Phosphorylation of the desmoplakin COOH terminus negatively regulates its interaction with keratin intermediate filament networks

Desmoplakins (DPs) are the most abundant proteins in the innermost portion of the desmosomal plaque and have been proposed to play a role in the attachment of intermediate filaments (IF) to cell-cell contact sites. Our previous results suggest that the globular end domains of DP perform dual functions: first, to target DP to the desmosome via the NH₂ terminus and second, to attach IF to the desmosomal plaque via the COOH terminus. When ectopically expressed in most cultured cells, the COOH terminus plus the rod domain (DP.ΔN.SerC23) exhibits striking coalignment with keratin IF networks. However, in certain cell types (e.g. PtK2) or in cells treated with forskolin to activate protein kinase A, DP.ΔN.SerC23 exhibits a diffuse cytoplasmic distribution. A variant molecule (DP.ΔN.GlyC23) in which a serine located 23 amino acids from the COOH terminus is altered to a glycine, thereby disrupting a protein kinase A consensus phosphorylation site, co- localizes with keratin IF networks regardless of cell type or forskolin treatment. Analysis of the phosphopeptide maps of these DP variants and endogenous DP is consistent with the phosphorylation of the serine 23 residues from the COOH terminus. These results suggest that phosphorylation of a specific residue in the DP COOH terminus may negatively regulate its interaction with keratin IF networks.