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 α-splice variant of p73 (p73α) 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 μM 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 15N, 1H- 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.
['Neira|José L|JL|', 'Rizzuti|Bruno|B|', 'Ortega-Alarcón|David|D|', 'Giudici|A Marcela|AM|', 'Abián|Olga|O|', 'Fárez-Vidal|María Esther|ME|', 'Velázquez-Campoy|Adrián|A|']
D051186:Armadillo Domain Proteins D006801:Humans D008958:Models, Molecular D062105:Molecular Docking Simulation D051191:Plakophilins D011485:Protein Binding D011487:Protein Conformation D000072417:Protein Domains D054730:Protein Interaction Domains and Motifs D000071682:Sterile Alpha Motif D000072160:Tumor Protein p73
NMR Isothermal titration calorimetry Molecular dynamics Protein-protein interactions