The armadillo-repeat domain of plakophilin 1 binds the C-terminal sterile alpha motif (SAM) of p73

Background: Plakophilin 1 (PKP1) is a component of desmosomes, which are key structural components for cell-cell adhesion, and can also be found in other cell locations. The p53, p63 and p73 proteins belong to the p53 family of transcription factors, playing crucial roles in tumour suppression. The ?-splice variant of p73 (p73?) has at its C terminus a sterile alpha motif (SAM); such domain, SAMp73, is involved in the interaction with other macromolecules. Methods: We studied the binding of SAMp73 with the armadillo domain of PKP1 (ARM-PKP1) in the absence and the presence of 100 mM NaCl, by using several biophysical techniques, namely fluorescence, far-ultraviolet circular dichroism (CD), nuclear magnetic resonance (NMR), isothermal titration calorimetry (ITC), and molecular docking and simulations. Results: Association was observed between the two proteins, with a dissociation constant of ~5 ?M measured by ITC and fluorescence in the absence of NaCl. The binding region of SAMp73 involved residues of the so-called "middle-loop-end-helix" binding region (i.e., comprising the third helix, together with the C terminus of the second one, and the N-cap of the fourth), as shown by N, H- HSQC-NMR spectra. Molecular modelling provided additional information on the possible structure of the binding complex. Conclusions: This newly-observed interaction could have potential therapeutic relevance in the tumour pathways where PKP1 is involved, and under conditions when there is a possible inactivation of p53. General significance: The discovery of the binding between SAMp73 and ARM-PKP1 suggests a functional role for their interaction, including the possibility that SAMp73 could assist PKP1 in signalling pathways.