, PKP2 associates with DP in plaque precursors that form in the cytoplasm and translocate to nascent desmosomes. Here, we provide evidence that PKP2 governs DP assembly dynamics by scaffolding a DP–PKP2–protein kinase Cα (PKCα) complex, which is disrupted by PKP2 knockdown. The behavior of a phosphorylation-deficient DP mutant that associates more tightly with IF is mimicked by PKP2 and PKCα knockdown and PKC pharmacological inhibition, all of which impair junction assembly. PKP2 knockdown is accompanied by increased phosphorylation of PKC substrates, raising the possibility that global alterations in PKC signaling may contribute to pathogenesis of congenital defects caused by PKP2 deficiency

(A) PKP2 reexpression enhances DP border localization during PKP2 knockdown

Silencing-resistant FLAG-tagged PKP2 was coexpressed with PKP2 or nontargeting (NT) siRNA in SCC9 cells. Cells were stained for DP (right), FLAG, and PKP2 (using the same secondary antibody; left). Intensity of DP border fluorescence at borders between pairs of transfected cells or pairs of untransfected cells is shown on the far right. *, P < 0.001. (B) PKC activation rescues DP border localization during PKP2 knockdown. PKP2 siRNA or NT siRNA cells were treated with 15 nM PMA for 30 min and stained for DP. DP border fluorescence quantitation is shown on the bottom. **, P < 0.001. (C) PKC expression rescues DP border localization during PKP2 knockdown. SCC9 cells transfected with PKP2 or NT siRNA were infected with wild-type PKCα or constitutive active PKCα retrovirus and stained for PKP2 and DP. (C, top) Overexpression of PKCα rescues DP border localization. (C, bottom) DP localization at cell–cell borders was assessed for robustness by taking into account the percentage of occupied border, border continuity, and fluorescence intensity. Borders were scored on a graded scale of 1–5 and plotted as percentages of total borders counted. (C, bottom right) Representative borders scored from 1 to 5. 5 represents the most mature border, with most continuous and intense DP fluorescence, and 1 represents the least mature border, with minimal to no DP fluorescence. Graphs represent mean values from three independent experiments. NT and PKP2 siRNA experiments were performed at same time but separated in the graphs for clarity. Error bars represent SEM. Bars, 10 μm.<p><b>Copyright information:</b></p><p>Taken from "Plakophilin 2: a critical scaffold for PKCα that regulates intercellular junction assembly"</p><p></p><p>The Journal of Cell Biology 2008;181(4):605-613.</p><p>Published online 19 May 2008</p><p>PMCID:PMC2386101.</p><p></p