401 PP2A-B55α dephosphorylates desmoplakin's C-terminus to regulate cell adhesion

Critical for the maintenance of epidermal stability and function are attachments between intermediate filaments (IF) and intercellular junctions called desmosomes. The desmosomal cytoplasmic plaque protein desmoplakin (DP) is essential for anchoring IF to the junction. DP-IF interactions are regulated by a phospho-regulatory motif within the DP C-terminus. Previously we showed that hypo-phosphorylated DP increased DP-IF interaction strength, impairing desmosome formation due to DP retention on IF, but generated stronger, more stable desmosomes in mature cell sheets. Thus, DP’s phospho-regulation provides a mechanism to finely tune junction assembly dynamics and intercellular adhesion strength. Our group identified GSK3b as capable of phosphorylating DP’s C-terminus; however, the phosphatase responsible for DP’s dephosphorylation was unknown. Protein phosphatase 2A (PP2A) comprises a scaffolding subunit (A), a catalytic subunit (C), and one of 23 different regulatory subunits (B) responsible for binding PP2A substrates. Here, we identify the PP2A-B55a holoenzyme as capable of dephosphorylating DP’s C-terminus. Using co-immunoprecipitation and colocalization analysis we demonstrate B55a as a new DP binding partner in keratinocytes and uncover a novel DP-dependent membrane-bound fraction of B55a in both 2D- and 3D- epidermal models. Furthermore, we show that PP2A-B55a activity increases tissue integrity in a manner similar to expression of a hypo-phosphorylated DP variant, S2849G DP. Finally, PP2A signaling is required for proper DP recruitment during desmosome assembly in vitro and in vivo in embryonic epidermis of B55a-deficient mice. Together, our data uncovers a potential mechanism for PP2A-B55a’s regulation of the DP-IF association and intercellular adhesion in the epidermis.